T Trilobites of New York
A N
I L L U S T R A T E D
Thomas E. Whiteley Gerald J. Kloc Carlton E. Brett with a forewo...
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T Trilobites of New York
A N
I L L U S T R A T E D
Thomas E. Whiteley Gerald J. Kloc Carlton E. Brett with a foreword by Rolf Ludvigsen
Published in cooperation Paleontological Research
COMSTOCK
with
Institution,
PUBLISHING
CORNELL
UNIVERSITY
Ithaca
London
and
the Ithaca,
New
York
A S S O C I A T E S , a division o f
PRESS
G U I D E
C o p y r i g h t © 2002 by Cornell University
All rights r e s e r v e d . Except for brief quotations in a review, this book, or parts thereof, must not be r e p r o d u c e d in any form without permission in writing from the publisher. For information a d d r e s s Cornell University Press, Sage House, 512 East State Street, Ithaca, N e w York 14850.
First p u b l i s h e d 2 0 0 2 by Cornell University Press Published in c o o p e r a t i o n with the Paleontological Research Institution, Ithaca, N e w York
Printed in the United States of A m e r i c a
Library of C o n g r e s s Cataloging-in-Publication Data Whiteley, T h o m a s E. ( T h o m a s Edward), 1 9 3 2 Trilobites of N e w York : an illustrated guide / Thomas E. Whiteley, G e r a l d J. Kloc, a n d Carlton E. Brett, p.
cm.
Includes b i b l i o g r a p h i c a l references and index. ISBN 0-8014-3969-9 (acid-free paper) 1. Trilobites—New York (State)
I. Kloc, Gerald J.
II. Brett, Carlton E. (Carlton Elliot)
III. Paleontological
Research Institution (Ithaca, N.Y.)
IV. Title.
QE821 .W397
2002
565'.39'09747—dc21 2001054767
Cornell University Press strives to use environmentally r e s p o n s i b l e suppliers a n d materials to the fullest extent possible in the p u b l i s h i n g of its b o o k s . Such materials include v e g e t a b l e - b a s e d , low-VOC inks a n d acid-free papers that are r e c y c l e d , totally chlorine-free, or partly c o m p o s e d of n o n w o o d fibers. For further information, visit our website at www.cornellpress.cornell.edu.
Cloth printing
10
9 8 7 6 5 4 3 2 1
Contents
Text Figures vii
Chapter
Text Tables ix
THE
Foreword xi
NEW YORK
Preface xv
4
PALEOZOIC
GEOLOGY
OF 43
Overview 4 3 C a m b r i a n Period 4 6
Chapter
1
Ordovician Period 1
BACKGROUND INFORMATION
Silurian Period 81 D e v o n i a n Period
Historical Notes 1
56
95
Trilobite Names 2
Chapter Chapter
2
THE T R I L O B I T E S 4
THE B I O L O G Y OF T R I L O B I T E S Exoskeleton Ontogeny
5
117
O r d e r Redlichiida
4
118
Order Corynexochida
13
Soft B o d y Parts
O r d e r Agnostida
117
O r d e r Lichida
17
124
Order Phacopida
L i f e - M o d e 21
O r d e r Proetida Evolution and Cladistics 30
O r d e r Asaphida
128 148 157
O r d e r Ptychopariida
Chapter
118
163
3 32
TAPHONOMY
Appendix A: Trilobites and T h e i r E n v i r o n m e n t s
Death, Decay, and Disarticulation 32
Appendix B : T h e P h o t o g r a p h y
Transport and Reorientation
Glossary
35
172
Fragmentation and Biased Preservation 36
References
Abrasion, C o r r o s i o n , and E n c r u s t a t i o n
Trilobite Index
37
Fossil Diagenesis: G e o c h e m i c a l Processing of Potential Fossils 37
168
170
177 191
Index 201 Plates
204
Trilobite Taphofacies 4 0 Trilobite Lagerstatten
40
v
Text Figures
2.1
Trilobite structure using Eldredgeops rami using Kettneraspis
5
2.2
Trilobite structure
2.3
T h e structure of the trilobite c e p h a l o n using Calymene species
tubercidata
6
2.4
Trilobite eyes 9
2.5
Cephalic sutures 9
2.6
Hxoskeletal pits or circular perforations
2.7
Ventral a n a t o m y of the exoskeleton
2.8
O n t o g e n y of the trilobite
2.9
Trilobite exoskeletons with attached fauna or injury
2.10
Trilobite appendage reconstruction and n o m e n c l a t u r e
2.11
Ventral a n a t o m y and appendages 20
8
11
12
14
2.12
Internal a n a t o m y of the trilobite 22
2.13
Trilobite shapes and functions 23
18 19
2.14
Trilobite traces 25
2.15
Trilobite injuries 27
2.16
Cryptolithus, the most-studied trilobite genus f o u n d in New York
3.1
fossil assemblages reflecting various c o n d i t i o n s and t i m i n g of burial
3.2
C o n d i t i o n s for the f o r m a t i o n of pyritized, well-preserved fossils 39
28 34
4.1
T i m e scale for Earth history and for the Paleozoic rocks in New York 44
4.2
P r e c a m b r i a n Grenville ( 1 . 0 B P ) m e t a m o r p h i c / i g n e o u s rocks 4 6
4.3
M a p showing the extent of the Grenville belt in eastern N o r t h A m e r i c a 47
4.4
Paleomagnetic reconstruction of the supercontinenl Rodinia
4.5
Position of the Laurentian plate and n e i g h b o r i n g Baltica and Avalonia
48
terranes from the C a m b r i a n to the Devonian 49 4.6
C a m b r i a n rocks in New York 50
4.7
New York in the C a m b r i a n 52
4.8
C l o s e - u p o f Upper C a m b r i a n Potsdam S a n d s t o n e 5 3
4.9
Upper C a m b r i a n limestones 54
4.10
Simplified stratigraphy of the C a m b r i a n - O r d o v i c i a n rocks in the T a c o n i c allochthon
55
4.11
Lower C a m b r i a n a l l o c h t h o n o u s beds of the low T a c o n i c M o u n t a i n s
4.12
Stratigraphic chart of the Ordovician exposures in New York 57
56
4 . 1 3 A - C New York during the Early and early M i d d l e O r d o v i c i a n 58 4.13D-F
New York during the Middle O r d o v i c i a n 59
4.14
Details of the stratigraphy of the Chazy G r o u p in northeastern New York 63
4.15
Flute casts on Austin Glen graywacke 65
4.16
Black River limestones 66
4.17
Middle Ordovician Black River G r o u p 67
4.18
Close-up o f sharp Black River/Trenton ( W a t e r t o w n - N a p a n e e ) c o n t a c t 6 8
4.19
Close-up of storm beds in the Middle Ordovician Kings Falls L i m e s t o n e 69
viii
TEXT 4.20
M a p s of New York during late Middle Ordovician times 70
4.21
M i d d l e Trenton at Trenton Falls 72
4.22
M i d d l e Ordovician Trenton G r o u p limestone 7 3
4.23
Trenton at Trenton Falls 74
FIGURES
4.24
M i d d l e O r d o v i c i a n Dolgeville and Utica rocks 77
4.25
New York maps during the late M i d d l e Ordovician and the Upper
4.26
Ordovician/Silurian u n c o n f o r m i t i e s
4.27
Stratigraphic chart of the Silurian rocks in New York 82
4.28
O r d o v i c i a n - S i l u r i a n ( C h e r o k e e ) u n c o n f o r m i t y / O r d o v i c i a n - L o w e r Silurian
4.29
M a p s o f New York during W e n l o c k times
4.30
I r o n d e q u o i t - R o c h e s t e r b i o h e r m at the upper part of the Lower Silurian
Ordovician
succession
79 81
84
Clinton Group
87
88
4.31
M i d Silurian successions 91
4.32
Silurian c a r b o n a t e s 92
4.33
D o m a l stromatolites
4.34
C o m p o s i t e stratigraphic chart for the n o r t h e r n and central parts of the
4.35
N e w York during the Early D e v o n i a n 97
Appalachian Basin
94
96
4.36
Upper Silurian/Lower Devonian rocks
4.37
New York during the early M i d d l e D e v o n i a n
99
4.38
Upper Silurian/Onondaga Limestone
103
104
4.39
M i d d l e D e v o n i a n rocks
4.40
New York d u r i n g the Middle D e v o n i a n H a m i l t o n deposition
4.41
Devonian successions
4.42
M i d d l e D e v o n i a n / U p p e r Devonian successions
4.43
Stratigraphy of the Upper Devonian and the upper Middle Devonian of New
5.1
Lichids from the lower and lowest M i d d l e Devonian of New York
125
5.2
Key cephalic differences between D a l m a n i t i d a e and Synphoriidae
136
5.3
Trilobites of the families D a l m a n i t i d a e and Synphoriidae
5.4
Lower D e v o n i a n and lower Middle Devonian p h a c o p i n s
5.5
Features of New York Cryptolithus
York
106 108
113 114
115
164
138 143
Text Tables
5.1
Trilobites of the suborder Agnostina
118
5.2
Trilobites of the suborder Eodiscina
119
5.3
Defining features of the New York asteropygins
5.4
Features of the genera Bellacartwrightia and
129
5.5
Features of the Lower Devonian phacopids f r o m New York
5.6
Features of the Middle Devonian phacopids of New York
5.7
Features of the genus Odontocephalus of New York
5.8
Features of the described proetids of New York
Greenops
153
130
147
141 142
Foreword
T o m Whiteley, a n accomplished a m a t e u r paleontologist, has
" l o v e r " ) are those who pursue an activity out of interest instead
taken the lead in c o m p i l i n g a m u c h - n e e d e d popular a c c o u n t
of financial gain. A m a t e u r s , like professionals, tend to specialize.
of the trilobites of New York.
S u m p t u o u s l y illustrated with
In New York, as in O n t a r i o , O h i o , I n d i a n a , O k l a h o m a , and
generous photographs of c o m p l e t e s p e c i m e n s of New York trilo-
U t a h — r e g i o n s that have a lot of fossiliferous Lower Paleozoic
bites, this b o o k is m o r e than a regional field guide. It also testi-
o u t c r o p — m a n y a m a t e u r s and fossil collectors c o n c e n t r a t e their
fies to G e r r y Kloc's expertise in preparation and Carlton Brett's
efforts on trilobites. T h e y have collected and prepared trilobite
keen insight about the rocks and c o m p l e x facies of the state. In
s p e c i m e n s that, in quality and c o m p l e t e n e s s , rival those described
essence, the b o o k reprises the w o r k of Charles Walcott, a n -
by a c a d e m i c paleontologists. T h e extent of p o p u l a r interest in
other accomplished a m a t e u r paleontologist of a c e n t u r y and a
trilobites is exemplified by h u n d r e d s of websites on the Internet,
quarter ago.
almost all of t h e m hosted by a m a t e u r s .
T h e technical literature on trilobites is vast. It is dispersed
T h e p h o t o g r a p h s in this publication reveal why trilobites have
through n u m e r o u s paleontological j o u r n a l s , in specialized b o o k s ,
long been considered to be a m o n g the m o s t desirable and curious
and as century-old m o n o g r a p h s . Even a dedicated trilobite pale-
of fossils. A late Paleolithic h u n t e r in w h a t - i s - n o w central France
ontologist with access to a university library would have difficulty
carried o n e a r o u n d his neck as a p e n d a n t suspended from a
retrieving all of it. T h e popular literature on trilobites is a lot
leather t h o n g . A n c i e n t C h i n e s e philosophers called t h e m " s t o n e
easier to access, if only because there is so little of it. Few profes-
s i l k w o r m s " and " b a t s t o n e s . " T h e Pahvant Ute tribe of western
sional paleontologists have considered it i m p o r t a n t to write field
Utah knew t h e m as timpe khanitza pachavee, m e a n i n g "little water
guides to fossils along the lines of the popular regional guides to
bug like stone house." Strung as amulets, they were thought
flowers, m u s h r o o m s , trees, insects, and birds that are available in
to possess magical powers. In 1 6 9 8 , Edward Lhwyd, c u r a t o r of the
every bookstore across A m e r i c a . Fewer still have written b o o k s
A s h m o l e a n M u s e u m in O x f o r d , included in the pages of the
exploring the natural history of fossils.
Philosophical
Trilobites of New York is a lavishly illustrated bestiary of New
Transactions of the Royal Society a
plate with
a
finely
lined oval fossil f r o m the shales exposed near Llandeilo in south-
York trilobites in which o n e can sense the spirit of S a m u e l Latham
ern Wales. He t h o u g h t it was a "flat fish." In nearby areas in Wales
Mitchill's m o n o g r a p h , The Fishes of New York, published in 1 8 1 5 .
suffused with the legend of King Arthur, such fossils are called
Both books are basically encyclopedic and scientific in their
" s t o n e butterflies" and are widely believed to have been e n t o m b e d
approach, but each includes snippets of practical i n f o r m a t i o n .
in rock by spells cast by the wizard Merlin.
Mitchill advised fishermen that the blackfish will run in the spr-
European naturalists were fascinated by these fossils. Although
ing when the dogwood b l o s s o m s o p e n . W h i t e l e y suggests that a
they described and figured t h e m in considerable detail, they
3 5 - m m T I F F c o l o r image of a trilobite is a b o u t 25 megabytes
had great difficulty in d e t e r m i n i n g what they were and how they
in size and should be stored on a C D - R O M . T i m e s c h a n g e !
should be
Here is a s u m m a t i o n of nearly two centuries of discovery and
applied
classified.
the
term
The
great
Entomolithus
naturalist paradoxus
Carolus
Linnaeus
("paradoxical
stone
study of New York trilobites by a succession of paleontologists
i n s e c t " ) to such fossils f r o m Sweden; others t h o u g h t they were
and as an exposition of the natural history of these fascinating
fossil crabs or possibly weird mollusks.
fossils. T h e b o o k will be w e l c o m e d especially by A m e r i c a n pale-
W i t h their evocative and disparate n a m e s , these stony objects
ontologists, professional as well as amateur, and by anyone who
were corralled in
delights in the exquisite beauty of these ancient fossils.
Ernst
Professional paleontologists working on New York fossils have
Immanuel
Naturgeschichte
der
1771 Walch
w h e n the G e r m a n naturalist lohann published
Versteinerungen
the
third
[Natural
volume
History
of Der
of Petrifac-
always been greatly o u t n u m b e r e d by a m a t e u r paleontologists and
tions]. In it he proposed a collective n a m e derived f r o m the most
avocational fossil collectors. Although often ( a n d mistakenly) dis-
obvious f e a t u r e — t h e i r t h r e e - l o b e d appearance. Eventually natu-
missed as dilettantes, amateurs ( t h e root is the Latin amator, for
ralists studying fossils ( w h o were now b e g i n n i n g to be called pale-
XI
FOREWORD
XII
ontologists) accepted that trilobites c o m p r i s e a distinct group of
support from the state legislature to prepare a single volume on
fossil a r t h r o p o d s .
the fossils of the state. Hall had greater a m b i t i o n s , and entirely
By the early years of the n i n e t e e n t h century, trilobites of m a n y
due to his s t u b b o r n d e t e r m i n a t i o n and ability to browbeat legis-
different types had been d o c u m e n t e d f r o m Britain, G e r m a n y ,
lators, this v o l u m e b e c a m e the first of no fewer than 13 quarto
Scandinavia, and B o h e m i a . T h e n , as now, they attracted attention
volumes of the m o n o g r a p h series Palaeontology of New York. T h e
because of their peculiar shape, their striking o r n a m e n t a t i o n ,
series c o m p r i s e d thousands of pages and m a n y hundreds of plates
and their great age. A m o n g the m o s t ancient of fossils, they were
published over the next half century. To pursue his work, Hall
f o u n d mainly in the sectors of Earth history s o o n to be n a m e d
amassed
the C a m b r i a n and Silurian systems.
Beaverkill in Albany. Of Hall it can truly be said that he never met
huge fossil
collections at his laboratory along the
Trilobites are the most lifelike of f o s s i l s — m a n y well-preserved
a fossil he didn't covet. He hired a succession of assistants to
specimens belie their great antiquity and seem almost ready
collect, prepare, describe, and draw the specimens. Hall h i m s e l f
to arch their bodies, peer a b o u t with their c o m p o u n d eyes, and
described m a n y trilobites from the state in his Palaeontology, but
crawl forward as if to c o n t i n u e a j o u r n e y that was interrupted
independently o n e of his assistants m a d e the class Trilobita
hundreds of millions of years ago. A trilobite is an ancient a r t h r o -
his o w n .
pod, but it is certainly not a lesser a r t h r o p o d .
Charles Doolittle Walcott was a young m a n of 26 when he
Trilobite discoveries in the New W o r l d followed closely on
started to work for Hall in 1876. B o r n at New York Mills in the
those in the Old. By 1832 so m a n y different kinds of trilobites
M o h a w k Valley, he had little formal education but a wealth of
had been collected that J a c o b G r e e n , a professor at Jefferson
practical knowledge about fossils. A few years previously he had
Medical College in Philadelphia, had sufficient material to write
sold a collection of trilobites he had compiled from Trenton Falls
a 9 3 - p a g e m o n o g r a p h detailing all the species then k n o w n . He
on West C a n a d a Creek to the M u s e u m of Comparative Z o o l o g y
might have titled it Trilobites of New York instead of A Monograph
at Harvard University for $ 5 0 0 0 . At night, after he had finished
of the
his w o r k for Professor Hall, Walcott polished sections of tightly
Trilobites of North America,
because
of the
32
species
he
dealt with, all but 7 c a m e f r o m that state.
enrolled s p e c i m e n s of the trilobite Ceraurus in an a t t e m p t to
New York was h o m e to the first A m e r i c a n s c h o o l of geology.
d e t e r m i n e the structure of its infolded limbs. This was e n o r -
1824 Stephen Van Rensselaer provided the funding that
m o u s l y difficult—akin to attempting the restoration of an orchid
allowed A m o s Eaton to start the Rensselaer S c h o o l in the town of
by slicing serially through a rolled-up f l o w e r — a n d , not sur-
In
Troy. T h i s school left a solid m a r k on early A m e r i c a n geology
prisingly, resulted in inaccurately reconstructed trilobite limbs.
and
of
Walcott also collected f r o m localities in New York where few trilo-
g e o l o g i s t s — o n e that effectively d o m i n a t e d A m e r i c a n geological
bites had been k n o w n b e f o r e . He m a d e large collections f r o m
paleontology.
It graduated
a
remarkable contingent
surveys—along with a few paleontologists who ensured that New
deformed
York remained the paleontological heartland of North A m e r i c a
and f r o m Upper C a m b r i a n limestones near Saratoga Springs.
for the rest of the nineteenth century. O n e of the Rensselaer
However, Walcott chafed at the treatment he received f r o m the
graduates was sure that he had f o u n d a living trilobite.
mercurial Hall, w h o after a few years dumped him for a younger
As a m e m b e r of the United States " E x p l o r i n g Expedition
Lower C a m b r i a n
rocks of the Taconic M o u n t a i n s
and m o r e c o m p l i a n t assistant. Walcott benefited by gaining a
o f 1830," James Eights o f Albany was the f i r s t A m e r i c a n scien-
position
tist to study the m a r i n e animals, l a n d f o r m s , and geology of
b e c a m e director of the survey, and rose to b e c o m e the secretary
in
the newly f o r m e d
U.S. Geological Survey, t h e n
Antarctica and its surrounding islands. A m o n g his discoveries
of the S m i t h s o n i a n Institution. Walch might have n a m e d it, b u t
from the shallow seas a r o u n d the bleak S o u t h Shetland Islands
it was Walcott w h o conceived the trilobite taxon. He was the first
was a peculiar creature that he n a m e d Brongniartia
trilobitoides
to suggest that the class Trilobita (or, as s o m e paleontologists now
Brongniartia
favor, phylum Trilobita) was a group of arthropods quite distinct
had
f r o m crustaceans.
and boltoni.
illustrated B.
alongside
boltoni was
a
its
large
presumed fossil
relative,
trilobite
that
been
described f r o m ancient Silurian shales scooped up by tarriers digging the Erie Canal near Rochester. And if B.
New York had to relinquish its primacy in matters trilobitic
boltoni was
in the early years of the twentieth century as the research focus
a stony extinct trilobite, then surely the lively trilobitoides m u s t
shifted to o t h e r parts of the c o n t i n e n t — t o the Great Basin of
b e living m e m b e r s o f that a n c i e n t clan. T h e Antarctic a n i m a l ,
Nevada and Utah, to N e w f o u n d l a n d , to Virginia, to the Upper
however, t u r n e d o u t to be a c r u s t a c e a n — a n isopod of the genus
Mississippi Valley, and to the southern C a n a d i a n Rockies. In the
Serolis.
But even today Serolis trilobitoides (Eights)
is cited as a
1960s a new crop of paleontologists applying m o d e r n paleonto-
t e x t b o o k e x a m p l e o f the convergent evolution o f isopods and
logical ideas sparked renewed interest in the New York trilobites
trilobites.
that had been described a century earlier. A m o n g these scientists
New York p a l e o n t o l o g y entered a new state-sanctioned phase
were F r a n c o Rasetti, the paleontologist/nuclear physicist from
in the early 1840s w h e n James Hall, the state paleontologist and
J o h n s H o p k i n s University w h o focused on C a m b r i a n trilobites
the b e s t - k n o w n graduate of the
from the tortured rock of the Taconic region; Harry W h i t t i n g t o n ,
Rensselaer S c h o o l , received
FOREWORD
XIII
the Woodwardian professor of geology at C a m b r i d g e University,
ing us full circle as they update and e n h a n c e the story of the trilo-
who restored the a n a t o m y of pyritized
bites of New York, bringing new visions and fresh perspective to
Triarthrus f r o m Upper
Ordovician shales near R o m e ; and Niles Eldredge, the p a l e o n t o l -
these wonderful creatures.
ogist at the American M u s e u m of Natural History in New York City, whose work ensured the centrality of M i d d l e Devonian Phacops
(now Eldrcdgeops)
in
the
new
evolutionary
model
of
punctuated equilibria. So the authors of this b o o k are to be congratulated for bring-
ROLF
LUDVIGSEN,
Head
D e n m a n Institute for Research on Trilobites D e n m a n Island, British C o l u m b i a , C a n a d a
Preface
S c i e n t i s t s estimate that life on Earth may have begun as early as three billion years ago. life
was
confined
to
For m u c h of its history, however,
single-celled
bacteria.
Stromatolites,
New York State is and has long been a m a g n e t for trilobite hunters. Historically, New York was of central i m p o r t a n c e in the study of Paleozoic fossils, and New York's trilobites were
layered m o u n d s of sediment trapped by mats of blue-green
among
cyanobacteria, are the p r e d o m i n a n t fossils for nearly two billion
York strata are the source of m a n y s p e c i m e n s accepted worldwide
years
as the best of their kind. T h e s e fossil
of geologic
history.
Finally,
in
the
late
Precambrian
the
first
illustrated
fossils
in
North
America.
New
remains are actively
Ediacarian Period, about 5 7 0 million years ago, e n i g m a t i c soft-
sought, studied, and traded. W i t h its extensive shale deposits,
bodied forms of multicellular life first appeared as impressions in
New York is a particularly rich source of trilobites, m a n y of
sandstone.
which are shown for the first t i m e in this v o l u m e . M a n y out-
About 543 million years ago life m a d e a further p r o f o u n d
standing localities in New York State, from the majestic O r d o v i -
change in direction. A sudden burst of new o r g a n i s m s with hard
cian limestone bluffs of Trenton Falls to the Silurian beds in the
skeletons in the fossil record has b e e n called the " C a m b r i a n
great gorge of Niagara River to the D e v o n i a n shale cliffs of Lake
Explosion." T h e earliest fossil skeletons were simple tubes, p r o b -
Erie, c o n t i n u e to yield a b u n d a n t and spectacular trilobite fossils.
ably made by w o r m s ; shell material clearly m a d e by c o m p l e x
New York strata have also yielded m o r e trilobites with preserved
living creatures such as mollusks appeared a b o u t 5 4 0 million
appendages and o t h e r "soft p a r t s " than almost any other region
years ago. T h e n , beginning about 5 2 0 million years ago, highly
o f the world. T h e rarity and aesthetic beauty o f c o m p l e t e o u t -
sophisticated
stretched or enrolled trilobites gives trilobite fossils special value
skeletons
of trilobites,
early
representatives
of
Earth's most a b u n d a n t c o m p l e x a n i m a l s — t h e Phylum A r t h r o -
to
p o d a — a p p e a r e d in m a r i n e strata worldwide. T h e trilobites not
ranging f r o m a few millimeters to nearly a h a l f - m e t e r in length,
only appeared dramatically in the fossil record but for millions of
are featured in m u s e u m s all over the world; s o m e extraordinary
years they d o m i n a t e d it. Trilobites are the quintessential archaic m a r i n e animals. Few
collectors.
Spectacular,
ornate
trilobites
from
New York,
examples are prized by collectors and have been sold for t h o u sands of dollars.
if any other invertebrate fossils have attracted m o r e attention
Yet despite the f a m e of New York State's trilobites, no recent
from paleontologists and fossil collectors than these ancient
text has a t t e m p t e d to d o c u m e n t comprehensively these remark-
arthropods, distant relatives of today's crustaceans and insects.
able fossils. W i t h a little effort o n e can find trilobites in New
Paleontologists have learned a great deal about trilobites because
York State rocks ranging in age f r o m the t i m e of their earliest
they were ubiquitous in the oceans and seas of the early Paleo-
o c c u r r e n c e in the Early C a m b r i a n up to their last t i m e of
zoic Era and because they possessed readily preserved hard skele-
m a j o r a b u n d a n c e in the Devonian Period, a b o u t 3 7 0 million
tons. In the mid to late 1800s lithographed images of trilobites
years ago. T h u s , although New York strata do not d o c u m e n t the
became symbolic of the rapidly developing field of paleontology
entire evolutionary history of trilobites, the a b u n d a n t , high-
in New York State as well as in E n g l a n d , two hotbeds of early
quality material available in this area offers a rare o p p o r t u n i t y to
research by serious amateurs and professional scientists when
discover and study these intriguing representatives of early life
interest in the nascent field of geology was first b e g i n n i n g to
history.
burgeon. T h e beautifully preserved, segmented exoskeletons of
Trilobites of New York is
intended
to be a
nearly complete
trilobites—in shades of saddle brown and blue gray to b l a c k —
c o m p i l a t i o n of the trilobite species f o u n d in New York: a review
are truly spectacular o b j e c t s , but perhaps above all it is the well-
of the biology of the trilobite; insight into trilobite preservation
developed, c o m m o n l y c o m p o u n d eyes of trilobites that have
in the rocks; a s h o r t course on the Paleozoic geology of New York,
made them attractive to paleontologists and lay persons alike.
emphasizing trilobite-bearing strata; and a collection of high-
Trilobites were certainly a m o n g the first organisms to f o r m rela-
quality images of representative New York trilobites. T h e b o o k
tively clear images of their world.
is not and was never intended to be a field guide or identificaxv
PREFACE
XVI
tion m a t r i x to trilobites. As s u c h , there is no specific local-
the basis of quality, rarity, and representation of the material
ity i n f o r m a t i o n , although
present in the New York rocks.
tographs very useful
in
m a n y readers will
find
identification and in
the p h o -
differentiating
similar species.
Anyone w h o collects fossils of any kind understands that finding a c h o i c e specimen is only part of the process. Most trilo-
T h i s work started m o r e than 20 years ago as an attempt by
bites are e m b e d d e d in a m a t r i x of shale or limestone, and to really
T o m Whiteley to c o m p i l e illustrations of the trilobites found in
appreciate their quality o n e must prepare t h e m by removing the
New York. Although New York has a history of trilobite discov-
stone f r o m the part of the trilobite that is to be displayed. Most
ery and research since 1824, references on trilobites are scattered
of the illustrated s p e c i m e n s were prepared or " t o u c h e d u p " by
and often not available except in the libraries of large universi-
Gerald Kloc. Even s o m e m u s e u m specimens were worked o n ,
ties. T h e only collective works on New York trilobites were the
with the m u s e u m ' s permission, to bring out the details concealed
classic volumes by James Hall, and the last of these was published
by m a t r i x .
in 1888. It soon b e c a m e apparent that the New York Paleozoic
As work on this project progressed, it b e c a m e clear that to
exposures are too varied for o n e person to really understand all
c o m p i l e data and images on trilobites was not e n o u g h . A listing
the trilobites and their l o c a t i o n s . H e n c e , Gerald K l o c b e c a m e
of nearly 5 0 0 separate species, while interesting to a few special-
involved with this project for his knowledge of the Silurian and
ists, is not very helpful to the collector or the student. To be really
Devonian exposures and their trilobites and for his contacts in
useful, we needed to include i n f o r m a t i o n on why the trilobites
the amateur c o m m u n i t y .
are f o u n d where they are and how their preservation c o m e s
As in all research p r o g r a m s , b a c k g r o u n d literature is an essen-
a b o u t . To this e n d , Carlton Brett provided a review of trilobite
tial starting point. T h e r e are a few texts on trilobites that provide
t a p h o n o m y , as well as an overview of the geological history of the
m o r e in-depth i n f o r m a t i o n on the a n i m a l itself than we include.
New York Paleozoic.
T h e works
of Johnson
(1985),
Levi-Setti
(1975),
Ludvigsen
T h e r e m a i n i n g issue c o n c e r n e d the intended audience, or who
( 1 9 7 9 b ) , and W h i t t i n g t o n ( 1 9 9 2 ) are g o o d references for addi-
is expected to read the b o o k . Dr. Warren Allmon of the Paleon-
tional reading. Every trilobite publication has references, and
tological Research Institution suggested that high-school earth
these references lead to o t h e r p u b l i c a t i o n s , which in turn lead to
science teachers represented the right level for content, as this
m o r e references and s o o n . H u n d r e d s o f publications were e x a m -
level would provide i n f o r m a t i o n useful to the collector, teacher,
ined, and the relevant o n e s were put into a database. Fieldwork
and student. Dr. Allmon also made the first contacts with Cornell
was also carried out in the m o r e p r o m i s i n g exposures. However,
University Press.
this fieldwork resulting f r o m literature surveys was limited and
As already m e n t i o n e d , it was necessary in the course of this
nowhere near the h o u r s and days of work spent by m a n y p r o -
work to visit m a n y of the m a j o r natural history m u s e u m s in
fessionals and amateurs in the field collecting each individual
the northeast United States. T h e A m e r i c a n M u s e u m o f Natural
specimen. S p e c i m e n s of the quality illustrated in this b o o k are
History, M u s e u m o f C o m p a r a t i v e Z o o l o g y (Harvard), Peabody
u n c o m m o n , even rare. A n u m b e r of the trilobite s p e c i m e n s are
M u s e u m (Yale), New York State M u s e u m , National M u s e u m of
unique in their quality of preservation and preparation, and very
Natural History ( S m i t h s o n i a n ) , Rochester M u s e u m and Science
few like them exist anywhere.
Center, Royal O n t a r i o M u s e u m , and Paleontological Research
T h e trilobite collections in a n u m b e r of m a j o r n o r t h e a s t e r n
Institution were visited, s o m e m a n y times, and their collections
United States m u s e u m s were e x a m i n e d carefully, and s p e c i m e n s
carefully e x a m i n e d . T h e c o o p e r a t i o n o f the m u s e u m s ' m a n a g e -
that were unusual or of high quality were p h o t o g r a p h e d and the
m e n t and their collections managers in particular was unreserved.
a c c o m p a n y i n g i n f o r m a t i o n recorded. T h e s e m u s e u m collections
W i t h o u t their help, this work would not have been possible.
represent the efforts of dozens of individuals over a period of
The
cooperation
and
assistance
of
Fred
Collier,
Jan
m o r e than 150 years. A n u m b e r of a m a t e u r collectors m a d e their
T h o m p s o n , Ed Landing, Niles Eldredge, Janet Waddington, T i m
s p e c i m e n s available for photography, which was helpful as the
W h i t e , Wendy Taylor,
best and most c o m p l e t e material is often n o t in a m u s e u m . In
Westrop, and G e o r g e M c i n t o s h were all i m p o r t a n t to this work.
Paul
Krohn,
Fredrick
Shaw,
Stephan
a few cases research paleontologists m a d e their p h o t o g r a p h s of
Fred Collier, in particular, while at the United States National
uncommon
M u s e u m , greatly influenced the early direction of these efforts
material available
for
reproduction.
The
photo-
graphic procedures are provided in A p p e n d i x B, b u t in general
with his professionalism and enthusiasm.
the p h o t o g r a p h s were taken in a m u s e u m or in a l a b o r a t o r y envi-
We gratefully acknowledge the collectors w h o m a d e their
r o n m e n t . O f t e n the s p e c i m e n s were whitened with a m m o n i u m
s p e c i m e n s available for p h o t o g r a p h y and in s o m e instances
c h l o r i d e to b r i n g o u t detail. T h e images were then scanned into
donated these s p e c i m e n s to a m u s e u m . T h e y are William P i n c h ,
a digital file, and all of the final preparation of images was d o n e
Kent S m i t h , Lee Tutt, Paul K r o h n , James Scatterday, Gregory
on c o m p u t e r . No i n f o r m a t i o n was added or subtracted from the
Jennings, Fred Barber, Kym Pocius, S a m Insalaco, Kevin Brett,
digital image at any t i m e . Of the thousands of p h o t o g r a p h s , o n l y
Steve Pavelsky, Tod C l e m e n t s , Douglas DeRosear, Fred Wessman,
a b o u t 2 0 0 could be selected for the b o o k . Selection was m a d e on
G o r d o n Baird, and William Kirchgasser.
PREFACE
xvii
Rolf Ludvigsen, Nigel Hughes, and G e o r g e M c i n t o s h read early drafts of the b o o k and m a d e m a n y valuable suggestions. Warren Allmon made the first contacts with
Sandoval, Lou R o b i n s o n , and C a n d a c e Akins also provided valuable assistance. To all we express o u r gratitude.
Peter Prescott,
science editor of Cornell University Press, w h o was i n s t r u m e n t a l
T h o m a s E. Whiteley
in giving the b o o k focus and helped turn what was a collection
Gerald J. Kloc
of information and pictures into s o m e t h i n g publishable. Alyssa
Carlton E. Brett
T Trilobites of New York
1
Background Information
Historical Notes
Lyceum of Natural History of New York was established in New York City, and in 1823 the Albany Lyceum of Natural History
E. Lhwyd provided the first record of trilobites in the litera-
was f o u n d e d .
Isotelus gigas was described
of Natural History (New Y o r k ) .
in
the Journal of the
ture in 1 6 9 8 , with the publication of plates depicting two Welch
Lyceum
trilobites, identified as fish. In 1 7 7 1 , J. I. Walch originated the
the Trenton L i m e s t o n e of central New York, particularly Trenton
T h e Isotelus fossils
from
use of the n a m e " t r i l o b i t e " as a distinct class of a n i m a l . L. D.
Falls, were long k n o w n and collected for sale by the local residents
H e r r m a n n , however, used the term trilobus as part of the n a m e
and b e c a m e part o f m a n y early natural history collections. T h e
for a trilobite fossil, as early as 1 7 1 1 . ( F o r the very early trilobite
Arctinurus s p e c i m e n described by Bigsby was first f o u n d during
references, see the publications by H. B u r m e i s t e r ( 1 8 4 3 , 1 8 4 6 ) . ) In 1822, C. Stokes was the first to describe North A m e r i c a n
the digging of the Erie Canal locks in what is n o w L o c k p o r t , New York.
from
In 1 8 3 6 , New York began a general natural history survey of
Canada. J. E. DeKay provided the first unequivocal description
the state, including its geology and mineralogy, and at the same
of a New York trilobite, Isotelus gigas from Trenton Falls ( n o r t h
t i m e f o r m e d the New York Geological Survey. T h a t same year the
trilobites,
with
Asaphus
(now
Isotelus)
platycephalus
of Utica, New York), in 1824. T h i s report was followed by that of
first state paleontologist was n a m e d , T. A. C o n r a d , a Philadelphia
Arctinurus
conchologist.
boltoni
by
Bigsby
in
1825.
Qf
the
40
trilobites
described in the classic works by the Philadelphia physician J. Green in 1832 and 1835, most were from New York. New York State
took an
early leadership
role
For the geological survey the state was divided into four districts, and the results from each district were published as sepa-
in
North
rate v o l u m e s . Starting in 1 8 4 2 , the first of these was published.
American Paleozoic invertebrate paleontology, in part due to
The
the n u m b e r of lower Paleozoic exposures within the state and also
b e g i n n i n g of the career of New York's s e c o n d and most well-
Geology of the Fourth
District of New
York
(1843)
was
the
to the history of the state itself. T h e early 1800s saw a general
k n o w n state paleontologist, James Hall. Hall was also the princi-
expansion westward within the United States. New York par-
pal a u t h o r of the e i g h t - v o l u m e Palaeontology of New York, issued
ticipated both by pressing settlement into the rich farmlands
between 1847 and 1 8 9 4 . V o l u m e s 1, 2, 3, and 7 (written with
of western New York and by aggressively seeking to b e c o m e the
J. M. Clarke) c o n t a i n significant trilobite i n f o r m a t i o n and are the
c o m m u n i c a t i o n route to the nation's Midwest. Roads, canals,
p r i m a r y references for early trilobite work in New York. Hall had
and permanent c o n s t r u c t i o n were all part of these goals, and
a n u m b e r of assistants w h o began their careers with h i m : F. B.
all needed building stone to succeed. L i m e s t o n e was the ideal
M e e k , F. V. Hayden (future director of the United States Geolog-
material both for buildings and for the c e m e n t and m o r t a r to
ical Survey), C. A. W h i t e (future state paleontologist for Iowa),
hold them together. T h u s , small and large limestone quarries
W. A. G a b b , R. P. Whitfield, C. Calloway, C. D. Walcott (future
became c o m m o n along the H u d s o n - M o h a w k River corridor.
director of the United States Geological Survey and the secretary
T h e state government also was c o n c e r n e d about its knowledge
of the S m i t h s o n i a n I n s t i t u t i o n ) , C. E. Beecher (future professor
of the natural treasures c o n t a i n e d within its b o r d e r s . In 1818 the
at Yale University), and J. M. Clarke (future state paleontologist 1
BACKGROUND
INFORMATION
for New York). Hall was a difficult m a n to work with and c o n s e -
usually derived
quently had a high turnover in assistants. It is claimed that Hall
Greek, or I n d o - E u r o p e a n and are selected by the describing
t o o k on s o m e of his assistants to gain access to their personal
author. N a m e s are often latinized, and the gender of the species
fossil collections (Yochelson, 1 9 8 7 ) .
follows that of the genus. In other words, if the genus n a m e is
T h e turn o f the c e n t u r y introduced additional
important
c o n t r i b u t o r s to the knowledge of New York trilobites, such as R. R u e d e m a n n and P. R a y m o n d . By the early twentieth century,
from
Latin, ancient Greek, latinized ancient
c h a n g e d , the ending on the species n a m e must s o m e t i m e s change to agree with it in gender. G e n e r a , that are considered to be closely related on the basis
New York was no longer a m a j o r area for new trilobite discover-
of shared characteristics, are grouped into families.
ies, as the focus had shifted westward with the general expansion
families are larger and m o r e distinctive, it is often easier to deter-
of the United States. However, significant c o n t r i b u t i o n s are still
m i n e the family to which a new trilobite belongs than to deter-
m a d e today, for example, in the u n d e r s t a n d i n g of s o m e less well-
m i n e its genus. New genera are constantly being erected, and
described areas such as the Middle O r d o v i c i a n Chazy G r o u p and
trilobite species are frequently moved a r o u n d as descriptive
the C a m b r i a n in eastern New York. T h e general shift in e m p h a -
m e t h o d o l o g y b e c o m e s m o r e sophisticated and new classification
sis f r o m discovery to understanding still keeps New York trilo-
standards are adopted.
Because
bites in the limelight. In later chapters we will point out the
Families are further collected into orders, again based on
i m p o r t a n c e of New York trilobite beds in o u r understanding of
inferred evolutionary relationships. T h e r e are currently eight
trilobite biology and fossil preservation.
orders in the class Trilobita of the phylum Arthropoda (Kaesler, 1 9 9 7 ) . T h e r e are additional t a x o n o m i c relationships rarely used herein, such as superfamily and suborder. Family n a m e s always
Trilobite Names
have the suffix -idae; superfamily names, -acae; suborder, -ina;
Trilobites are n a m e d using the rules of zoological n o m e n c l a ture published in English and French in the International Code of Zoological Nomenclature ( I C Z N ) . T h i s c o d e is used worldwide by
and order, - i d a . * The common
New York trilobite
Eldredgeops rana
is taxo-
nomically described as follows:
all scientists irrespective of the language of p u b l i c a t i o n . In dealing with the names of trilobites ( o r any o t h e r kind of o r g a n i s m ) , it
phylum
helps to understand the basic rules, how n a m e s c o m e a b o u t , and
Arthropoda
how they can change over t i m e .
class
For example, and as m e n t i o n e d already, in 1824 J. E. Dekay first reported Isotelus gigas Dekay,
Trilobita
1 8 2 4 , a n a m e that remains
order
unchanged to this day. T h e first n a m e , with a capital first letter,
Phacopida
Isotelus, is the genus n a m e and refers to a group of a n i m a l s in
family
which similar characteristics indicate a close evolutionary rela-
Phacopidae
tionship. G e n u s names must be original and not used for any
genus
other grouping of fossil or living a n i m a l . T h e second mme, gigas,
Eldredgeops
which is not capitalized, is the species n a m e and ideally should
species
refer only to a coherent group of interbreeding populations.
rana
Species n a m e s do not have to be u n i q u e , except within the s a m e genus. T h e r e can be only o n e gigas within the genus Isotelus, but
To take a n o t h e r , s o m e w h a t m o r e c o m p l e x example, Eldred-
species n a m e s can and do reoccur in different genera (plural of
geops
genus). Accordingly there are seven different New York trilobite
(short for variety) rana by J. Green in 1832. (Green in the s a m e
genera with the species n a m e trentonensis.
T h e proper n a m e ,
rana
publication
(Green, had
1832)
described
was
first
named
Calymcne bufo
Calymene bufo from a
var.
poorly pre-
genus and species, is italicized in print. In descriptive literature
served phacopid s p e c i m e n in a float boulder. T h e c o n d i t i o n of
the n a m e of the describing author (Dekay in our e x a m p l e ) and
the fossil was too p o o r to use for determining clear relationships
s o m e t i m e s the date of publication ( 1 8 2 4 in o u r e x a m p l e ) follow
and the n a m e subsequently was a b a n d o n e d . ) In 1860 E m m o n s
the n a m e . C h a n g e s to the genus n a m e are not u n c o m m o n , and
changed the species to Phacops bufo because of the greater simi-
in these cases the n a m e of original a u t h o r and date are given in
larity of the genus to the European genus Phacops than to Caly-
parentheses.
mene.
T h e e t y m o l o g y of trilobite n a m e s often refers to a m o r p h o logical
feature.
Hall ( 1 8 6 1 ) first called the c o m m o n trilobite from the
Hamilton
shales
and
limestones,
Phacops rana.
Green's
name
Isotelus m e a n s " s i m i l a r " (iso-) " e n d " or " t a i l "
and publication date now appear in parentheses because of the
(-telus), referring to the similarity between the head and tail of
c h a n g e in his original genus designation. T h i s story is further
this species, a n d gigas m e a n s " l a r g e " or "giant," referring to the
complicated by the assignment of s o m e new phacopids to s u b -
large size of this species c o m p a r e d to most trilobites. N a m e s are
species of Phacops rana such as Phacops rana milleri Stewart,
1927
TRILOBITE
3
NAMES
and Phacops rana rana ( G r e e n , 1 8 3 2 ) . ( T h e designation of s u b -
representative, then this specimen b e c o m e s a lectotype. Other
species is not often used with trilobites because strictly speaking
s p e c i m e n s in the original series b e c o m e p a r a l e c t o t y p e s , and the
the term implies geographically separated populations that could
t e r m syntype can no longer be applied. If the species has no orig-
interbreed given the opportunity. T h i s is nearly impossible to
inal type s p e c i m e n s that can be f o u n d and a sufficient taxonomic
determine from fossils, and most authors prefer the single species
purpose is present, an a u t h o r may designate a specimen to rep-
names Mich as Phacops rami ami Phacops milleri.) Struve ( 1990)
resent the type for the species. T h i s specimen is called a neotype.
re-examined the New World "Phacops" species and f o u n d t h e m
If at all possible, the n e o t y p e should be from the same location
significantly different from the type species Phacops latifrons that
and horizon as the originally described material. T h e description
was originally described from the Devonian of Germany. ( T h e
and n a m e m u s t be publicly issued as a p e r m a n e n t scientific
type species is the single species used to describe and define the
record and available in multiple, identical copies. All these rules
genus.)
are t h o r o u g h l y spelled o u t in the I C Z N ( 1 9 8 5 ) .
He erected
a
new genus, Eldredgeops, with
Eldredgeops
milleri as the type species. T h e very familiar f o r m e r Phacops rana is thus now properly referred to as Eldredgeops rana.
O t h e r type designations are c o m m o n l y used in collections but do not bear I C Z N r e c o m m e n d a t i o n . H y p o t y p e is a specimen that
Species n a m e s are never changed by later a u t h o r s , not even to
was referred to, usually in publications, to extend or correct the
correct spelling errors. T h e exceptions to this are if the n a m e has
knowledge of a species. T o p o t y p e refers to a specimen from
already been used for a closely related animal in the s a m e genus
the type locality, and p l e s i o t y p e refers to s p e c i m e n s very close to
or if the same animal has been n a m e d by a n o t h e r person in an
the type. P l a s t o t y p e is an artificial cast of the original type.
earlier publication. In almost all cases, priority is with the n a m e given by the first author.
A difficulty o n e often e n c o u n t e r s with trilobites, as well as o t h e r fossils, is that species were originally n a m e d when only a
T h e n u m b e r of trilobite species and genera is constantly
partial s p e c i m e n was available. T h e r e are a n u m b e r of instances
increasing due to b o t h new finds and redescriptions of previously
where, for e x a m p l e , the pygidium of an u n c o m m o n trilobite bore
collected material. T h e r e are also differing a p p r o a c h e s to the
o n e n a m e and the c e p h a l o n a different o n e . Also in the 1800s
concept of species. S o m e authors view speciation on the basis of
trilobites were often identified with the s a m e n a m e s as species
small external changes and tend to propose new species based on
f r o m o t h e r locations but of the same geological age. A n u m b e r of
these differences. O t h e r s view m a n y small external differences
New York trilobites, for e x a m p l e , were given the s a m e names as
as
species from the Midwest, particularly O h i o , as well as s o m e from
within
the
normal
intraspecies
variation
and
include
a
wider variety of s p e c i m e n s within a single species. T h e concept
Europe. S o m e o f these n a m e s are o n l y n o w being corrected a s
of species is not u n a m b i g u o u s in extant animals, and in the case
careful studies are m a d e .
of fossils morphological features are usually all there is available
A n o t h e r interesting situation is that early in the twentieth
for evaluation. Statistical evaluation of fossils using m e a s u r e -
century, scientists going through m u s e u m collections saw differ-
ments of key features is often currently used to define intraspecies
ences in s p e c i m e n s and gave t h e m new n a m e s , or listed t h e m as
variability, and the c o m p a r i s o n of derived (uniquely shared)
subspecies, by n o t i n g the n a m e on a label and leaving it with the
characteristics between closely related species is used to deter-
s p e c i m e n . T h e s e " m u s e u m l a b e l " n a m e s are c o m m o n l y e n c o u n -
mine their evolutionary relationship. S y s t e m a t i c s is the study of
tered with the Ordovician trilobites at the National M u s e u m of
the similarities and differences in o r g a n i s m s and their related
Natural History ( S m i t h s o n i a n o r U S N M ) . M u s e u m label names
species. A. B. S m i t h ( 1 9 9 4 ) presented an excellent in-depth review
are not recognized by the I C Z N and have no priority or recogni-
of systematics for the fossil record.
tion, except when subsequent a u t h o r s c h o o s e to use t h e m to
T h e rules of zoological n o m e n c l a t u r e now require that an
name specimens.
Later a u t h o r s
recognized
these
unpublished
men that clearly exemplifies this new species. T h i s specimen
s c r i p t " ) . T h e official date for the n a m e is when it is published,
should then be deposited in an appropriate public collection,
however, not when the m u s e u m label was m a d e . An example of
such as a m u s e u m . T h i s o n e specimen is called the h o l o t y p e .
such a n a m e is Isotelus walcotti Ulrich in C. D. Walcott, 1918. In
Other specimens of a
reference g r o u p
this case E. O. Ulrich saw differences between the specimens from
hypodigm)
the
(the " t y p e series" or
n a m e s with
sometimes
author, when describing a new species, must designate a speci-
the designation " M S " (for " m a n u -
called
the New York Trenton L i m e s t o n e , which were being called Isotelus
paratypes. In the 1800s and early 1900s authors often illustrated
iowensis, and the a u t h e n t i c trilobite f r o m Iowa. He n a m e d the
their new specimens but failed to designate a single type speci-
New York species /. walcotti on a m u s e u m label, and C. D. Walcott
men and its repository. Later authors in referring to these speci-
recognized the n a m e in a subsequent publication in 1918. T h e r e
mens, provided they could be f o u n d , consider the m e m b e r s of the
is no specific rule or protocol for the recognition of m u s e u m label
from
which
holotype
was
chosen
are
type series to be syntypes. T h e term c o t y p e is also seen in c o l -
n a m e s , and it is solely to the discretion of the publishing author
lections; it is a synonym for syntype or paratype and its use is dis-
whether the n a m e is recognized or not. Walcott chose to recog-
couraged by the I C Z N . Should an a u t h o r need to c h o o s e a single
nize Ulrich; thus it is appropriate, but not required, to add his
specimen from the designated syntypes to be the single species
n a m e to the final f o r m a l trilobite n a m e .
The Biology of Trilobites
T r i l o b i t e s are the earliest u n a m b i g u o u s arthropods found in the
Exoskeleton
fossil record, evidently because they were the first a r t h r o p o d s to develop the mineralized skeleton necessary for frequent preser-
T h e word trilobite, freely translated from Latin, m e a n s " h a v i n g
vation. M o d e r n arthropods have o r g a n i c e x o s k e l e t o n s that are
the nature of three lobes." T h e n a m e refers to the three length-
often strengthened with minerals such as calcite. In contrast,
wise, lateral parts or lobes of the trilobite body (Figure 2.1 A ) , not
trilobites had an exoskeleton c o m p o s e d primarily of calcite,
the three parts m a k i n g up the b o d y — t h e cephalon or head
although it undoubtedly was modified with organic materials or
(Figure 2.1 D ) , t h o r a x (Figure 2 . 1 E ) , and pygidium or tail (Figure
other minerals. T h e appendages, legs, and a n t e n n a e were p r o b a -
2 . I F ) . T h e central of the three lobes is referred to as the axial lobe
bly an organic material like the chitin of extant a r t h r o p o d s , which
(Figure 2 . I B ) and the side lobes of the thorax and pygidium, as
is o n l y preserved when replaced by minerals under special c o n -
the pleural lobes (Figure 2 . 2 C ) .
ditions (see C h a p t e r 3 ) . N o n e of the organic skeletal materials
Trilobites
and
other
arthropods
probably
evolved
from
have survived directly in the fossil record. T h e word a r t h r o p o d
annelid wormlike ancestors. T h i s is reflected in their segmented
m e a n s " j o i n t e d foot or leg," and the phylum A r t h r o p o d a includes
b o d y and the observation that each segment, whether fused
trilobites, crabs, lobsters, spiders, horseshoe crabs, centipedes,
together or j o i n t e d , carries a pair of appendages. T h e segments in
millipedes, and the largest of all living a n i m a l groups, the insects.
the front of the b o d y are fused to form the cephalon, and those
T h i s large group includes animals with strong external skeletons
in the rear of the b o d y are fused to f o r m the pygidium. T h e
that must be shed periodically to enable physical growth. M a n y
central segments f o r m i n g the thorax or trunk were j o i n e d by flex-
also go through o n t o g e n e t i c phases in their growth, which can
ible tissue that enabled m a n y trilobites to flex inward to the point
include free-ranging n e a r - m i c r o s c o p i c larvae.
of enrolling (Plate 1 0 9 ) , but probably did not allow for m u c h , if
T h i s chapter is divided into four parts: the external skeleton
any, sideway flexing. S o m e trilobites are found arched or flexed
or exoskeleton, the growth phases or ontogeny, the appendages
dorsally, which
and internal a n a t o m y ( t h e unmineralized or soft b o d y p a r t s ) , and
culation. For a m o r e detailed a c c o u n t of the elements of trilo-
indicates the flexibility of the trilobite arti-
the m o d e of life of trilobites. T h i s order roughly c o r r e s p o n d s to
bite articulation, see the publications by Whittington ( 1 9 9 2 ) ,
the level of knowledge about the biology of trilobites, as most is
B e r g s t r o m ( 1 9 7 3 ) , Levi-Setti ( 1 9 7 5 , 1 9 9 3 ) , M o o r e ( 1 9 5 9 ) , and
k n o w n a b o u t the exoskeleton and least is known about the life-
Kaesler ( 1 9 9 7 ) . T h e work by M o o r e ( 1 9 5 9 ) , Treatise on Inverte-
m o d e . T h e last part, l i f e - m o d e , is inferred from knowledge of
brate Paleontology, Part
trilobites in the fossil record and observations of living a r t h r o -
simply as the Treatise. Kaesler's publication ( 1 9 9 7 ) is the first of
O, A r t h r o p o d a
1, often
is referred
to
pods. All of the illustrations were c h o s e n from trilobite genera
a three-part revision in progress but only the first part has been
f o u n d in New York.
released. F o u r t e r m s are used repeatedly in describing trilobites. T h e s e are dorsal, m e a n i n g the uppermost surface of the body in the animal's life position; ventral, the lower surface of the body;
•1
FIGURE 2 . 1 . Trilobite structure using Eldredgeops rana (PRI 4 9 6 5 6 , w h i t e n e d ) . A. The three lobes from w h i c h the n a m e trilobite w a s d e r i v e d . B. The axial lobe. The axial lobe is s u b d i v i d e d into the thoracic axis a n d the p y g i d i a l axis. C. The pleural lobes. These may be further d e f i n e d as thoracic pleurae a n d p y g i d i a l pleurae. D. The c e p h a l o n or h e a d of the trilobite. E. The thorax. F. The p y g i d ium or tail. G. The glabella. H. The eyes. I. The p a l p e b r a l lobes on top of the eyes.
FIGURE
2 . 2 . Trilobite
structure
using
Ketlneraspis
tuberculata
(GJK
collection,
whitened).
A.
The
exoskeleton without the right free c h e e k . B. The genal spine e x t e n d i n g off the free cheek (arrow). C. Lateral thoracic pleural s p i n e s (arrows). D. Pygidial spines (arrow). E. Anterior c e p h a l i c s p i n e s (arrow). F. Axial n o d e s , raised areas on the thoracic axis (arrows). G. Axial o c c i p i t a l n o d e (arrow). H. Pustules, small, r a n d o m l y scattered raised areas (arrow).
7
EXOSKELETON a n t e r i o r , toward the front; and p o s t e r i o r , toward or at the rear of
T h e glabella is the dorsal covering of the s t o m a c h of the trilo-
the body part being described. Dorsal and ventral are absolute
bite. It usually has lateral grooves in its surface known as lateral
terms in the sense that the dorsal and ventral surfaces are the
glabellar f u r r o w s (Figure 2 . 3 G ) . T h e furrows rarely cross the
same regardless of the part being described or its o r i e n t a t i o n .
surface completely but can be deep, f o r m i n g p r o m i n e n t areas on
Anterior and posterior relate to the particular part or parts being
the glabella called lateral glabellar l o b e s (Figure 2 . 3 F ) or simply,
described. T h u s , a suture, or inflexible j o i n i n g , might be anterior
glabellar lobes. Because of the use of furrows and lobes in trilo-
to o n e body part and posterior to another.
bite identification, they are designated starting f r o m the occipital
T h e hard mineral exoskeleton, also called the c u t i c l e , is a
ring and occipital furrow. T h e furrow or sulcus separating the
complex structure. T h e external surface is variously o r n a m e n t e d
occipital ring f r o m the glabella is labeled S O ; the next most ante-
with ridges, terrace lines, n o d e s (Figure 2.2F, G ) , pustules (Figure
rior glabellar furrow is S I , a n d so o n . ( T h e O in SO and LO refers
2 . 2 H ) , tubercles, and spines (Figure 2 . 2 B , C, D ) . Terrace lines are
to " o c c i p i t a l " a n d is n o t a zero.) T h e lobes are similarly desig-
c o m m o n on the trilobite exoskeleton and are described in s o m e
nated, with the occipital ring called L O , the lobe directly anterior
detail by Miller ( 1 9 7 5 ) . Nodes are discrete, r o u n d e d , raised areas
to it L I , and so forth. It is not always easy to distinguish the most
on the dorsal exoskeleton, which are usually bilaterally s y m m e t -
anterior lateral lobes because the furrows can be very faint; thus,
rical (Plates 45 and 1 3 4 ) . Pustules, on the o t h e r h a n d , are raised
the most a n t e r i o r p o r t i o n of the glabella is called La, for anterior
areas that are m o r e frequent on the surface and also tend to be
glabellar lobe. T h e furrow separating the glabella from the cheek
more r a n d o m l y distributed (Plates 24 and 2 5 ) . Large pustules
area (Figure 2 . 3 H ) is the glabellar furrow. On m a n y trilobites the
are often called tubercles (Plate 2 7 ) . T h e r e c o m m o n l y are c h a n -
edge of the c e p h a l o n is distinctive (Figure 2.31) and is called the
nels from the surface of these structures to the interior, and it is
border.
believed
that
these
channels
served
for
sensory
capability,
T h e cheeks generally bear the eyes of the trilobite (Figure
enabling the trilobite to sense currents and chemical changes
2 . 1 H ) . ( T h e r e are eyeless, presumably b l i n d , trilobites but these
to the e n v i r o n m e n t (Figure 2 . 6 ) . S o m e of these perforations also
are an exception a n d will be n o t e d in the specific descriptions.)
may have been follicles for sensory hairs. Dorsal spines are the
T h e eye is often p r o m i n e n t l y raised, actually being on a stalk, in
physically extended equivalents to nodes (Plates 46 and 1 2 8 ) .
a few species. T h e area between the eye and the glabella is called
Spines, which extend or radiate from the edges of body parts, will
the palpebral area (Figure 2.11) and can have its o w n furrows and
be discussed later.
lobes.
T h e cephalon is the most c o m p l e x and the m o s t i m p o r t a n t
T h e surface of the eye has multiple lenses to form images,
part of the trilobite to be understood by the student or collector
a n d this surface can be very distinctive. Eyes with a closely
because it is often the m o s t easily recognized evidence for trilo-
packed optical structure a n d a s m o o t h , c o n t i n u o u s o u t e r surface
bites in the rocks.
The center of the cephalon is the glabella
( c o r n e a ) are called h o l o c h r o a l (Figure 2 . 4 A ) . Although they have
(Figure 2 . 1 G , 2 . 3 E ) , a raised p o r t i o n with characteristics often
a s m o o t h a p p e a r a n c e , these are c o m p o u n d eyes, similar to those
important to the recognition of trilobite species. Although the
in s o m e m o d e r n insects. Eyes with discrete individual lenses,
glabella is treated here as separate from the occipital ring (Figure
each with its o w n corneal surface, are called s c h i z o c h r o a l (Figure
2.3F, the lobe labeled L O ) , the t e r m i n o l o g y used in the Treatise,
2 . 4 B ) . In addition, the lenses in schizochroal eyes are separated
most trilobite workers today include the occipital ring as part of
by a thick interlensal sclera. T h e r e is a third type of eye, resem-
the glabella (this latter protocol is used in the Kaesler ( 1 9 9 7 ) . Lat-
bling the schizochroal eye, f o u n d in the family Pagetidae, called
erally outward from the glabella are the cheeks or genae (sin-
a b a t h o c h r o a l (Jell 1 9 7 5 ) . A b a t h o c h r o a l eyes lack the deep inter-
gular, gena). T h e s e areas usually have sutures (Figure 2.5) that
lensar scleral p r o j e c t i o n and the intrascleral m e m b r a n e of schizo-
separate t h e m into free cheeks (Figure 2 . 3 C ) and fixed cheeks
chroal eyes.
(Figure 2 . 3 D ) , called librigenae and fixigenae, respectively. T h e
Holochroal eyes arc found in the majority of trilobites, w h i l e
librigenae usually separate from the cephalic area on m o l t i n g , and
the schizochroal eyes are f o u n d only in the s u b o r d e r Phacopina,
the fixigenae remain p e r m a n e n t l y attached. T h e glabella, together
which arose in the O r d o v i c i a n and disappeared by the end of the
with the fixagenae, is called the c r a n i d i u m (Figure 2 . 3 B ) . M a n y
D e v o n i a n . P h a c o p i n s are well represented in New York, and this
trilobite species, particularly in the C a m b r i a n , are differentiated
type of eye structure is often observed. T h e n u m b e r of optical
on the basis of details in their cranidia. T h e posterior part of the glabella, the occipital ring, is a raised portion almost always separated from the main body of the
elements in schizochroal eyes is usually less, often far less, than that in h o l o c h r o a l eyes. In a series of elegant e x p e r i m e n t s , Towe
(1973)
demon-
glabella by a groove. S o m e occipital rings have a central p r o m i -
strated that the eyes in at least two trilobites were single crystal
nently raised area, n o d e , or spine that is very characteristic for
calcite oriented to give high-quality imaging. Towe mounted
particular genera or species. Spines are often overlooked because
the eye surface of Eldredgeops rana and an Isotelus species and
of the ease with which they are broken o f f and lost during the
demonstrated
process of removing the trilobite from the rock.
species) and each lens of a schizochroal eye (E. rana) gave an
that
each
facet
of a
holochroal
eye
(Isotelus
FIGURE 2.3. The structure of the trilobite c e p h a l o n
using
Calymene s p e c i e s (S.
Insalaco collection,
w h i t e n e d ) ( c e p h a l o n only). A. C e p h a l o n lacking the right free c h e e k . B. The c r a n i d i u m (arrow). C. Free c h e e k (arrow). D. Fixed c h e e k s (arrows). E. The g l a b e l l a (arrow). F. The glabella, with the glabellar lobes n u m b e r e d (arrows). G. The glabella, with the lateral glabellar furrows n u m b e r e d (arrows). H. The glabellar furrows (outlined, with arrows). I. The c e p h a l i c border (arrows).
9
EXOSKELETON
FIGURE
2.4.
Trilobite
eyes.
holochroal e y e s (arrow). B.
FIGURE 2.5.
A.
C e p h a l i c sutures. A.
c e p h a l i c sutures (arrows),
Monodechenella
macrocephala
(G.
Jennings
collection,
whitened)
with
Viaphacops bombifrons ( G J K c o l l e c t i o n , w h i t e n e d ) with s c h i z o c h r o a l eyes (arrow).
Ceraurus pleurexanthemus (GJK c o l l e c t i o n , w h i t e n e d ) s h o w i n g p r o p a r i a n
both e n d s e m e r g i n g anterior to the g e n a l a n g l e .
B.
Calymene niagarensis (S.
Insalaco collection) with g o n a t o p a r i a n c e p h a l i c sutures (arrows), the posterior e n d e m e r g i n g at the g e n a l angle. O n e free c h e e k has b e e n lost on the right side. C. Isotelus maximus (PRI 4 9 6 5 1 ) with o p i s t h o p a r i a n c e p h a l i c sutures (arrows), w h e r e the posterior e n d e m e r g e s a l o n g the rear of the c e p h a l o n . D. Calyptaulax callicephalus (GJK c o l l e c t i o n , w h i t e n e d ) , a trilobite in w h i c h the facial sutures are f u s e d a n d do not s e p a r a t e u p o n molting. This is c o m m o n in the order P h a c o p i d a .
inverted image when viewed from the rear, just as a simple glass
showed clear evidence of optical fibers extending f r o m the lens
lens does.
into the central c e p h a l o n . O n e asteropyge in their study also had
S t u n n e r and Bergstrom ( 1 9 7 3 ) studied the internal structure
similar fibers e x t e n d i n g f r o m the lens. T h e s e structures were c o n -
of the eyes of Phacops s p e c i m e n s f r o m the H u n s r i i c k slates in
sidered c o m p a r a b l e to similar structures in extant a r t h r o p o d s .
Germany. S o m e of the soft tissue of the trilobites was replaced by
Structures of this kind were reported previously in asteropyges
the mineral pyrite (i.e., it was pyritized), a n d X - r a y p h o t o g r a p h s
but not c o n f i r m e d until this study.
10
THE
BIOLOGY
OF
TRILOBITES
On m a n y trilobites there is a medial, glabellar n o d e on the
Naroidae to 40 or m o r e . Each segment has a central or axial
meraspid (juvenile) that generally disappears by the holaspid
p o r t i o n and a lateral part on each side of the body called the
(adult) phase or early in the holaspid growth. T h i s n o d e is inter-
pleura (plural, pleurae). T h e usually grooved pleurae also may
preted as having a visual or light-sensing capability ( R u e d e m a n n
extend laterally beyond the body into short rounded extensions
1 9 1 6 b , Jell 1 9 7 5 ) . Cryptolithus is a c o m m o n O r d o v i c i a n trilobite
called lappets (Plates 50 to 5 6 ) or even into m o r e extended and
genus in New York that lacked n o r m a l eyes b u t retained this
pointed pleural spines (Figure 2 . 2 C ) . T h e distinction between
median glabellar n o d e into the m a t u r e holaspis (Plates 163 to
lappets and spines is qualitative, and in s o m e cases it is not clear
1 6 7 ) . A review of the evolution of trilobite eyes with references
whether the extensions should be termed long lappets or short
for detailed reading is given by C l a r k s o n ( 1 9 7 5 ) . M o r e will be said
spines (Plates 46 and 4 8 ) . O t h e r structures can increase flexibil-
about eye function later in this chapter when the possible m o d e s
ity and s o m e t i m e s tight enrolling, but these are beyond the scope
of life of trilobites are discussed.
of this b o o k . T h e s e structures are covered in detail in the work
In p o s t - C a m b r i a n and m o s t C a m b r i a n trilobites there is a
by Bergstrom ( 1 9 7 3 ) .
suture r u n n i n g across the upper edge of the eye, separating the
T h e most posterior part of the trilobite is the pygidium
lens surface f r o m the palpebral area. T h i s suture or separation
(Figure 2 . I F ) . T h e pygidium, similar to the thorax, has segments
continues to the edge of the c e p h a l o n in b o t h directions and
with axial and pleural p o r t i o n s . However, the pygidial segments
results in a portion of the cheek area (librigena) that can separate
are totally fused so there is no flexibility a m o n g the parts. T h e
from the c r a n i d i u m . T h i s facial or cephalic suture separates the
n u m b e r of axial segments, the n u m b e r of pleurae, and the general
free cheek (librigena) f r o m the fixed cheek (fixigena).
shape of the pygidium are often diagnostic to species. Pygidia are
In most C a m b r i a n trilobites an additional suture runs below
c o m m o n in the trilobite fossil record, and these features are very
the eye and j o i n s the facial suture, f o r m i n g the c i r c u m o c u l a r
i m p o r t a n t to the identification of trilobites. T h e pygidium can
suture so that the visual surface separates on m o l t i n g and is lost.
have marginal lappets or marginal spines (Figure 2 . 2 D ) as exten-
It is also c o m m o n in articulated calymenids for the visual surface
sions of the pleurae, and s o m e t i m e s has a central posteriorly
to be missing. T h i s a b s e n c e suggests that a suture s u r r o u n d e d the
directed spine called the t e r m i n a l axial spine (Plate 8 8 ) . In addi-
visual surface or that it was weakly mineralized, if at all.
t i o n , there is s o m e t i m e s a narrow featureless area around the
T h e a n t e r i o r margin of the c e p h a l o n is rounded and the pos-
margin of the pygidium simply called the border. In s o m e genera
terior margin is less curved laterally, resulting in the f o r m a t i o n of
of trilobites, the pygidium is almost featureless, and when f o u n d
a c o r n e r at the posterolateral e x t r e m i t y k n o w n as the genal angle.
separate, it looks like n o t h i n g m o r e than a dark t h u m b n a i l on or
I )epending on the species, this structure varies f r o m a blunt well-
in the rock. T h i s finding should not be overlooked as an indica-
rounded angle to a genal spine (Figure 2 . 2 B ) that extends back
tor o f the presence o f particular species.
along the body. O n e end of the facial suture crosses the genal area
On the surface of m a n y trilobites, there are pitted areas that
and emerges o n the anterior m a r g i n o f the c e p h a l o n , and the
often penetrate the exoskeleton. T h e exoskeleton of the large
o t h e r e n d emerges either in front or b e h i n d the genal angle. If
trilobite Dipleura dekayi is literally covered with pits, seen
both ends of the sutures emerge a n t e r i o r to the genal angle, it is
Figure 2.6A as small white specks. Figure 2 . 6 B shows a broken
known as a p r o p a r i a n (Fig 2 . 5 A ) suture; if o n e end emerges on
area of this trilobite, with the pits as complete perforations
the posterior cephalic m a r g i n , the suture is called opisthoparian
through the cuticle. Isotelus gigas is similarly covered with pits
in
(Figure 2 . 5 C ) . As o n e might expect, there are trilobites where the
(Figure 2 . 6 D ) . Pits such as on these two trilobites may have served
suture emerges precisely o n , or very near, the genal angle, and that
a sensory purpose and may have contained sensory hairs. A dif-
condition is called g o n a t o p a r i a n (Figure 2 . 5 B ) . In s o m e trilobites
ferent kind of pit is seen on the cephalic border of Cryptolithus
the cephalic suture (Figure 2 . 5 D ) is fused and does not o p e n on
species (Figure 2 . 6 C ) . T h e s e are also perforations but go c o m -
molting.
pletely through the b o r d e r area. T h e i r role is u n k n o w n , but they
T h e area immediately in front of the glabella varies from
m a y have served to sense water currents or m o v e m e n t .
nearly nonexistent to a broad b o r d e r platform or b r i m . In s o m e
T h e exoskeleton covering the entire dorsal surface of the trilo-
families, particularly the calymenids (Figure 2.31), the shape of
bite s o m e t i m e s curls under at the edges of the cephalon and
this area is i m p o r t a n t to the identification of genera.
pygidium to form the doublure (Figure 2 . 7 H , I ) , a fiat terrace
S o m e t i m e s this anterior b o r d e r has a lateral furrow. T h e area
a r o u n d the ventral edges of the cephalon and pygidium. In addi-
between this preglabellar furrow and the glabellar furrow is the
tion to the d o u b l u r e , there are two important pieces of the
preglabellar field (Plates 139 a n d 1 4 2 ) . T h e area between the
exoskeleton on the ventral side of the cephalon. T h e rostral plate,
preglabellar groove and the a n t e r i o r margin
is the a n t e r i o r
cephalic border.
found on m a n y but not all trilobites, is a c o n t i n u a t i o n of the d o u blure b u t is separated from the rest of the cephalon by sutures; it
T h e t h o r a x is divided into a n u m b e r of segments that f o r m a
is located directly under the front central part of the cephalon
highly flexible part o f the exoskeleton. T h e n u m b e r o f t h o r a c i c
(Plate 65 shows a displaced rostral plate immediately in front of
segments can range from zero in the unusual C a m b r i a n family
the c e p h a l o n ) . Under the a p p r o x i m a t e center of the cephalon is
FIGURE 2.6.
Exoskeletal pits or circular perforations. A. Dipleura dekayi(PR\ 4 9 6 2 9 ) . White s p e c k s over
the b o d y are sediment-filled pits. The arrow points to the area s h o w n in B. B. C l o s e - u p of Dipleura pits, showing
that they e x t e n d
through
the
exoskeleton
(arrow).
C.
Cryptolithus
lorettensis (PRI
49657,
whitened), s h o w i n g pits on the c e p h a l i c border (arrow). These are actually perforations that e x t e n d all the way through the border. D. Isotelus gigas (TEW collection, w h i t e n e d ) . The exoskeleton' is heavily pitted (arrow),
a d i a g n o s t i c character of this s p e c i e s .
E.
Greenops grabaui (F.
Barber collection,
whitened), s h o w i n g rows of circular perforations characteristic of the New York a s t r o p y g i d s . F. Greenops? species (GJK collection, w h i t e n e d ) . U n n a m e d , new?, s p e c i e s of Greenops in w h i c h the circular perforations are d e g e n e r a t e .
12
THE
BIOLOGY
OF
TRILOBITES
FIGURE 2.7. Ventral a n a t o m y of the exoskeleton. A-D.
Ceraurus pleurexanthemus
(PRI
49658,
whitened), p r e p a r e d to show the ventral exoskeleton. B. The h y p o s t o m e (arrow). C. Two of the a p o d e m e s (arrows). The a p o d e m e s are a r r a n g e d along the ventral surface u n d e r the axial furrow. There is a pair of a p o d e m e s for e a c h pair of c e p h a l i c a p p e n d a g e s , two for e a c h pair of thoracic a p p e n d a g e s ( w h i c h also c o r r e s p o n d to the n u m b e r of thoracic s e g m e n t s ) , a n d a p o d e m e s a s s o c i a t e d with the p y g i d i a l a p p e n d a g e s . The often large n u m b e r s of pygidial a p p e n d a g e s are not as well reflected in prominent a p o d e m e s . D. The arrows point out the entrance into the hollow exoskeletal genal spines, pleural s p i n e s , a n d pygidial spines. E-l. Isotelus s p e c i e s (Kevin Brett collection), ventral exoskeletal anatomy. A s p e c i m e n from C a n a d a . F. The h y p o s t o m e (arrow), the anterior m a r g i n a n d " w i n g s " of w h i c h are under the d o u b l u r e . G. The a p o d e m e s (arrows) of Isotelus, w h i c h are far less prominent than t h o s e of Ceraurus. Given that these represent m u s c l e a t t a c h m e n t , the shape must represent their use a n d consequently the life-mode of the trilobite. H. The cephalic d o u b l u r e (arrow). I. The p y g i d i a l doublure (arrow), w h i c h is i n c o m p l e t e in this specimen.
a
rounded plate called the h y p o s t o m e
(plural, h y p o s t o m a )
(Figure 2 . 7 B , F ) . T h e m o u t h was a t the rear o f the h y p o s t o m e . T h e h y p o s t o m e is a m o r e i m p o r t a n t feature. F o r s o m e species
T h e a t t a c h m e n t of the h y p o s t o m e is proposed to have significance for the h i g h - o r d e r classification of trilobites (Fortey [1990a]
using observations
m a d e by Fortey and C h a t t e r t o n
it is very robust, distinctive, and a not u n c o m m o n part of the
[ 1 9 8 8 ] ) . Natent h y p o s t o m a are those separated from the cephalic
fossil record. (In at least o n e rare O r d o v i c i a n trilobite, Hypodi-
d o u b l u r e , or rostral plate when present, by a gap and are dis-
cranotus, the presence of its h y p o s t o m e is a very g o o d indicator,
placed or absent in m o s t trilobite specimens. C o n t e r m i n a n t
and o n e o f the only indicators, o f its stratigraphic range.) T h e
h y p o s t o m a are closely j o i n e d to the cephalic d o u b l u r e or rostral
significance of the h y p o s t o m e will be pointed o u t for the indi-
plate and consequently are m o r e likely to be present on speci-
vidual species. T h e h y p o s t o m e is directly under the glabella, and
m e n s (Figure 2 . 7 B , F ) . In both of the above cases, the anterior
together they f o r m an envelope covering and protecting the
margin of the glabella is directly above the anterior margin of the
s t o m a c h . T h e m o u t h of the trilobite was at the posterior central
h y p o s t o m e . T h e third type of h y p o s t o m e , i m p e n d e n t , is when
n o t c h of the h y p o s t o m e (Plates 3 7 , 4 7 , 77, 117, 153, and 1 5 7 ) .
the anterior margin of the h y p o s t o m e is not close to the anterior
13
ONTOGENY margin of the glabella and the anterior margin of the glabella
O n e needs very sharp eyes to spot t h e m a m o n g the n o r m a l fossil
coincides with the cephalic margin. Examples of trilobites with
debris, or to spend hours with a s t e r e o m i c r o s c o p e scanning likely
the different types of h y p o s t o m e a t t a c h m e n t are as follows:
surfaces. M o s t protaspides that have been studied are those in
n a t e n t — s o m e proetids; c o n t e r m i n a n t — / . gigas (Plates 153 and
which their exoskeletons have been replaced by silica, since silici-
155)
fied fossils survive acidic dissolution of a limestone or shaly
£.
and
Cerauruspleurexanthemus
(Plate
77);
and
impendent—
rana.
m a t r i x . O n e then uses the m i c r o s c o p e to pick out the important
Under the lateral sides of the ventral axial region of the t h o -
material f r o m the insoluble residue, including protaspides. T h e
racic segments, glabella, and pygidium are thickened areas called
advantage of this procedure is that in silicified material, very fine
apodemes (Figure 2 . 7 C , G ) o r s o m e t i m e s a p p e n d i f e r s . T h e a p o -
details, including spines and in s o m e cases h y p o s t o m a , are often
demes are believed to have been points of muscle a t t a c h m e n t and
preserved.
may provide evidence for the lifestyle of the trilobite. On s o m e trilobites,
such
as
Ceraurus
pleurexanthemus
(Plate
77),
T h e shape and o t h e r physical features of the protaspis are
the
u n i q u e for specific fossil families and genera, and for this reason
apodemes extend down from the ventral surfaces of the segments
the protaspides are used for t a x o n o m i c assignments and confir-
to form p r o m i n e n t ridges or posts, yet in others, as illaenids, the
m a t i o n ( C h a t t e r t o n and Speyer 1 9 9 7 ) . Protaspides are assigned
apodemal area is fairly s m o o t h . T h e long a p o d e m e s suggest g o o d
to specific trilobites a n d growth series through association with
leverage for the attached muscles and a high level of l i m b m o b i l -
pieces of the m o r e m a t u r e fossil in the same debris and through
ity, perhaps for s w i m m i n g , rapid crawling, or digging.
the prior knowledge of protaspis-adult relationships. Using the presence of m a t u r e trilobites, however, is not always a secure way to assign protaspis-adult relationships.
Ontogeny
It is p r o b a b l e that the pre-protaspid and protaspid phases of
O n t o g e n y is the biological life cycle of an animal; for the trilo-
the trilobite growth cycle served to disperse the trilobites in their
bite this would be from the presumed egg to the smallest larvae
e n v i r o n m e n t , similar to the situation with m a n y m o d e r n m a r i n e
and the various intermediate stages, to the end of its life cycle.
Crustacea. S o m e protaspides were benthic ( b o t t o m dwelling) and
Considerably m o r e is k n o w n about the adult phase of the trilo-
others planktic (in the p l a n k t o n ) , drifting in the Paleozoic seas.
bite life cycle because the vast p r e p o n d e r a n c e of the fossil record
Because o f their differing l i f e - m o d e and preservation, s o m e p r o -
is c o m p o s e d of the pieces of the exoskeleton representing late
taspides m a y never be f o u n d or never be associated with their
growth stages. Careful workers have f o u n d , however, a significant
later growth and adult phases.
a m o u n t of i n f o r m a t i o n related to trilobites' earlier growth stages.
Parts B and C in Figure 2.8 show two protaspides of trilo-
Chatterton and Speyer ( 1 9 9 7 ) provide an extensive and current
bites f r o m the Lower D e v o n i a n of New York. T h e o n e in B is a
review of the present state of knowledge of trilobite ontogeny.
lichid (family Lichidae) and the o n e in C is a phacopid (family
Trilobites most likely began life as an egg, which was either laid
P h a c o p i d a e ) . T h e y are magnified to the s a m e scale and each is
outside the body or hatched within the a n i m a l , although there is
a b o u t a m i l l i m e t e r in actual length. At this phase of growth,
no unequivocal evidence of trilobite eggs. C. D. Walcott ( 1 8 7 7 c ) ,
morphological
in a study of remarkably well-preserved trilobites in New York,
in the adult, are given the profo-prefix. T h u s , the protaspis has a
features, which
represent
a
future body part
found spherical objects within the cross sections of s o m e trilo-
protocephalon,
bites, which he identified as eggs. T h i s and earlier reports a b o u t
adults, and a p r o t o p y g i d i u m . Trilobites also had growth stages
described
similarly
to
the
cephalon
of the
trilobite eggs by Barrande in 1852 apparently have not been inves-
within the protaspid phase. T h i s is observed by increases in the
tigated completely.
n u m b e r of lateral lobes on the glabellar axis and the general size
T h e well-defined phases of trilobite growth are labeled the
(Figure 2 . 8 A ) . T h e p r o t o c e p h a l o n and p r o t o p y g i d i u m , however,
protaspid, meraspid, and holaspid phases. Fortey and M o r r i s
are fused and r e m a i n together in the molted protaspid exoskele-
Lower
ton, although the free cheeks and the h y p o s t o m e may be deta-
Ordovician as a pre-protaspid phase of trilobite called phaselus.
c h e d . Each successive molt is called an instar (Figure 2 . 8 A ) .
Chatterton and Speyer ( 1 9 9 7 ) also found phaseluses in beds with
W h e t h e r or not there is a direct correlation between the n u m b e r
silicified trilobite remains. It is still not clear whether these are
of instars and the n u m b e r of m o r p h o l o g i c a l l y defined phases is
from trilobites or a n o t h e r fossil a r t h r o p o d .
not clear.
(1978)
described
some
small
exoskeletons
from
the
T h e first well-defined phases in growth of the trilobite, after
W h e n the trilobite larvae clearly begin to display the separa-
the presumed egg and the phaselus, is the protaspid phase. ( T h e
tion of the p r o t o c e p h a l o n f r o m the p r o t o p y g i d i u m , they are
name protaspis (plural, protaspides)
was given to the individual
referred to as being in the meraspid phase (Figure 2 . 8 E ) . T h e
silicified exoskeletons found in or on rocks by C. Beecher ( 1 8 9 3 a ,
segments develop from the anterior of this transitory pygid-
1893b).) Protaspides are difficult to find because of their small
i u m . ( O n c e the s e g m e n t a t i o n is definite, the protopygidium is
size. T h e larger ones are about the size of the " o " in the printed
no longer " p r o t o " but is still not the final pygidium, as the t h o -
word "protaspis," and m o s t are between that a n d h a l f that size.
racic segments are being f o r m e d anterior to this new pygidial
FIGURE 2.8.
O n t o g e n y of the trilobite. A. Flexicalymene senaria p r o t a s p i d e s from the O r d o v i c i a n of New York. These silicified s p e c -
imens w e r e p r e p a r e d a n d reported on by Chatterton et al. (1990). R e p r o d u c e d with p e r m i s s i o n . B. Possibly a lichid protaspid from the w o r k of B e e c h e r (1893a) a n d r e p r o d u c e d by Whittington (1957). C. A p h a c o p i d p r o t a s p i d from the s a m e source as B. D. The p r o t a s p i d of Isotelus gigas reported on by Chatterton a n d Speyer (1990). R e p r o d u c e d with permission. The s h a p e p r e c l u d e s this p r o t a s p i d from b e i n g a b o t t o m dweller. It p r o b a b l y w a s planktic, living a n d drifting near the surface of the sea. E. Triarthrus m e r a s p i d instars of d e g r e e 1, 2, a n d 4 from Whittington (1957). This is part of a nearly c o m p l e t e g r o w t h series c o l l e c t e d a n d reported on by Walcott (1918). The m e r a s p i d e s are r e p r o d u c e d at a b o u t eight times their life size. F. A growth series of Isotelus gigas holaspides from R a y m o n d (1914). The h o l a s p i d s are natural size. Young Isotelus h o l a s p i d e s have prominent genal s p i n e s , w h i c h are lost, in New York s p e c i m e n s , w h e n they r e a c h a b o u t 5 0 m m l o n g . G. A molting s e q u e n c e for c a l y m e n i d trilobites p r o p o s e d by Mikulic a n d Kluessendorf (2001). The trilobite on the right first p u s h e s its p y g i d i u m into the surface as an anchor, the c e p h a l i c sutures o p e n , and the animal c r a w l s f o r w a r d a n d out. The c e p h a l i c parts held in position by the ventral integument fall b a c k together, leaving a molt with the c e p h a l o n a n d p y g i d i u m c u r v e d d o w n w a r d a n d the thorax in a c o n c a v e curve d u e to the p u s h i n g up of the c r a n i d i u m during the p r o c e s s .
15
ONTOGENY s t r u c t u r e — h e n c e , the n a m e transitory pygidium.) In trilobites the
125 mm
early meraspides show a line of separation f r o m the cephalon on
Trenton age rocks of New York.
(5
inches)
are c o m m o n
in
the
Middle Ordovician
the anterior margin of the transitory pygidium but no t h o r a c i c
As in all a r t h r o p o d s , growth in trilobites required that the
segments. This is referred to as a "degree 0 meraspis." D u r i n g suc-
exoskeleton be shed or molted at regular intervals. In modern
cessive molts, as the segments detach from the transitory pygid-
a r t h r o p o d s , m o l t i n g (ecdysis) begins by the f o r m a t i o n of a new
ium and can be considered separate, the meraspis grows in size
cuticle or shell beneath the current o n e , separation of these shells
and degree number. Each detached t h o r a c i c segment is c o u n t e d ,
by a space filled with m o l t i n g fluid, and resorption of much of
and this n u m b e r is the degree assigned to the meraspis. T h e fact
the old cuticle to provide the base chemicals to finish the new
that segments are added from the anterior side of the transitory
o n e . W h e n this process is c o m p l e t e , the old shell splits and
pygidium is ascertained by the growth of trilobites with pleural
the animal emerges with a soft cuticle in place. By swelling this
or axial spines on specific t h o r a c i c s e g m e n t s . T h e spine first
soft cuticle through the intake of liquids, the new body rapidly
appears on a segment adjacent to the transitory pygidium, and
b e c o m e s larger. T h e new exoskeleton then hardens and the
each succeeding segment is added behind it
animal has grown o n e m o r e i n c r e m e n t . Because o f the resorption
until the adult
n u m b e r of segments is reached. There are usually p r o f o u n d changes in the shape of the
of s o m e of the old cuticle, the molted shell is significantly thinner and m o r e fragile than it was on the a n i m a l . T h i s process also
cephalon during the meraspid phase. W h e n the meraspid trilo-
lowers the energy r e q u i r e m e n t s of growth because the resorbed
bite molted, the cephalon and transitory pygidium separated, and
chemicals are available for the new exoskeleton.
thus they are found as distinct e l e m e n t s , unlike the o n e - p a r t p r o -
Trilobite molts, on the o t h e r h a n d , are robust and at least as
taspis molts. T h e meraspis can grow until it gains the n u m b e r of
thick as m o l t s attributed to living a n i m a l s that died and were pre-
free thoracic segments f o u n d in the adult, at which t i m e it is
served. T h i s i n f o r m a t i o n indicates that trilobites did not resorb a
called a
significant a m o u n t o f the minerals o f the old exoskeleton and
holaspis.
Figure 2.8E illustrates a limited growth series of Triarthrus
must have e m e r g e d from the m o l t i n g process with a rather thin
eatoni, part of the only nearly c o m p l e t e series k n o w n from a New
cuticle that was little m o r e than a soft template within which the
York trilobite. T h e meraspides appear to be missing their free
new mineralization t o o k place. Very thin a n d compressed or
cheeks and are probably molts. Meraspids are also k n o w n for
wrinkled trilobite fossils are k n o w n and usually are attributed
Elliptoccphala asaphoides from
the Lower C a m b r i a n of New York
to the exoskeletal remains or impressions of recently molted or
(Ford 1877, 1 8 7 8 ) , but articulated holaspids are rare. Parts D and
" s o f t - s h e l l e d " individuals. T h e r e is at least o n e e x a m p l e of a trilo-
F of Figure 2.8 illustrate the m o r p h o l o g i c a l changes in Isotelus
bite preserved in the Burgess Shale beds that is completely
gigas from the protaspid (Figure 2 . 8 D ) through the early holaspid
unmineralized and believed to be a very recently molted individ-
(Figure 2 . 8 F ) . It is surprising, given the n u m b e r of adult trilobite
ual ( W h i t t i n g t o n 1 9 8 5 ) . T h i s finding suggests that trilobites were
remains found in New York, that so few examples of growth series
vulnerable to predators and external t r a u m a for an extended time
have been recorded. Either the exoskeleton on the juvenile f o r m s
and that building the new exoskeleton required significant energy
of most trilobites is too fragile and not easily preserved in the
from the a n i m a l . It also suggests that as trilobites gained size, they
fossil record, or the juveniles did not o c c u p y areas where their
b e c a m e increasingly vulnerable d u r i n g m o l t i n g and that the
remains could be readily preserved or f o u n d .
n u m b e r of trilobites that might grow to an exceptional size for
T h e trilobite exoskeleton did not necessarily b e c o m e fixed in
the species was severely limited. Early trilobite predators such as
its physical structure at the early holaspid phase. T h e achieve-
Anomalocaris species have been identified in the Middle C a m -
ment of the holaspid phase generally only m e a n s that no m o r e
brian, and the n u m b e r of potential predators such as cephalopods
thoracic segments were added during c o n t i n u e d growth. T h e r e
and fish increased t h r o u g h o u t the Paleozoic.
are, however, variations in the n u m b e r of t h o r a c i c segments in
T h e r e are m a n y possible strategies for trilobite molting, and
the holaspis of a very few species; in Aulacopleura koninki, from
the cephalic sutures play a part in m o s t of t h e m . H e n n i n g s m o e n
the Silurian of B o h e m i a , t h o r a c i c segments were added after the
( 1 9 7 5 ) , M c N a m a r a and Rudkin ( 1 9 8 4 ) , Speyer ( 1 9 8 5 , 1990b,
holaspid phase was reached ( H u g h e s and C h a p m a n 1 9 9 5 ) . F o r
1 9 9 0 c ) , and W h i t t i n g t o n ( 1 9 9 2 , 1 9 9 7 ) discussed specific molting
all other trilobites the segment n u m b e r was stable. Trilobites did
strategies in detail.
not reach maturity, or at least their final body p r o p o r t i o n s , until
fossils in m a n y New York rocks. S i n c e m o s t of these parts are
they grew substantially from the first holaspis. In s o m e trilobites
attributable to molts, significant i n f o r m a t i o n is gained from their
such as Eldredgeops species, the growing holaspis changed very
e x a m i n a t i o n . For e x a m p l e , in Eldredgeops rana, the very c o m m o n
little and the small ones looked essentially like the adults. Isotelus
trilobite of the Middle Devonian of New York, c o m p l e t e cephala
gigas, on the other hand, possessed long genal spines in meraspids
and pygidia are the parts most often f o u n d . T h i s evidence shows
and early holaspides (Figure 2 . 8 F ) and did not lose the spines
that the facial sutures were fused and that the c o n n e c t i o n s
until it reached about o n e - t h i r d the size of a full m a t u r e speci-
between the cephalon and thorax and between the thorax and
men. Isotelus tergites or molt remains from specimens longer than
pygidium were o p e n e d or weakened during the molting process.
Disarticulated exoskeletons are c o m m o n
THE
16 T h e a n i m a l generally emerged through the split between the cephalon and t h o r a x .
BIOLOGY
OF
TRILOBITES
spine is on the lower lamella. An articulated specimen of Cryptolithus without the genal spines can be reasonably assumed to be
O n e often-illustrated set of molt remains is the upright t h o r a x
a molt (Plates 163 and 1 6 5 ) . In s o m e extant arthropods like the
and pygidium, with the inverted c e p h a l o n just in front of it. In
h o r s e s h o e c r a b , however, which uses this same molting technique,
this case the molting animal must have pushed forward after the
the suture can reseal after m o l t i n g and the molted exoskeleton
cephalon and t h o r a x parted, with the forward margin of the
remains whole. T h u s , when o n e finds a whole, articulated Cryp-
cephalon pushed down in the s e d i m e n t . T h i s action would pivot
tolithus s p e c i m e n with
the cephalon molt over upside d o w n , and the a n i m a l could then
absolute certainty it is not a molt.
its genal spines, o n e c a n n o t say with
emerge with the thorax and pygidium almost intact and right
M o s t authors agree on the generalities of trilobite molting, but
side up. That the pygidium is usually f o u n d separate suggests that
s o m e unanswered questions are rarely discussed. T h e roles of the
the c o n n e c t i o n is weakened d u r i n g the m o l t i n g process a n d
ventral, unscleritized i n t e g u m e n t and the scleritized appendages
s o m e t i m e s separates f r o m the t h o r a x d u r i n g or shortly after
have been largely ignored ( b u t see W h i t t i n g t o n 1 9 9 2 ) . It is not
ecdysis. T h e r e are other a r r a n g e m e n t s , besides the o n e just dis-
surprising that there is little preserved evidence for the ventral
cussed, of the m o l t remains of E. rana f o u n d in western New
m e m b r a n e ; there is so little soft tissue evidence from the fossil
York. S. E. Speyer ( 1 9 9 0 c ) , w h o has studied these m o l t r e m a i n s ,
record, and only o n e of the k n o w n soft tissue preservation sites
s u m m e d it up well: "Trilobites, like m o d e r n a r t h r o p o d s , displayed
contains any significant ventral anatomical information beyond
a variety of m o u l t behaviors which vary according to ecological
the appendages (Walcott 1 8 8 1 , 1 9 1 8 ) . In the trilobites in which
considerations (e.g., substrate consistency) and individual c o n v e -
the loss of the free cheeks is an i m p o r t a n t first step in ecdysis, the
nience."
inversion of these parts helps d e m o n s t r a t e the actual process
M o s t other trilobites, however, lose their free cheeks during
( M c N a m a r a and Rudkin 1 9 8 4 ) . B r i e f m e n t i o n is m a d e of the pos-
the m o l t i n g process. T h e most c o m m o n fossil remains of /. gigas,
sibility that the free cheeks m a y still have been attached to the
for e x a m p l e , are cranidia, free cheeks, h y p o s t o m a , and pygidia.
ventral m e m b r a n e and would have been inverted as they c a m e
Separated
specimens
away f r o m the a n i m a l . M c N a m a r a and Rudkin as do others,
without their free cheeks are very rarely f o u n d in the fossil record.
explain the inversion of the cephalon in molt remains as evidence
/. gigas evidently molted by the facial sutures o p e n i n g and the
that the trilobite pushed its cephalon down in front while arch-
whole
cephala
of Isotelus, or articulated
free cheeks separating, with the suture between the c r a n i d i u m
ing its t h o r a x to break away the cephalon at the c e p h a l o t h o r a x
and the d o u b l u r e o p e n i n g , possibly along with a break in the c o n -
suture. As the trilobite c o n t i n u e d to m o v e forward, the cephalon
nection between the t h o r a x and the c e p h a l o n . T h e s e breaks per-
inverted. M a n y trilobites are f o u n d with the inverted cephalon
mitted ecdysis by the trilobite m o v i n g straightforward. T h i s
under the t h o r a x . M a n y of these also have long genal spines.
molting strategy is i m p o r t a n t because /. gigas in the early phase
S o m e trilobite genal spines are totally enclosed except for the
had quite long genal spines, and it had to be able to free t h e m
area at the genal angle. For this m e c h a n i s m to take place, the soft
to molt properly. T h e m o s t efficient way to do this was to
spines must be dragged from their exoskeleton prior to total
emerge in a forward direction through the o p e n e d sutures in the
inversion of the free cheeks. Such a m e c h a n i s m is illustrated by
cephalon.
W h i t t i n g t o n ( 1 9 9 2 , Figure 9 ) . Not illustrated is the final struggle
Spines on the trilobites were hollow, and the tissue inside had to be withdrawn during m o l t i n g (Figure 2 . 7 D ) . Any m o l t i n g
of the a n i m a l during the ecdysis process, which drags the dorsal cuticle forward over the now-inverted free cheeks.
strategy of an individual trilobite must a c c o m m o d a t e the phy-
Based on the observation that calymenid trilobites from the
sical shape of the a n i m a l . Since the newly m o l t e d a n i m a l was
Silurian of W i s c o n s i n and Illinois are often found with the
unmineralized and soft, there must have been s o m e strategy to
cephalon and pygidium curled somewhat down, with a distinct
provide s o m e protection while the new exoskeleton b e c a m e suit-
concave sway to the t h o r a x , Mikulic and Kluessendorf ( 2 0 0 1 )
ably mineralized and hardened.
p r o p o s e that the trilobite molted by pushing its pygidium into
It is not always possible to distinguish the m o l t e d parts from the disarticulated
remains
of a
dead
trilobite.
A
complete
the substrate to a n c h o r it, by the sutures between the free cheeks and c r a n i d i u m as well as the anterior cephalic suture o p e n i n g ,
exoskeleton, with free cheeks, is almost always the fossil of the
and by the animal crawling forward, leaving the old exoskeleton
carcass of a trilobite. As in any rule, however, there are exceptions.
b e h i n d . T h e y further propose that since the upper and lower p o r -
Trinucleids and harpids do not have dorsal facial sutures. T h e
tions of the cephalon are held in place by the ventral integument,
dorsal cheeks and preglabellar areas are separated from the
after the m o l t i n g process the parts fall back into place, leaving a
ventral d o u b l u r e by a suture that runs parallel to the horizontal
molt that may be indistinguishable from a carcass (Figure 2 . 8 G ) .
plane. In o t h e r words, there is a suture separating the dorsal
T h e same process is seen in extant horseshoe crabs, which leave
surface from the ventral surface, and the suture line is a r o u n d the
behind an intact m o l t e d exoskeleton.
edge o f the c e p h a l o n . M o l t i n g o c c u r s b y the o p e n i n g o f this
G r o w t h was rapid during the early holaspid phase and could
suture and the trilobite emerging forward. In trinucleids the genal
be expected to slow as the animal reached maturity. In a few rare
SOFT
BODY
PARTS
17
cases, long periods between molts of larger individual species have been inferred. Tetreault ( 1 9 9 2 ) , Kloc ( 1 9 9 3 ,
only five localities worldwide, including the really remarkable
1 9 9 7 ) , and
new C a m b r i a n sites in C h i n a ( S h u et al. 1 9 9 5 ) . Two of these five
Brandt ( 1 9 9 6 ) observed epizoans (encrusting a n i m a l s ) on whole,
sites are in New York and o n e of t h e m , the Walcott-Rust Quarry,
articulated exoskeletons of several species of trilobites.
Such
was not included in Briggs and Allison's list. I n f o r m a t i o n on soft
encrusters must have been on the living a n i m a l , as articulated
or weakly skeletonized b o d y parts is very rare, and because of this
trilobites would not remain whole unless buried a very short t i m e
these data are generalized to a wide range of trilobites. T h e rarity
after death and epizoans would have little o p p o r t u n i t y to b e c o m e
of this s o f t - b o d i e d i n f o r m a t i o n is exemplified by the remarkably
attached to the buried carcass. T h e s e observations indicate either
preserved biota of the Burgess Shale in British C o l u m b i a . Most
a terminal molt, after which there is little growth in the animal
of what is k n o w n of s o f t - b o d i e d a n i m a l s in the C a m b r i a n initially
with no further molting, or long intervals between molts of the
c a m e f r o m these beds, yet of the 22 species of trilobites k n o w n
mature species. Tetreault further observed that b r a c h i o p o d s on
from the Burgess Shale beds, apparently only 4, so far, have
the exoskeleton of Arctinurus boltoni were in
four distinct size
yielded appendage i n f o r m a t i o n and in o n e of these it is from a
classes.
F r o m this he d e d u c e d , a s s u m i n g these b r a c h i o p o d s
single s p e c i m e n . W h e n you are reading through the following
spawned o n c e a year, that the largest b r a c h i o p o d s were 4 years
descriptions, r e m e m b e r that all the soft tissue data c o m e from a
old and that in m a t u r e Arctinurus animals m o l t i n g was terminal
very few sites and only a bare handful of trilobite species.
or occurred at as m u c h as 4-year intervals.
An indirect relationship of trilobites to the annelid w o r m s was
Many collectors find populations of trilobites that are signi-
introduced earlier. T h i s evolutionary trail is s u p p o r t e d by the
ficantly larger than the n o r m . Excellent s p e c i m e n s of E. rana 5
m u l t i s e g m e n t e d b o d y o f the trilobite a n d the observation that
to 6 . 4 c m (2 to 2.5 inches) long and /. gigas 13 to 1 5 c m (5 to
each recognizable s e g m e n t bears a pair of appendages. This
6 inches) long are not u n c o m m o n . S p e c i m e n s of E. rana of 10 to
observation includes the c e p h a l o n and pygidium, in which the
13 cm (4 to 5 inches) and 7. gigas of 3 0 . 5 cm ( 1 2 inches) and larger
segments are fused. Careful studies, by C. D. Walcott ( 1 8 7 6 , 1 8 8 1 ,
are found, but rarely. T h e largest reported articulated trilobite is
1918, 1 9 2 1 ) , of the s p e c i m e n s he had available established that
an Ordovician asaphid from the Arctic. It is 72 cm ( 2 8 inches)
the appendages are b i r a m o u s (Figure 2 . 1 0 A ) . In o t h e r words,
long.
growth
each individual appendage is divided into two parts, o n e part for
throughout life, as do extant lobsters, and could achieve an excep-
walking, the e n d o p o d (Figure 2 . 1 0 D ) , and o n e part, the e x o p o d
This
indicates
that
some
trilobites
continued
tional size in favorable e n v i r o n m e n t s . R a y m o n d ( 1 9 3 1 ) , describ-
(Figure 2.1 OB), possibly an apparatus similar to the gill of fishes,
ing an unusually large h y p o s t o m e of an Isotelus species from the
for breathing.
Ordovician Chazy limestones, estimated the length of the trilo-
T h e o n l y trilobite appendages that are not b i r a m o u s are the
bite at 61 to 6 4 c m ( 2 4 to 26 i n c h e s ) . Estimates of sizes of o t h e r
most anterior, which are modified into a n t e n n a e (Figure 2 . 1 1 A ,
New York trilobites, from molt remains or partial s p e c i m e n s ,
B , C ) . C . E . B e e c h e r ( 1 8 9 3 c , 1 8 9 4 a , 1 8 9 4 b , 1 8 9 6 ) , after years o f
listed by R a y m o n d are as follows:
study on meticulously prepared Triarthrus eatoni from Beecher's Trilobite Bed, published the m o s t f a m o u s , and m o s t often re-
Basilicas Isotelus Isotelus Isotelus Terataspis Trimerus Coronura
whittingtoni "giganteus" gigas maximus grandis major myrmecophorus
- 1 2 inches
(30 cm)
- 2 4 - 2 6 inches
(61-64 cm)
2 . 1 1 A ) . Beecher, possibly following Walcott's lead, gave T. eatoni
- 1 7 inches
(43 c m )
an extra set of appendages u n d e r the c e p h a l o n , but this does not take away from his r e m a r k a b l e a c h i e v e m e n t .
p r o d u c e d , illustration of the trilobite ventral a n a t o m y (Figure
- 1 8 - 1 9 inches
(46-48 cm)
- 2 0 - 2 4 inches
(51-61 cm)
- 1 5 - 1 6 inches
(38-41 cm)
and
- 1 5 inches
(38 c m )
Raymond
T h e works o f Walcott a n d o f Beecher have been modified augmented
by a
(1920a),
number
Stormer
of later workers,
(1939,
1951),
particularly
Sturmer
(1970),
B e r g s t r o m ( 1 9 6 9 , 1 9 7 2 , 1 9 9 0 ) , B e r g s t r o m and Brassel ( 1 9 8 4 ) ,
Soft Body Parts
Cisne ( 1 9 7 5 , 1 9 8 1 ) , W h i t t i n g t o n ( 1 9 8 0 , 1 9 9 2 ) , and W h i t t i n g t o n and A l m o n d ( 1 9 8 7 ) .
T h e unmineralized or soft parts of the trilobite b o d y are very
B i r a m o u s appendages are also the rule in extant crustaceans.
rarely preserved in the fossil record. Allison and Briggs ( 1 9 9 3 )
In the trilobites all the appendages, with the exception of the
made a listing of sites of exceptional fossil preservation, called by
a n t e n n a e , are very similar, differing primarily in size. All but o n e
the G e r m a n n a m e Konservat-Lagerstatten. T h e y recognized 19
of the trilobites recently studied have three pairs of b i r a m o u s
marine sites worldwide in the Paleozoic, where soft b o d y fossils
appendages in the c e p h a l o n , a pair for each thoracic segment,
are preserved. Nine of these sites are in the United States, and six
and multiple pairs in the pygidial section. Four pairs of b i r a m -
of them yield trilobites. O n l y o n e site in the United States in their
ous cephalic appendages reportedly were f o u n d in o n e trilobite
listing has significant trilobite appendage and o t h e r soft body
( B e r g s t r o m and Brassel
information: Beecher's Trilobite Bed in New York. In fact, most
finding is q u e s t i o n a b l e . In the pygidium the segmentation has to
of what we know about the soft parts of trilobites c o m e s f r o m
be inferred. For e x a m p l e , the n u m b e r of appendage pairs under
1 9 8 4 ) , b u t s o m e workers think this
FIGURE 2.9. Trilobite exoskeletons with a t t a c h e d f a u n a or injury. A. Arctinurus boltoni (USNM 449453). This trilobite has b r a c h i o p o d s of different sizes (arrows) a t t a c h e d , indicating the length of time b e t w e e n molts of mature s p e c i m e n s . Arctinurus s p e c i m e n s with a t t a c h e d b r a c h i o p o d s are not rare in the Rochester Shale b e d s , where these c a m e from. It is unlikely that they settled on the exoskeleton after death b e c a u s e the exoskeleton w o u l d have to have b e e n b u r i e d to remain a r t i c u l a t e d . B. The s a m e s p e c i e s of trilobite as in A with healed injuries to the exoskeleton. Three areas (arrows) have b e e n d a m a g e d a n d b e e n t h r o u g h at least one molt (PRI 42095). C. Dalmanites limulurus (F. Barber collection, w h i t e n e d ) . This trilobite has a t t a c h e d b r a c h i o p o d s (arrows). This b r a c h i o p o d a t t a c h m e n t is very unlikely to have o c c u r r e d p o s t m o r t e m for the s a m e reasons. D. The s a m e trilobite as in C, with the eye area e n l a r g e d to better s h o w the b r a c h i o p o d s (arrow). E. The s a m e trilobite, with the thoracic area e n l a r g e d to s h o w the b r a c h i o p o d s (arrows).
FIGURE 2.10.
Trilobite a p p e n d a g e reconstruction a n d n o m e n c l a t u r e . A - l . Triarthrus eatoni b i r a m o u s
a p p e n d a g e after Starmer (1939). B. The exite or brachial a p p e n d a g e (arrow) with the c o m b l i k e structures. C. The basis (arrow), also c a l l e d the coxite in early literature. D. The w a l k i n g leg or t e l e p o d i t e (arrow). This leg has seven s e g m e n t s , i n c l u d i n g the foot, in all trilobites w h e r e the a p p e n d a g e s have b e e n s t u d i e d . E. A p o d o m e r e or individual s e g m e n t (arrow) of the w a l k i n g leg. F. Endites (arrows), small triangular i n w a r d i n g - f a c i n g projections on the p o d o m e r e . T h e s e w e r e possibly u s e d to help transport f o o d a l o n g to the mouth at the rear of the h y p o s t o m e . G. Setae (arrow), hairlike projections on the exites. H. G n a t h o b a s e s (arrow) are s h a r p projections on the basis that may have b e e n used to masticate, to r e d u c e the size of f o o d particles. I. The foot (arrow) with its setae. J. A r e c o n struction of the filaments of the e x o p o d of Ceraurus pleurexanthemus by Starmer (1939). K. A partial axial view, looking toward the rear, of Triarthrus as r e c o n s t r u c t e d by Whittington a n d A l m o n d (1987, p. 42, Fig. 43). R e p r o d u c e d with p e r m i s s i o n .
20
THE
BIOLOGY
OF
TRILOBITES
FIGURE 2 . 1 1 . Ventral anatomy a n d a p p e n d a g e s . A.
Triarthrus eatoni, ventral
anatomy. The first essentially correct reconstruction of a trilobite's ventral s u r f a c e a n d a p p e n d a g e s . After B e e c h e r (1896). B. Ceraurus
pleurexanthemus,
ventral
anatomy.
This reconstruction from R a y m o n d (1920a) w a s from cross sections m a d e by Walcott in the late 1900s. C. Ceraurus pleurexanthemus,
ventral
anatomy.
This
reconstruction by Stormer (1951) w a s from s p e c i m e n s c o l l e c t e d b y Walcott a n d uniquely p r e p a r e d . D.
Triarthrus eatoni, a p p e n d a g e
structure d e v e l o p e d by Cisne (1975, p. 4 9 , Fig. 3) from high-resolution r a d i o g r a p h s of pyritized s p e c i m e n s . R e p r o d u c e d from Fossils a n d Strata, www.tandf.no/fossils, by J. L. Cisne, 1975, vol. 4, 4 5 - 6 3 , by permission of Taylor a n d Francis AS. E. Triarthrus eatoni,
a p p e n d a g e structure
d e v e l o p e d b y Whittington a n d A l m o n d (1987, p. 3 1 , Fig. 41), from direct o b s e r v a t i o n of very carefully p r e p a r e d s p e c i m e n s . R e p r o d u c e d with p e r m i s s i o n . F. Ceraurus pleurexanthemus,
appendage
structure
d r a w n by B e r g s t r o m (1972) from information d e v e l o p e d by Stormer (1939, 1951). R e p r o d u c e d with p e r m i s s i o n . G.
Cryptolithus
bellulus, a p p e n d a g e structure d e v e l o p e d by Bergstrom (1972, 1973) from pyritized s p e c i m e n s p r e p a r e d by Beecher. R e p r o d u c e d with p e r m i s s i o n . H. Phacops cf. P. ferdinandi, a p p e n d a g e structure d e v e l o p e d by B e r g s t r o m (1969) u s i n g the radiographs of pyritized s p e c i m e n s from the Hunsruck Shale in Germany. It is a s s u m e d that the p h a c o p i d trilobites of New York will have similar structures. R e p r o d u c e d w i t h permission.
the pygidium in T. eatoni is significantly m o r e than the n u m b e r
e n d o p o d . Until the work of Cisne ( 1 9 7 5 , 1 9 8 1 ) , all reconstruc-
of axial furrows on the dorsal surface of the pygidium, showing
tions of the appendages included a precoxa from which the
a weak relationship between s e g m e n t s a n d axial furrows.
e x o p o d extended ( S t o r m e r 1 9 3 9 , Figure 1, p. 155). This view is
A m o r e detailed look at an individual trilobite appendage
incorrect, based on all the material recently examined; both the
illustrates that it is a c o m p l e x structure. T h e a t t a c h m e n t to the
e x o p o d and the walking leg are attached to the s a m e apparatus,
ventral surface of the trilobite body is through a part called the
the basis. T h e e x o p o d has a series of thin, flattened filaments
basis (Figure 2 . I O C ) . ( F o r a current view on appendage n o m e n -
extending f r o m it, giving it a feather- or c o m b - l i k e appearance,
clature, see the article by RamskoTd and E d g e c o m b e ( 1 9 % ) . ) T h e
and it is carried up under the pleurae. O n l y in 7' eatoni are the
e x o p o d or outer b r a n c h is attached to the basis, which in turn is
exopods k n o w n to be long enough to extend well out from the
attached to the ventral m e m b r a n e of the trilobite. Beneath the
lateral edge of the dorsal exoskeleton. T h e large surface area of
e x o p o d , and also attached to the basis, is the walking leg, or
the small filaments led most a u t h o r s to believe that they have
LIFE-MODE
21
served for breathing, as an external gill. Bergstrom ( 1 9 6 9 ) argued
hepatopancreatic
that the filaments are t o o small to support an effective circulatory
equivalent of a liver, under the genal areas.
organs
(Figure
2.12A,
label
h),
probably
the
system, and instead they may have served as a filter of f o o d , as a
T h e gut (Figure 2 . 1 2 A , B, C, label g) passes from the stomach
means to circulate water over gill m e m b r a n e s on the ventral
through the axial region of the t h o r a x and terminates at the
surface, or possibly as a s w i m m i n g f u n c t i o n . In all the studies the
anus just under the posterior area of the pygidium (Plate 7 8 ) . T h e
exopods are drawn with the filaments lateral and posterolateral.
position of the gut is k n o w n f r o m several s p e c i m e n s of trilo-
S t o r m e r ( 1 9 3 9 ) actually f o u n d the filaments of Ceraurus pleurex-
bites because the ingested sediment often survives in place (Figure
anthemus pointed forward, but he rotated t h e m in his r e c o n -
2 . 1 2 D , label g) and has a different texture or c o l o r than the sur-
struction. Bergstrom ( 1 9 6 9 )
believed that they were pointed
r o u n d i n g stone ( R a y m o n d 1920a; Cisne 1 9 7 5 ; W h i t t i n g t o n 1993;
forward in life (Figure 2.1 I F ) . S t o r m e r ( 1 9 3 9 ) reconstructed the
Whiteley et al. 1 9 9 3 ; Brett et al. 1 9 9 9 ) . It is assumed that s o m e
filaments of the
f o r m of circulatory and nervous systems also o c c u p i e d the axial
exopod
of C.
pleurexanthemus
(Figure
2.10J),
illustrating the high surface area that supports their use as a brachial organ.
interior region. Small rodlike structures seen in radiographs (Cisne 1981) and
The endopod is multiply jointed with distinct sections called
in unusually well-preserved s p e c i m e n s ( W h i t t i n g t o n 1993) are
podomeres (Figure 2 . 1 0 E ) . T h e r e is general a g r e e m e n t that there
believed to represent muscles, and r e c o n s t r u c t i o n of the m u s c u -
are seven podomeres on all trilobites e x a m i n e d . On s o m e of the
lature related to the appendages has been proposed (Figure 2 . 1 2 A ,
podomeres, starting with the ones closest to the basis, are p r o -
B, C, label m ) . S o m e s p e c i m e n s of E. rana show s y m m e t r i c a l dark
jections, endites (Figure 2.1 OF), with hairlike setae (Figure 2 . 1 0 G )
spots on the exoskeleton ( B a b c o c k 1 9 8 2 ) . T h e s e are interpreted
near or on their tip. On
as m u s c l e a t t a c h m e n t areas.
Triarthrus, and probably most o t h e r
genera, the last p o d o m e r e is a footlike tip to the walking leg (Figure 2.101).
Two
soft-bodied
arthropods,
Naraoia
compacta
and
N.
spinifer, originally discovered from the Burgess Shale, are now
T h e most thoroughly e x a m i n e d trilobite appendages are those
regarded as trilobites and assigned to the family Naraoiidae. T h e
of T. eatoni and C. pleurexanthemus, b o t h from the Ordovician
Naraoiidae now includes five genera (Fortey and T h e r o n 1 9 9 5 ) .
of New York. T h e r e are m o r e s p e c i m e n s available of these trilo-
Two of these are O r d o v i c i a n and o n e survived to Late O r d o v i -
bites with preserved appendages than there are of any others.
cian. If, as s o m e a u t h o r s believe, the heavier exoskeleton of
Figure 2.11A is the Beecher reconstruction of T. eatoni and parts
p o s t - C a m b r i a n trilobites is an evolutionary response to m o r e
B and C of Figure 2.11 are reconstructions of C. pleurexanthemus
advanced predators, then it is unlikely that s o f t - b o d i e d trilobites
by R a y m o n d ( 1 9 2 0 a ) and S t o r m e r ( 1 9 5 1 ) . S t o r m e r ' s figure is
survived past the O r d o v i c i a n . N o n e of these genera are known
modified to show the dorsal a n a t o m y on the right and the ventral
f r o m New York, but given the special c o n d i t i o n s necessary for
on the left. Parts D and E of Figure 2.11 are r e c o n s t r u c t i o n s of
their preservation, this does not prove that they were not present.
the legs of T. eatoni by Cisne ( 1 9 8 1 ) and by W h i t t i n g t o n and A l m o n d ( 1 9 8 7 ) , respectively. Parts F, G, and H are drawings of the legs of Ceraurus,
Cryptolithus hellulus, and
Phacops cf.
P. fer-
dinandi, all as reconstructed by Bergstrom ( 1 9 6 9 ) .
Life-Mode T h e following discussion of l i f e - m o d e is based almost exclu-
Figure 2 . 1 0 K shows T. eatoni as reconstructed by W h i t t i n g t o n
sively on circumstantial evidence. As s u c h , it is highly interpre-
and A l m o n d ( 1 9 8 7 ) . T h e view is from about the midline of the
tive. Fortey ( 1 9 8 5 ) p o i n t e d out that using the s a m e body of
trilobite looking to the rear and illustrates the orientation of the
knowledge, trilobites in the family Agnostidae have been hypoth-
walking legs and their parts. T h e bases are shown with toothlike
esized to be pelagic, b e n t h i c , parasitic, a n d epifaunal, possibly
adaxial projections or gnathobases (Figure 2 . 1 0 H ) . F o o d was
attached to algal strands. T h e fossil record does not often permit
probably collected, masticated, or pulled apart, and passed along
clear, u n a m b i g u o u s c o n c l u s i o n s . However, o n e might assume
forward to the m o u t h at the posterior of the h y p o s t o m e by use
that " f o r m follows f u n c t i o n " and that a trilobite's m o r p h o l o g y is
of the e n d o p o d s and bases. Trilobites so equipped would be effec-
often a g o o d indicator of its life habits.
tive b o t t o m feeders, both as predators and as scavengers. Since
M o s t paleontologists c o n t e n d , by analogy with crustaceans,
this kind of i n f o r m a t i o n on appendages is k n o w n for so few trilo-
that for m a n y trilobites the protaspid phase was planktic. That is
bites, it is difficult to extrapolate too far as to the l i f e - m o d e of the
to say that the larvae floated and drifted in the sea, ensuring a
others.
wide dispersal of the species.
Pyritized trilobites from New York and G e r m a n y that were
Trilobites that were primarily b e n t h i c as adults probably
studied by high-definition X-ray p h o t o g r a p h y provide m u c h of
settled to the sea b o t t o m s o m e t i m e during the meraspid phase.
what we know about
1939;
Pelagic, or f r e e - s w i m m i n g trilobites, may never have left the open
Sturmer and Bergstrom 1 9 7 3 ; Cisne 1 9 7 5 , 1 9 8 1 ) . T h e i r s t o m a c h
the internal a n a t o m y ( S t o r m e r
seas during their t r a n s f o r m a t i o n to adults. All through these
(Figure 2.12A, label c ) , crop, or foregut is located in the glabella.
changes, the i m m a t u r e trilobite was very vulnerable to predators
Along with
and e n v i r o n m e n t a l stress, as the m o l t i n g process left the animal
the stomach
in
the cephalon are organs called
THE
22
FIGURE 2.12.
BIOLOGY
OF
TRILOBITES
Internal a n a t o m y of the trilobite. A. Internal o r g a n s of the c e p h a l o n of Triarthrus d e t e r m i n e d by
Cisne (1975, p. 55, Fig. 9): m, m u s c l e s ; c, c r o p or s t o m a c h ; g, gut; h, h e p a t i c o p a n c r e a t i c o r g a n . R e p r o d u c e d from Fossils a n d Strata, www.tandf.no/fossils, by J. L. Cisne, 1975, vol. 4, 4 5 - 6 3 , by permission of Taylor a n d Francis AS. B. Cross section of the thorax with the internal o r g a n s (Cisne 1975, p. 53, Fig. 7): d, dorsal vessel or "heart"; m, m u s c l e s ; g, gut. R e p r o d u c e d from Fossil a n d Strata, www.tandf.no/fossils, by J. L. Cisne, 1975, vol. 4, 4 5 - 6 3 , by p e r m i s s i o n of Taylor a n d Francis AS. C. Reconstruction of s o m e internal anatomy of Ceraurus pleurexanthemus by R a y m o n d (1920a): d, dorsal vessel or heart; g, gut; m, muscles. D. S p e c i m e n of C. pleurexanthemus that c l e a v e d to s h o w the gut (g) as a dark ferruginous stain ( M C Z 111716).
with little in the way of defense for a period of t i m e and the
(Figure 2 . 8 F ) is well d o c u m e n t e d . B o t h forms are well suited to
energy b u r d e n of continually building new exoskeletons was c o n -
their respective m o d e s of life. T h e change to a b o t t o m - d w e l l i n g
siderable. I n extant a r t h r o p o d s Clarkson ( 1 9 7 9 ) n o t e d that 8 0 %
( b e n t h i c ) f o r m c o m e s at the m e t a m o r p h o s i s f r o m protaspis to the
t o 9 0 % o f m o r t a l i t y c o m e s during the m o l t i n g process. T h e m o r e
meraspis, which is shaped like the adult but with long genal
c o m m o n trilobites had to p r o d u c e large n u m b e r s of larvae in
spines. As m e n t i o n e d earlier, the long genal spines last well into
order for significant n u m b e r s to reach maturity.
the early juvenile holaspis.
asaphid trilobite, Isotelus,
S o m e trilobite species are widely dispersed throughout the
f r o m the planktic protaspid (Figure 2 . 8 D ) to the b e n t h i c adult
world, suggesting that they were planktic or pelagic as adults and
T h e m e t a m o r p h i c c h a n g e in an
23
LIFE-MODE
FIGURE 2.13. Trilobite s h a p e s a n d functions. A. Isotelus gigas ( M C Z 311). This trilobite has a s m o o t h s h a p e suitable for shallow p l o w i n g of the surface m u d s for f e e d i n g . B. Hypodicranotus striatulus ( M C Z 100986). The streamlined s h a p e a n d the 1 8 0 - d e g r e e visual c a p a b i l i t y s u g g e s t a trilobite that m i g h t have b e e n a g o o d swimmer a n d w a s p e r h a p s pelagic. C. Achatella achates (PRI 4 9 6 5 9 ) . This trilobite has a fairly flat b o d y with eyes raised well a b o v e the rest of the c e p h a l o n . In a n a l o g y with b o t t o m dwellers with raised eyes, one might e x p e c t this trilobite to rest on the b o t t o m , with the b o d y just u n d e r the s u r f a c e of the substrate a n d the eyes a b o v e it. This position is a d e f e n s e against p r e d a t o r s a n d possibly a m e a n s of lying in wait for prey. D.
Cryptolithus bellulus (PRI 4 9 6 5 4 ) . This trilobite has a prominent, robust
c e p h a l o n c o m p a r e d to the light exoskeletal material on the thorax a n d p y g i d i u m . This feature a n d other e v i d e n c e ( C a m p b e l l 1975) s u g g e s t a sedentary lifestyle a n d filter f e e d i n g habit. E. Triarthrus eatoni (TEW collection). The a p p e n d a g e , i n c l u d i n g the exite or brachial b r a n c h , e x t e n d s well b e y o n d the e d g e of the thoracic shield. This configuration aids the trilobite to survive in the d y s o x i c , d e e p - w a t e r c o n d i t i o n s s u g g e s t e d by the dark shales in w h i c h they are f o u n d .
moved freely t h r o u g h o u t the seas. Planktic larval f o r m s would
and it is reasonable to suggest that they t o o buried themselves
also serve to disperse species but in a m o r e limited m a n n e r . D i s -
under a thin layer of sediment.
persal into new areas by the n o r m a l l y b e n t h i c trilobites is e x -
B o d y shape suggests the l i f e - m o d e of m a n y trilobites. Fortey
pected to be followed by speciation, so o n e would not expect the
( 1 9 8 5 ) defined three m o r p h o l o g i e s o f pelagic species: ( 1 ) large-
species identity to be m a i n t a i n e d if dispersal was into areas not
eyed, epipelagic, s l o w - s w i m m i n g trilobites; ( 2 ) pelagic, stream-
previously occupied by the species.
lined, faster s w i m m e r s ; and
There are wide variations in the size of trilobite eyes, the
(3)
possible s w i m m i n g Irvingella
types, remopleuridids, and progenetic types. T h e f a s t - s w i m m i n g
n u m b e r of lenses, and the angle of vision. T h e s e variations are
trilobites have r o u n d e d streamlined shapes, which p r o m o t e d
certainly s o m e indication of the l i f e - m o d e , but m o d e r n analogy
b u o y a n c y and low drag while s w i m m i n g . T h e s e shapes are not
is often necessary to c o m e up with suggestions. Animals that b u r y
often found a m o n g New York trilobites, but the Middle O r d o v i -
themselves shallowly in the b o t t o m sediment have eyes that are
cian
raised above the plane of the head, enabling t h e m to see when
and is considered pelagic (Figure 3 . 1 3 B , Plate 1 6 0 ) .
they are slightly buried. S o m e trilobites have such raised eyes (e.g., Achatella
achates
(Figure
2.13C)
and
Dalmanites
species),
remopleurid
Hypodicranotus
striatulus
has
the
right
shape
Vaulted, s m o o t h exoskeletons such as that on /. gigas (Figure 2 . 1 3 A , Plate
1 5 0 ) , Dipleura dekayi (Plate 9 7 ) , and
Trimerus del-
24
THE
BIOLOGY
OF
TRILOBITES
phinocephalus (Plate 9 9 ) were well designed to plow through the
fossils such as tracks, burrows, and distinctive pits preserved in
upper sediment layers in search of f o o d . T. eatoni, with its thin
the fossil record are attributed to trilobites, as supported by the
exoskeleton and outer b r a n c h e s that extend b e y o n d the pleurae,
rare find of a trilobite at the end of a trackway or in o n e of the
was unsuited for shallow, t u r b u l e n t water and for plowing in sed-
burrows (Figure 2 . 1 4 A ) . Two c o m m o n t r a c e s — C r u z i a n a and
i m e n t and was better designed for surface scavenging in deeper,
Rusophycus—are generally preserved as molds
less oxygenated e n v i r o n m e n t s (Figure 2 . 1 3 E , Plate 1 7 2 ) .
basal c o n t a c t of sandstones or c a r b o n a t e beds in shales. O n e type
It has been proposed that to m a x i m i z e visual effectiveness,
(fillings)
on the
of deep, inscribed, horizontal track or furrow, Cruziana, shows
the plane of the upper and lower edges of the eyes should be
" V - l i k e " scratch patterns m a d e by the dactyls (claws) of the trilo-
parallel to the substrate (Plates 5 a n d 6 ) . In m a n y outstretched
bite and a central groove corresponding to an axial ridge of debris
trilobites it is apparent that the eyes are parallel to the s u b -
pushed up by the trilobite (Figure 2 . 1 4 B ) . T h e bilobed burrows,
strate. However, m a n y species of illaenids, when outstretched,
or resting pits also showing V-shaped scratches, are called Ruso-
have eyes that are angled upward and posteriorly. Westrop
phycus, and the trackways consisting of small " f o o t p r i n t s " on the
( 1 9 8 3 ) and B e r g s t r o m ( 1 9 7 3 ) , a m o n g others, argued that these
substrate
trilobites were i n f a u n a l , b u r y i n g themselves backward into a
1 9 9 0 , p. 1 6 1 ) , although O s g o o d ( 1 9 7 0 ) did not hold this t e r m in
soft b o t t o m with their cephalon on the surface at an angle to the
high regard.
thoracopygidium.
are
sometimes
known
as
Diplichnites
(see
Bromley
Trace fossils attributed to various burrowing animals (worms?)
In this attitude the eye base is parallel to the surface and gives
have been f o u n d ending in Rusophycus, suggesting the trilobite
the m a x i m u m all-around vision. T h e s e infaunal trilobites fed and
had attacked a n o t h e r burrower. T h e s e fossils indicate that m a n y
breathed by the exchange of water on the buried ventral anatomy.
trilobites walked a r o u n d on the b o t t o m and dug into the sediment
T h i s exchange o f water was the result o f the m o v e m e n t o f the
for b o t h food and resting places (Hall 1852, Plate 9, Figure 1).
appendages a n d a n upstream o r i e n t a t i o n o f the burrow. T h e r e
In the case of Cruziana the direction of travel for the trilobite
are other trilobites with this l i f e - m o d e , which Westrop termed
is toward the o p e n end of the V-shaped appendage traces. M o s t
" i l l a e n i m o r p h s . " W h i t t i n g t o n ( 1 9 9 7 b ) rebutted this view on the
Rusophycus traces f o r m a V, with the gape end or anterior being
basis of the high flexibility of the t h o r a x in illaenids and that they
wider than the posterior, suggesting that the trilobite rested (or
are m o r e suited to crawling a r o u n d the b o t t o m and over irregu-
h u n t e d ) with the cephalon toward the " g a p e " end. T h e r e is little
lar objects than living or resting primarily in burrows.
a r g u m e n t that Rusophycus traces are most readily explained as
Westrop also p o i n t e d out that on s o m e of the illaenimorphs
trilobite h u n t i n g or resting pits, but the same is not true for
there is a median tubercle on the glabella, midway between the
Cruziana. W h i t t i n g t o n ( 1 9 8 0 ) , based on his in-depth studies of
palpebral lobes on the sagittal line. T h i s tubercle is also a thin
Olenoides
spot and is characterized as having possible light-sensing p r o p -
readily allow for the type of traces represented by Cruziana and
erties. It is the highest point when the b o d y is in a n o r m a l life
that s o m e o t h e r a n i m a l , perhaps not even an a r t h r o p o d , may be
serratus,
believed
that
trilobite
appendages
do
not
position, thus covering any blind spots of the c o n v e n t i o n a l eyes.
responsible. However, m a n y Cruziana traces end in Rusophycus,
R u e d e m a n n ( 1 9 1 6 b ) f o u n d a significant n u m b e r trilobites with
and since the latter are u n a m b i g u o u s l y trilobite, this reference is
such median tubercles, even n o m i n a l l y blind trilobites such as in
questionable.
the genus Cryptolitluis.
It is reasonable to c o n j e c t u r e that the
In New York, traces attributed to trilobites are c o m m o n in the
tubercles had a light-sensing utility and played a role in the trilo-
Silurian C l i n t o n G r o u p , particularly on the base of sandstone
bite's life-mode. Such light sensing tubercles probably could sense
layers near the village of C l i n t o n , Oneida County. O s g o o d and
m o v e m e n t but not resolve o b j e c t s . In m o d e r n a r t h r o p o d s larger eyes are f o u n d on n o c t u r n a l
D r e n n e n ( 1 9 7 5 ) provided a g o o d description of these traces and their literature. In o t h e r strata they are far less well k n o w n , either
species or those adapted to low daylight levels. A n i m a l s with a
because c o n d i t i o n s were n o t right for their preservation
wide visual angle need it to watch for predators. Very large eyes
because little effort has been m a d e to find and identify t h e m in
and a wide visual angle are seen on s o m e pelagic trilobites (Figure
appropriate strata.
2 . 1 3 B ) . Fortey ( 1 9 8 5 ) considered the large-eyed trilobites as epipelagic and slow s w i m m e r s . C o m p o u n d eyes permit insects to be highly aware of m o v e -
or
Fortey and O w e n ( 1 9 9 9 ) proposed that trilobite feeding habits can be related to the position and a t t a c h m e n t of the h y p o s t o m e and other physical characteristics. Trilobites that are considered
m e n t but do not necessarily provide high visual acuity. R e d u c -
predatory (i.e., they fed o f f m a c r o f a u n a such as w o r m s ) had
tion in eye size has been noted for trilobite genera that m o v e d to
a c o n t e r m i n e n t h y p o s t o m e fixed or strongly supported at the
deeper water through t i m e . Blind trilobites, such as in the genus
anteroventral c e p h a l o n . T h e h y p o s t o m e provided a strong base
Cryptolithus (Figure 2 . 1 3 D ) , may have b u r r o w e d into sediment
for the appendages so the trilobite could manipulate and masti-
where sight would have been less i m p o r t a n t than o t h e r sensory
cate the prey. F o o d was passed forward along the ventral median
capabilities.
by the bases to the m o u t h at the posterior of the hypostome.
M o s t trilobites were b e n t h i c . T h e y passed m o s t of their life
Examples of this type of h y p o s t o m e a t t a c h m e n t in an Isotelus
on or in the u p p e r m o s t part of the sediment layer. S o m e t r a c e
species may be seen in Figure 2 . 7 E and Plates 153, 155, and 157.
25
LIFE-MODE
FIGURE 2 . 1 4 .
Trilobite traces. A. Rusophycus pudicum ( U C M 3 7 5 7 4 ) . S a n d s t o n e d e p o s i t s over trilo-
bite-rich b o t t o m m u d s often have c o n v e x , slightly V - s h a p e d traces on their lower s u r f a c e . These t r a c e s are known as Rusophycus a n d have b e e n long r e g a r d e d as trilobite "resting t r a c e s . " O s g o o d ( 1 9 7 0 ) reported on a remarkable Rusophycus that h a d the trilobite responsible for it still in p l a c e . B. Flexicalymene meeki. This trilobite w a s f o u n d on the Rusophycus in A. The trilobite is a b o u t 46 mm long a n d is from the U p p e r O r d o v i c i a n of Ohio. C. Trachomatichnus numerosum ( U C M 3 7 6 9 5 ) . A trilobite w a l k i n g trace attributed to Cryptolithus ( O s g o o d 1 9 7 0 ) . The illustration is life size. All figures r e p r o d u c e d with permission.
Impendent hypostoma that are also attached to the d o u b l u r e also
(weak)
suggest a predatory habit. Bellacartwrightia species (Plate 4 7 ) and
c e p h a l o n , as well as trace fossils clearly attributable to Cryp-
Calyptaulax callicephalus
tolithus resting ( a n d feeding) sites.
(Plate
117)
show this
mode
of h y p o -
stome a t t a c h m e n t .
thorax
and
thoracic
appendages
compared
to
the
Trilobites are f o u n d in a variety of e n v i r o n m e n t s , from fairly
Trilobites whose h y p o s t o m a were not strongly attached to the
shallow waters near s h o r e , to reefs, c o n t i n e n t a l shelves and slopes,
cephalon (i.e., natent) are considered to be particle feeders. T h e y
and m o d e r a t e l y deep basins. T h e observation has been m a d e
relied on m u c h smaller food particles swept up f r o m the b o t t o m ,
that trilobites from the shallower areas with m o r e wave turbul-
and the rigid h y p o s t o m e was unnecessary. T h e s e trilobites tend
ence have thicker exoskeletons (Fortey and W i l m o t 1 9 9 1 ) . T h e s e
to be smaller than the predatory ones because their food sources
thicker exoskeletons are possibly an evolutionary response to the
were not as rich and c o n c e n t r a t e d . T h e genera Harpidella (Plate
greater e n v i r o n m e n t a l energy. As m e n t i o n e d earlier, exoskeletons
128) and Triarthrus (Plates 170 to 174) represent this group of
are generally thicker in p o s t - C a m b r i a n trilobites, which may also
trilobites. The
last
signal the rise of better-developed predation, a n o t h e r form of feeding m o d e to
consider here
is filter-chamber
e n v i r o n m e n t a l stress.
feeding. This type of feeding is typical of trilobites with reduced
M a n y trilobites were gregarious, at least at s o m e point during
mobility and relatively large cephala. T h e s e trilobites settled in a
their life cycles. T h e large n u m b e r s of death and molt assem-
position and stirred up the sediment immediately under t h e m ,
blages, well illustrated by E. rana in New York, are no statistical
and then filtered out the m i n u t e food particles c o n t a i n e d in the
accidents. It is not u n c o m m o n , within a n u m b e r of different
top layer of sediment. M o v e m e n t o c c u r r e d only after the food
trilobite species, for a large n u m b e r of individuals to be found in
supply was exhausted. T h e genus Cryptolithus (Plates 163 to 167)
local " p o c k e t s " on the s a m e bedding plane or horizon, indicating
is the example used, and the evidence is the relatively small
s o m e f o r m o f group behavior (Plates 5 9 , 8 9 , 102, 128, 146, 147,
26
THE
BIOLOGY
OF
TRILOBITES
and 1 5 2 ) . All this suggests that it was c o m m o n for m a n y trilo-
d a m a g e . T h e s e m a l f o r m a t i o n s can c o m e about in several ways,
bites to congregate for breeding or m o l t i n g , or just because it was
but d a m a g e due to p r o b l e m s in molting and damage caused by
their n o r m a l l i f e - m o d e to be together in " s c h o o l s " (Speyer and
actual attack by a predator were probably the most c o m m o n . T h e
Brett 1985, Speyer 1 9 9 0 a ) .
m e c h a n i s m of a defect is not always clear, but healed punctures
Trilobites such as phacopids and calymenids were capable
and crescent-shaped m a l f o r m a t i o n s are readily attributed to pre-
of very tight e n r o l l m e n t , literally into a ball. T h i s position was
dation. Figure 2 . 1 5 illustrates four examples of exoskeletons with
undoubtedly a defense m e c h a n i s m resorted to in times of stress.
clear indication of predation. Panels A and B show " b i t e " marks
If the stress was an undersea s e d i m e n t flow t o o large to get out
out of trilobites from the Rochester Shale that have been through
of, the trilobite would be e n t o m b e d in the tightly enrolled posi-
at least o n e m o l t , as shown by the broken edges of the thoracic
tion. Most of the p o s t - C a m b r i a n trilobites in New York could
s e g m e n t s being r o u n d e d to a new termination. Panel C shows the
enroll, and the frequency of this position versus the o p e n posi-
unusual situation of Dalmanites limulurus with the pygidial spine
tion is perhaps an indication that it was a c o m m o n reaction to
missing ( c o m p a r e to Figure 2 . 1 5 A ) and the damage healed over.
stress for the species.
Panels D through F show a calymenid from the Rochester Shale
S o m e b e n t h i c trilobites u n d o u b t e d l y could swim. M o d e r n
with evidence of b o r i n g on its exoskeleton. This finding is par-
horseshoe c r a b s , a n o r m a l l y b e n t h i c species, are g o o d s w i m m e r s .
ticularly interesting because crinoids from the Rochester Shale
T h e y also swim upside d o w n , and it has been shown that the
have been described with similar boring marks (Brett
hydrodynamics of their s w i m m i n g works best in this attitude
1 9 8 5 ) . S i g n o r and Brett ( 1 9 8 4 ) and Pratt ( 1 9 9 8 ) s u m m a r i z e d the
(Fisher 1 9 7 5 ) . It is reasonable to a s s u m e that s o m e n o r m a l l y
possible
benthic trilobites could swim also and possibly quite well. S o m e
d o c u m e n t e d predator o f the C a m b r i a n , and many o f the circular
predators
in
the
Paleozoic.
Anomalocaris
is
a
1978, well-
likely swam upside d o w n . ( H . B u r m e i s t e r first proposed this
scars on trilobites are attributed to the circular m o u t h of Anom-
in 1843.) T h i s m o d e of s w i m m i n g may not have been involved in
alocaris. C e p h a l o p o d s b e c a m e a predation factor in the O r d o v i -
their
cian, and fishes b e c a m e p r o m i n e n t in the Devonian.
food
gathering,
but
they
could
move
from
place
to
place and evade danger by s w i m m i n g . In o n e trilobite bed, the
Babcock
(1993)
has collected
information
related to the
result of a burial event, at least 9 8 % of the trilobites with a wide
c o n c e p t of behavioral a s y m m e t r y . T h i s means that when a trilo-
variety of sizes are f o u n d buried upside down ( W h i t e l e y et al.
bite was attacked by a predator, its response was not r a n d o m ,
1993; Brett et al. 1 9 9 9 ) . T h i s observation applies to b o t h the lower
b u t there was a preference to m o v e in a m a n n e r that resulted in
surface and internal to the l i m e s t o n e . For m o r e on this, see
m o s t d a m a g e to the right side of the animal. Conversely, pre-
Chapter 3.
ferred d a m a g e to the right side of the trilobite could suggest the
Spininess in trilobites has invoked a n u m b e r of explanations,
attack strategy of the predator.
most o f t h e m probably correct for o n e species o r another. T h e r e
G e n e t i c or embryological abnormalities cannot be easily diag-
seems to be little need to invoke j u s t a single e x p l a n a t i o n , any
nosed in fossils. Pathological abnormalities are due to disease and
m o r e than there is o n e explanation for spininess in m o d e r n
parasites. Disease is impossible to d o c u m e n t in fossils, but para-
species of a r t h r o p o d s . Spines can be a defense m e c h a n i s m against
sitic attack is seen by the presence of w o r m - s h a p e d borings and
predators, provide support on a soft substrate, assist the a n i m a l
gall-like swellings. It is m o r e difficult to determine if borings were
in burying itself when
The
m a d e on the living animal (versus on the exuviae or p o s t m o r t e m )
Lower Devonian s t r a t u m in New York ( a n d the Devonian strata
than to recognize a healed injury. Swellings are known from a
necessary, and sensory devices.
of O k l a h o m a and M o r o c c o ) has trilobites with elaborate spines,
n u m b e r of different trilobite families, and s o m e can be diagnosed
s o m e curling up and over their t h o r a x . T h e s e spines of the genus
as the result of parasites based on the direct presence of w o r m -
Dicranurus c o m m o n l y carried algae and o t h e r encrusting org-
like structures.
anisms on t h e m , possibly as a defense m e c h a n i s m ( K l o c 1 9 9 3 ,
M c N a m a r a and Rudkin ( 1 9 8 4 ) d o c u m e n t e d the death of a
1 9 9 7 ) . T h e spines provided g o o d a t t a c h m e n t s and presumably
partially molted Pseudogygites latimarginatus.
helped break up the visual b o d y lines to provide c a m o u f l a g e .
the a n i m a l was o v e r c o m e and buried while molting.
S o m e m o d e r n a r t h r o p o d s do exactly this.
It seems likely that
Often trilobites are f o u n d in association with other fossils in
O n e can a s s u m e , based on m o d e r n analogy, that trilobites
rapid-burial deposits. If there is little indication of their being
were subject to injury, parasitism, and predation, as reflected in
t r a n s p o r t e d any distance, then this can be taken as evidence of
their preserved parts. O w e n ( 1 9 8 5 ) extensively reviewed trilobite
their ecology. An example is the o d o n t o p l e u r i d Meadowtownella
abnormalities.
He listed three general types of trilobite a b -
trentonensis from the Trenton G r o u p . T h i s small, spiny trilobite
n o r m a l i t i e s : injury, genetic or embryological m a l f u n c t i o n , a n d
is often f o u n d whole on fossil hash layers, particularly with
pathological a b n o r m a l i t i e s .
b r a n c h e d bryozoans, and also in burial event deposits with the
Not u n c o m m o n l y , trilobites are f o u n d with m a l f o r m a t i o n s
cystoid
Cheirocrinus
and
fenestrate
bryozoans.
M.
trentonensis
that are ascribed to healed injuries (Figure 2 . 9 B ) . M o s t of these
was probably a scavenger on b o t t o m s with animal debris accu-
injuries are seen on the pleura either by a s y m m e t r y or by healed
m u l a t i o n s and also in bryozoan thickets with their attached
FIGURE 2.15.
Trilobite injuries. A. Dalmanites limulurus with a semicircular portion of the
thorax missing. The d a m a g e has " h e a l e d " in that the e d g e s of the d a m a g e are r o u n d e d , indicating the trilobite has molted at least o n c e s i n c e the injury (K. Smith collection). B. Dicranopeltis nereus with an injury to the thorax. The trilobite has m o l t e d at least o n c e since the injury (K.
Smith collection).
C.
Dalmanites limulurus with the p y g i d i a l spine
missing. The e n d of the p y g i d i u m is r o u n d e d , indicating one molt s i n c e the loss of the spine. (K. Smith collection). D-F. Calymene s p e c i e s , s h o w i n g circular " b o r i n g s " (arrows) similar to those of Tremichnus on several different s p e c i e s of c r i n o i d s from the Rochester Shale reported by Brett (1985) (NYSM 16796).
THE
28
FIGURE 2.16.
BIOLOGY
OF
TRILOBITES
Cryptolithus, the
m o s t - s t u d i e d trilobite g e n u s f o u n d in New York. A. Cryptolithus tessellatus (TEW
collection,
whitened). This trilobite, in N e w York, is f o u n d only in the M i d d l e Ordovician Sugar River Formation. It is r e c o g n i z e d by the three rows of pits anterior to the c h e e k area (arrow).
B.
Cryptolithus
lorettensis
(PRI 49657, w h i t e n e d ) . In New York this trilobite is f o u n d only in the upper Sugar River Formation. It differs from C. tessellatus in that there are four rows of pits anterior to the c h e e k (arrow). The first approximately nine radial abaxial pit rows are well a l i g n e d . C. Cryptolithus
bellulus (PRI
49654,
latex pull, w h i t e n e d ) from the U p p e r O r d o v i c i a n Lorraine G r o u p in New York. This trilobite differs from C. lorettensis primarily in the poor radial alignment of the first four abaxial pit rows. D. A d r a w i n g of a silicified s p e c i m e n of C.
tessellatus by C a m p b e l l
(1975). R e p r o d u c e d with permission. E. A d r a w i n g of the same s p e c i m e n by B e r g s t r o m (1972) with the a p p e n d a g e s drawn in. The a p p e n d a g e information is from B e e c h e r s p e c i m e n s of C. bellulus. R e p r o d u c e d with p e r m i s s i o n . F. Raymond's (1920a) reconstruction of the ventral anatomy of C. bellulus using
specimens
p r e p a r e d by Beecher. G. Bergstrom's (1972) reconstruction of the s a m e s p e c i m e n s . R e p r o d u c e d with permission.
cystoids. A further possibility is that this trilobite fed o f f living
preserved in a wide variety of e n v i r o n m e n t s and the possibility
bryozoa.
of new i n f o r m a t i o n on the lifestyle and biology is enhanced.
Trilobites of the genus Cryptolithus are a m o n g the best u n d e r -
Cryptolithus
specimens
with
appendages
are
preserved
in
stood of the New York trilobite genera (Figure 2 . 1 6 ) . T h e r e are
Beecher's Trilobite B e d , which enables detailed observations of
several reasons for this. T h e genus is widely distributed g e o -
the ventral a n a t o m y ( B e e c h e r 1895a, R a y m o n d 1920a (Figure
graphically and geologically. W i d e distribution m e a n s that it is
2.16F), Bergstrom
1972 (Figure 2 . 1 6 G ) ) . At least o n e whole,
29
LIFE-MODE three-dimensional s p e c i m e n , preserved in silica, is k n o w n . Figure
covered by W h i t t i n g t o n ( 1 9 5 9 ) . P h o t o g r a p h s of this specimen
2 . 1 6 D is a drawing of the specimen by C a m p b e l l ( 1 9 7 5 ) , and
reproduced by C a m p b e l l ( 1 9 7 5 ) reveal that the ventral plane of
Figure 2 . 1 6 E is a drawing of the same s p e c i m e n by B e r g s t r o m
the t h o r a c o p y g i d i u m was well above the apparent plane of the
with the legs included. Walking and digging trace fossils unequiv-
ventral surface of the c e p h a l o n . In o r d e r to m o v e about on the
ocally assigned to Cryptolithus are k n o w n f r o m Kentucky. T h e
b o t t o m and to burrow, the trilobite must have had long, strong,
Cryptolithus protaspid is s o m e t i m e s found silicified and is easily
ventral walking legs or have developed o t h e r m o d e s of m o v e -
characterized. T h e unique c e p h a l o n with its heavily pitted b r i m
m e n t . C a m p b e l l ( 1 9 7 5 ) explored this in detail, but in s u m m a r y
is readily recognized in stratigraphic samples, and it is impossi-
Cryptolithus species could n o t have been
ble to mistake it for other genera. A n u m b e r of authors have
or deep burrowers (see Figure 2 . 1 6 G ) . O s g o o d ( 1 9 7 0 , Plate 5 8 ,
very active crawlers
studied the distribution of Cryptolithus so there is a large data-
Figure 1 and 2) figured the resting trace fossil Rusophycus cryptolithi, which because of size and the impressions of genal spines
base of i n f o r m a t i o n . In New York there are three distinct species or " m o r p h s " of
could only have been m a d e by Cryptolithus a n i m a l s . T h e s e traces
Cryptolithus ( W h i t t i n g t o n 1 9 6 8 , Shaw and Lesperance 1 9 9 4 ) : C.
indicate that the trilobite sat in a shallow depression, facing into
tessellatus (Figure 2 . 1 6 A ) , C. lorettensis (Figure 2 . 1 6 B ) , b o t h from
the c u r r e n t , and swept particles of detritus f r o m the b o t t o m into
the Middle Ordovician Sugar River F o r m a t i o n , and C. bellulus
its m o u t h .
(Figure 2 . 1 6 C ) from the Upper Ordovician Lorraine G r o u p , all in
T h e m o u t h of Cryptolithus species is at the rear of the small
New York. T h e y are distinguished primarily by the rows of pits
h y p o s t o m e in the ventral part of the c e p h a l o n . T h e s t o m a c h
or the pit a r r a n g e m e n t , or b o t h , so that it is justified to assume
occupies the glabella, a n d digestive capability extends out into the
that any biological i n f o r m a t i o n from o n e species is essentially the
genal area. T h e a l i m e n t a r y track lay along the axis of the t h o r a -
same for all of t h e m .
c o p y g i d i u m and o c c u p i e d a b o u t 2 0 % o f the width o f the axial
Trinucleids arose as a family in Europe and migrated to North America during the Middle O r d o v i c i a n
(Whittington
rings. T h e anus is at the posterior p o i n t of the pygidium. Fortey and O w e n s
(1999)
described
Cryptolithus species as
1 9 6 8 ) . T h e pelagic protaspis ( C h a t t e r t o n et al. 1994) ensured that
filter-chamber feeders, m e a n i n g that the resting trilobites stirred
o n c e in the North A m e r i c a n epicontinental sea, the trilobite
the sediment directly u n d e r t h e m , using the cavity between the
spread rapidly wherever currents t o o k the protaspis. Molts f r o m
cephalon and the substrate, and filtered o u t the food particles
protaspides and cephala of what is probably the meraspid phase
from the rest of the suspended s e d i m e n t . Fortey and O w e n s also
are c o m m o n l y found in the deep-water Frankfort Shales includ-
suggested that the cephalic perforations provided c h a n n e l s for
ing the Beecher's Trilobite Beds. T h e holaspid is blind except
water to flow out of this cavity while it was swept forward by the
for the possible eye spot or visual receptor centered on the
appendages. T h i s flow b r o u g h t the food particles forward to the
median line of the glabella. T h e r e is a wide b r i m on all but the
m o u t h and kept oxygenated water flowing over the exopods.
posterior of the cephalon. T h i s b r i m is bilaminar, separating into
As
in
all
specimens
of trilobites
with
preserved ventral
an upper and lower lamella through a plane parallel to the ventral
appendages, the b i r a m o u s limbs of Cryptolithus included walking
plane of the trilobite. T h e b r i m is perforated with holes, c o m -
legs, e n d o p o d s , and o u t e r b r a n c h e s , e x o p o d s , which may have
monly referred to as pits, that pass through b o t h lamellae. T h e
served as a breathing organ as well as provided s o m e ability to
pits are in circumradially arranged rows for the first three or four
sweep the surface under the trilobite. T h e exopods do not extend
circumferal rows. T h e r e are a n u m b e r of speculations as to the
b e y o n d the b o r d e r of the t h o r a c i c s e g m e n t s . T h e r e are three pairs
function of the pits. T h e m o s t current thinking is that they
of appendages under the c e p h a l o n and o n e pair under each
were sensory devices, perhaps to d e t e r m i n e c u r r e n t direction
t h o r a c i c segment. U n d e r the pygidium there are a large n u m b e r
(Campbell 1 9 7 5 ) .
o f m u c h - r e d u c e d appendages, reflecting the n u m b e r o f fused
T h e long genal spines are on the lower lamella. Cephala are
segments in the pygidium.
found with and without genal spines, as are n e a r - w h o l e a r t i c u -
A s s u m i n g the relationship of the appendages to the exoskele-
lated specimens. T h e trilobite molted by m o v i n g forward through
ton is correct, it is hard to see m e m b e r s of the genus Cryptolithus
a gap between the lamella, and this gap possibly closed after
as active surface crawlers.
molting (a m e c h a n i s m observed in extant h o r s e s h o e c r a b s ) .
On the w h o l e , o n e can a s s u m e that trilobites initially occupied
Alternatively, the lower lamella b e c a m e detached during the
m a n y of the e n v i r o n m e n t a l niches that are o c c u p i e d today by
molting process. Consequently, it is difficult to k n o w if a c o m -
m o d e r n m a r i n e a r t h r o p o d s . T h e ecological niche for the indi-
pletely whole articulated specimen with the genal spines intact is
vidual species enabled trilobites to b e c o m e part of a p a l e o c o m -
a molt or an animal killed and buried. T h e absence of genal spines
munity. W i t h i n this c o m m u n i t y the trilobites shared the local
and lower lamella, however, ensures it is a m o l t .
e n v i r o n m e n t with a variety of o t h e r a n i m a l s and plant life. W h e r -
T h e thorax and pygidium are m u c h reduced c o m p a r e d to the
ever o n e finds the necessary associations within the rocks, o n e can
cephalon. T h e r e are six t h o r a c i c segments and a small triangular
expect
pygidium. A nearly whole silicified Cryptolithus species was dis-
is no different than what is seen in extant, and undisturbed,
t o f i n d the o t h e r
m e m b e r s o f the c o m m u n i t y . T h i s
30
THE
BIOLOGY
OF
TRILOBITES
c o m m u n i t i e s today. S o m e trilobites, such as the illaenids, are
tion must offer a competitive advantage to the species. N o n c o m -
f o u n d in a fairly n a r r o w range of e n v i r o n m e n t a l c o n d i t i o n s ,
petitive genetic changes will s o o n disappear, as the animal is
and s o m e like in the genus Isotelus are f o u n d in a wide variety
unable to c o m p e t e for the o p p o r t u n i t y to pass the changes along
of c o m m u n i t i e s from shallow nearshore areas to deep r a m p -
to its offspring. Next, the new trait should o c c u r in an isolated
basin transition areas. Fortey ( 1 9 7 5 ) defined c o m m u n i t i e s in the
small p o p u l a t i o n so that it will not be " l o s t " in a m u c h , m u c h
northern
c o m m u n i t i e s , the
larger gene pool. Since m u t a t i o n occurs relatively frequently, a
cheirurid-illaenid and the olenid, fit m u c h of the New York
beneficial m u t a t i o n will happen at s o m e w h a t regular intervals,
Middle Ordovician very well.
statistically. In a stable e n v i r o n m e n t , however, with many others
Europe
Ordovician.
Two
o f his
T h e geographic restrictions of specific trilobite families and
of the same species, this change may not b e c o m e fixed. A better
genera also are used to identify p a l e o b i o g e o g r a p h i c provinces.
o p p o r t u n i t y for a new m u t a t i o n to be passed along is when a
W h i t t i n g t o n ( 1 9 6 1 b ) , W h i t t i n g t o n and Hughes ( 1 9 7 2 ) , and Ross
group, for s o m e reason, is o c c u p y i n g an e n v i r o n m e n t where there
( 1 9 7 5 ) , for e x a m p l e , used this a p p r o a c h to define the early posi-
is little o u t b r e e d i n g c o m p e t i t i o n or there are new environmental niches to occupy. Essentially populations are stable until a change
tions o f the p a l e o c o n t i n e n t s . T h e early evolutionary success of trilobites was probably due
takes place in an isolated e n v i r o n m e n t , and after this c h a n g e (or
to their development of a mineralized exoskeleton and their wide
c h a n g e s ) , the new species b e c o m e s competitive with the f o r m e r
geographic dispersal. T h e n u m b e r o f trilobite genera increased
stable species and displaces it or in s o m e cases occupies a differ-
from their first a p p e a r a n c e until the Late C a m b r i a n and then
ent available niche. T h u s , evolution is not a process of continual
decreased c o n t i n u o u s l y through the Paleozoic. T h i s is graphically
small changes b u t o n e of relative stability followed by abrupt step
shown in Ludvigsen ( 1 9 7 9 b , Figure 9 ) . A n u m b e r of explanations
changes. Eldredge and G o u l d ( 1 9 7 2 ) used the term punctuated
for this have been proposed. T h e n u m b e r of potential predators
equilibria to reflect their observation that evolution is not a
continually increased d u r i n g these s a m e times. It is likely that
s m o o t h transition f r o m o n e species to the other b u t rather is
the increased predation, increased n u m b e r o f c o m p e t i t o r s for
c o m p o s e d of periods of relative stability punctuated by periods
environmental niches, and the b u r d e n of m o l t i n g to a completely
o f rapid c h a n g e .
defenseless individual all played a part in the reduced success of
T h e observation
that s o m e
fossil
c o m m u n i t i e s s h o w re-
trilobites through t i m e . Trilobites disappeared completely at the
m a r k a b l e stability, s o m e t i m e s over millions of years, has led
end o f the P e r m i a n .
to s o m e i n - d e p t h investigations. In New York s o m e Middle Devonian fossil c o m m u n i t i e s , particularly in dysaerobic envir o n m e n t s , show this stability. As a result of these and o t h e r
Evolution and Cladistics
studies, Brett and Baird ( 1 9 9 2 , 1 9 9 5 ) coined the term coordinated
Fortey and O w e n s ( 1 9 9 7 ) reviewed the evolutionary history of
stasis for this f o r m of evolutionary stability (see also K a m m e r
trilobites. T h i s review should be considered the latest in what will
et al. 1 9 8 6 , M o r r i s et al. 1 9 9 5 , Brett et al. 1 9 9 6 ) . Similarly in
be an o n g o i n g series of a r g u m e n t s .
the C a m b r i a n , for e x a m p l e , there is evidence that populations
T h e phylogeny or evolution of trilobites is k n o w n f r o m the
of trilobites in the shallower water environments underwent a
first calcified r e m a i n s in the Lower C a m b r i a n rocks to the last
n u m b e r of e x t i n c t i o n s , and the area where they o n c e were was
trilobites in the Late P e r m i a n . It is generally believed that trilo-
repopulated with trilobites f r o m deeper water. T h e s e episodic
bites as a class are m o n o p h y l e t i c ; that is they are f r o m a c o m m o n
e x t i n c t i o n s and repopulations, called biomeres ( P a l m e r 1 9 6 5 ,
ancestor ( R a m s k o l d and E d g e c o m b e 1 9 9 1 ) . T h e earliest k n o w n
1 9 8 4 ) , are recorded from the C a m b r i a n of North A m e r i c a (see
trilobites are from the s u b o r d e r Olenellina f o u n d exclusively in
also E d g e c o m b 1 9 9 2 ) .
the Lower C a m b r i a n . T h e s e trilobites lack facial sutures, which is
Relationships of trilobites at all t a x o n o m i c levels are currently
considered a primitive characteristic (Fortey and W h i t t i n g t o n
being revised using cladistic methodology. Cladistics as used in
m e m b e r of
paleontology is the grouping of taxa by their shared physical c h a r -
this early group. Facial sutures first appear in the s u b o r d e r
acteristics. Close relationships are based on synapomorphies or
Redlichiina, also in the Lower C a m b r i a n . It is p r o b a b l e that there
"shared derived" characteristics. Traits may be divided into p r i m -
1989).
In
New York
Elliptocephala
asaphoides
is
a
are ancestral trilobites that did not have mineralized exoskeletons,
itive ( p l e s i o m o r p h i c ) or derived ( a p o m o r p h i c ) . For trilobites the
perhaps back into the late P r e c a m b r i a n . T h e r e is no c o m p e l l i n g
b i r a m o u s appendages are plesiomorphic or "shared primitive"
reason to believe that all trilobites sprang from olenellin a n c e s -
characteristics, m e a n i n g they are primitive to the group (i.e., they
tral stock.
represent a c o m m o n ancestral trait of a r t h r o p o d s not exclusive
F r o m these ancestors evolved the large n u m b e r of trilobite
to trilobites) and may be used only to c o m p a r e trilobites with
orders, families, and genera that are recognized today. Evolution,
o t h e r a r t h r o p o d s . Schizochroal eyes are only f o u n d in the subor-
however, is not s o m e t h i n g that proceeds s m o o t h l y with a calen-
der P h a c o p i n a , and thus this characteristic forms a derived char-
dar-like precision. For evolutionary c h a n g e to occur, at least two
acteristic or a p o m o r p h y . A n o t h e r example involves the trilobite
m a j o r factors must be in place. First, any genetic c h a n g e or m u t a -
h y p o s t o m e . Fortey ( 1 9 9 0 a ) developed the a r g u m e n t that hypos-
EVOLUTION
AND
CLADISTICS
31
t o m e a t t a c h m e n t , natent versus c o n t e r m i n a n t versus i m p e n d -
trilobites by the t i m i n g of the a p p e a r a n c e of the selected derived
ent, is an
characteristics. W i t h a large n u m b e r of characteristics, the g r o u p -
indicator of trilobite phylogeny.
other relationships, Fortey ( 1 9 9 0 a , Figure phylogeny/hypostome
attachment
chart
for
Using these a n d 19)
constructed a
trilobites,
ings are best d o n e with c o m p u t e r p r o g r a m s designed for cladis-
which
tics that search for the closest or most p a r s i m o n i o u s fit. Cladistics
By selecting a significant n u m b e r of derived characteristics
depth review, see the works by Fortey ( 1 9 9 0 b , 2 0 0 1 ) and Novacek
summarizes the current state of knowledge. and evaluating their presence or absence, o n e can group related
is clarifying m a n y relationships a m o n g trilobites. For a m o r e inand W h e e l e r ( 1 9 9 2 ) .
3
Taphonomy
Taphonomy is t h e study of processes that influence the preser-
a c c r e t i o n , the fossil is evidence of the death of an individual. C o n -
vation of potential fossils. T h i s field e n c o m p a s s e s the disciplines
versely, with trilobites, the presence of body parts is not a simple
of
indicator o f population density.
biostratinomy,
the
study
of
processes
affecting
organism
remains or traces prior to their final burial, and fossil diagenesis, the investigation of p h e n o m e n a affecting potential fossils after burial. Recently, t a p h o n o m y has developed b o t h as a m e a n s of assessing bias in the fossil record and m o r e positively, as a critical tool for p a l e o e n v i r o n m e n t a l analysis. T a p h o n o m i c analyses
Death, Decay, and Disarticulation Death
and fossil g e o c h e m i s t r y provide valuable i n f o r m a t i o n on the
T h e t a p h o n o m i c history o f most organisms generally begins
physical and chemical p a r a m e t e r s o f e n v i r o n m e n t s . T h e a s s u m p -
with their death, although in a r t h r o p o d s , including trilobites,
tions of u n i f o r m i t a r i a n i s m are applicable at this level because the
molted exuviae also b e c o m e a part of the preserved record. Death
physical and c h e m i c a l properties o f the skeletons o f o r g a n i s m s
of o r g a n i s m s m a y involve gradual n o r m a l mortality. However,
have probably been invariant t h r o u g h geologic t i m e , despite the
m a n y of the spectacular Lagerstatten occurrences involve mass
n o n u n i f o r m i t y i m p a r t e d b y evolution. T h e t a p h o n o m i c features
m o r t a l i t y of m a n y individuals and species due to environmental
of trilobites (e.g., articulation of delicate skeletal e l e m e n t s ) m a y
crises (Brett and Seilacher 1 9 9 1 ) . T h e s e crises may include s t o r m
provide u n a m b i g u o u s evidence for episodic s e d i m e n t a t i o n ; c o n -
and seismic s h o c k events, volcanic eruptions such as those at
versely, highly c o r r o d e d fossil material provides a distinctive sig-
Pompeii, overturn of the water c o l u m n , and anoxia. However,
nature o f l o n g - t e r m c o n d e n s a t i o n .
only the m o r t a l i t y events associated with episodes of burial will
Articulated s p e c i m e n s o f trilobites, while generally u n c o m m o n , may be a b u n d a n t in s o m e beds. Moreover, the totally m i n -
be recorded as such. M o s t bodies decay rapidly after death, with a resultant loss of soft parts.
eralized body parts of trilobites c o m m o n l y have been preserved
Scavengers of all sorts are also part of the normal fauna, and
nearly u n c h a n g e d for at least 5 2 0 million years. However, preser-
a dead animal can be expected to b e c o m e a food source for a wide
vational features of fossil s p e c i m e n s can be used to decipher the
variety of o t h e r animals, which in Paleozoic seas included other
t a p h o n o m i c history of these s p e c i m e n s and provide useful clues
trilobites. T h e result of this is that a dead trilobite exposed on the
as to original e n v i r o n m e n t .
seafloor can be expected to b e c o m e disarticulated and the body
T h e structural c o m p l e x i t y of the trilobite exoskeleton and the
parts scattered within a relatively short period of time. C o n s e -
process of m o l t i n g of the exoskeleton d u r i n g growth increase the
quently, very shortly after death the animal must be buried at least
n u m b e r of fossil parts that can be generated by a single individ-
deeply e n o u g h
ual. Trilobite skeletal parts can be f o u n d in a m a j o r i t y of fossilif-
oxygen inhibits scavenging and also favors the preservation of
erous localities in New York, and well-preserved cephala and
intact skeletons.
pygidia are generally identifiable to species. W i t h most o r g a n i s m s , like bivalves or gastropods in which the skeletal growth is by
32
to physically inhibit disarticulation.
Lowered
DEATH,
DECAY,
AND
Soft Tissue Decay
m o n t h s . As such, skeletons m a y be completely disarticulated
T h e most destructive process to affect the bodies of o r g a n i s m s is the decay of soft parts. T h e most volatile parts are tissues, such as internal organs and muscle, and as a result such tissues are very rarely encountered as fossils. W h e r e they are preserved, it is usually (but not always: Butterfield 1 9 9 0 ) the result of early diagenetic mineralization (Allison 1 9 8 8 b ) . Anoxia historically has been viewed as a prerequisite for the preservation of such tissues (e.g., W h i t t i n g t o n 1 9 7 1 b ) . However, the removal of e n v i r o n m e n t a l oxygen does not prevent microbial activity ( B e r n e r 1 9 8 1 b ; Allison 1988a; Allison and Briggs 1 9 9 1 a ) . Microbes simply utilize a variety of alternative oxidants for carbon degradation. In fact, it has been suggested that anoxia may retard the decay process by only a factor of two or three (Canfield and Raiswell 1 9 9 1 a ) . M o r e importantly, anoxia prevents scavenging and favors early mineralization. T h e microbial reactions that are involved in anaerobic decay also generate a series of reactive e l e m e n t s , which in s o m e c i r c u m s t a n c e s can go on to produce early diagenetic minerals. T h e s e minerals, in turn, may preserve the decaying tissues themselves. T h e minerals most frequently associated with soft-part preservation are pyrite (Allison
33
DISARTICULATION
1 9 8 8 a ) , phosphate (Allison
1 9 8 8 b ; Briggs and Kear
1 9 9 3 ) , and carbonates. T h e activity of anaerobic bacteria is a necessity for the f o r m a t i o n of all three. F o r m a t i o n of organic-clay complexes or clay coatings may also be i m p o r t a n t in soft-part preservation (Butterfield 1 9 9 0 ) . Most well-preserved b e n t h i c fossils are preserved approximately at their life sites, so b o t t o m - w a t e r anoxia can be ruled out as a preservational factor. Allison's ( 1 9 9 0 ) calculations demonstrate that most o r g a n i s m bodies b e c o m e anoxic m i c r o e n v i r o n m e n t s internally during early phases of decay. I n h i b i t i o n of scavenging in these e n v i r o n m e n t s may p r o l o n g the association of skeletal elements. 1 lowever, even under conditions of a n a e r o b i o sis or anoxia, bacterial decay of ligaments is rapid and the slightest currents will serve to disarray pieces (Allison 1988a, 1 9 9 0 ) . Not surprisingly, most cases of soft-part preservation a m o n g trilobites are associated with dysoxic m u d r o c k facies, typically dark, slightly organic-rich shales. T h e Burgess Shale ( M i d d l e C a m b r i a n , British C o l u m b i a ) , Frankfort Shale (Beecher's Trilobite Bed, Upper O r d o v i c i a n , New Y o r k ) , and the Hunsriick Shale (Lower Devonian, G e r m a n y ) are a few of the m o s t notable e x a m ples (Allison and Briggs 1991a, 1 9 9 1 b , 1 9 9 3 ; see b e l o w ) .
within a period of a few m o n t h s or less ( P l o t n i c k 1986, Allison 1988a). A variety of articulated trilobite remains are f o u n d . Speyer and Brett ( 1 9 8 6 ) recognized three basic categories: ( 1 ) partially articulated exoskeletons, ( 2 ) m o l t remains or exuviae, and (3) c o m pletely articulated skeletons representing carcasses (Figure 3 . 1 ) . Partially
articulated
remains,
such
as
groups
of
thoracic
segments, are of i n d e t e r m i n a t e origin and may represent either partially decayed remains of carcasses or molt parts. Exuviae in m a n y trilobites are recognizable by the absence of specific structures. For m o s t trilobites m o l t i n g involves the shedding of free cheeks. T h e r e f o r e , articulated exoskeletons with cranidia lacking free cheeks are almost certainly exuviae. P h a copids had fused facial sutures and shed the entire cephalic shield in m o l t i n g . T h u s , articulated thoracopygidia ("headless trilob i t e s " ) suggest m o l t parts. Molt e n s e m b l e s are groups of closely associated m o l t parts, such as free cheeks lying in close p r o x i m ity to articulated r e m a i n s with cranidia, or cephalic shields closely associated with thoracopygidia in p h a c o p i d s . Carcasses are represented by completely articulated exoskelet o n s , with the cephala intact a n d articulated (i.e., free cheeks are i n t a c t ) . T h i s category m a y be subdivided f u r t h e r into o u t stretched or p r o n e s p e c i m e n s and in s o m e trilobite species, partially enrolled and fully enrolled individuals. O r g a n i s m s with m u l t i e l e m e n t skeletons, including trilobites, are particularly sensitive indicators of rapid
and p e r m a n e n t
burial. Well-preserved, articulated trilobites typically o c c u r on certain bedding planes within
m u d r o c k s that would not be
recognizable as event-beds by other sedimentological m e a n s . Because such skeletons c a n n o t b e reworked, the o c c u r r e n c e o f even a single intact s p e c i m e n of a trilobite is an excellent indicator that the enclosing sediment a c c u m u l a t e d rapidly and was not subsequently disturbed. T h e o c c u r r e n c e o f large n u m b e r s o f completely
articulated
trilobites
provides
dramatic
evidence
for a p o p u l a t i o n of o r g a n i s m s that was abruptly wiped out. Conversely, the o c c u r r e n c e of well-preserved molt ensembles need not imply any m o r t a l i t y but rather indicates burial under relatively low-energy c o n d i t i o n s
that
prevented
scattering of
parts. Certain widely traceable levels in the Middle Devonian H a m i l t o n G r o u p are characterized by a b u n d a n t articulated molt ensembles but few, if any, c o m p l e t e trilobites (that would represent carcasses). Such findings may reflect rapid a c c u m u l a t i o n of thin layers of s e d i m e n t that did not kill or s m o t h e r living trilo-
Articulated Remains
bites but were sufficient to preserve molts (Speyer 1 9 8 7 ) . M a n y assemblages of well-preserved trilobites are also d e m o n -
Skeletons of organisms in which the skeleton is c o m p o s e d of
strably in situ ( b u r i e d in their living sites). A particularly sensi-
multiple elements weakly b o u n d together by ligaments or m u s -
tive indicator is the o c c u r r e n c e of trilobite molt ensembles, that
culature, such as trilobites, are o n l y rarely preserved intact.
is, associated, disarticulated m o l t parts. It is virtually impossible
M o d e r n experimental studies indicate that the degradation of
for different disarticulated p o r t i o n s of the skeleton to be trans-
soft tissues in arthropods occurs within a period of a few hours
ported any distance and still remain associated. T h e hydrody-
after death, while destruction of ligaments ensues in weeks to
n a m i c properties of whole exuviae versus free cheeks would
FIGURE 3 . 1 . Fossil a s s e m b l a g e s reflecting various c o n d i t i o n s a n d timing of burial. A. Low rates of s e d i mentation a n d slow burial a l o n g with reworking of the prefossilized hard parts. Sclerites are broken a n d scatt e r e d . B. Slow burial with disarticulation a n d m i x i n g . C. O b r u t i o n : burial, under a thin blanket of sediment, with s o m e disarticulation a n d infaunal s c a v e n g i n g . D. O b r u t i o n : s u d d e n burial under a thick blanket of s e d iment, resulting in g o o d preservation of articulated fossils a n d r e d u c e d infaunal s c a v e n g i n g . A - D from Brett a n d Baird (1993). E. Trilobites that exhibit s o m e d e g r e e of disarticulation b e c a u s e they were not b u r i e d d e e p l y e n o u g h or soon e n o u g h after mortality, PRI 4 9 6 6 1 . F. A g r o u p of w e l l - p r e s e r v e d trilobites as a result of r a p i d , relatively d e e p burial, PRI 4 9 6 6 2 . G. A trilobite that is tightly c o i l e d a n d well p r e s e r v e d , PRI 4 9 6 6 3 . Trilobites c o i l e d under stress, p r o b a b l y a s s o c i a t e d with the burial event.
TRANSPORT
AND
35
REORIENTATION
be so different that it is extremely unlikely they could be trans-
downward
ported and yet end up together. Molt ensembles thus constitute
that m a y be interpreted as indicators of storm-generated scour
p r o o f of life activity (i.e., m o l t i n g ) by trilobites in the precise site
and fill.
o f burial. O t h e r dramatic examples of in situ trilobites are the very rare
or lateral-oblique
positions
along
linear
features
M a n y skeletal e l e m e n t s , including articulated trilobites, as well as their cranidia and pygidia, have approximately concavo-
specimens of articulated skeletons directly associated with their
convex dish-shapes. R a n d o m o r preferred c o n v e x - u p o r convex-
trace fossils, such as famous s p e c i m e n s of Flexicalymene attached
down
to Rusophycus (see Figure 2 . 1 4 B ) . T h e o c c u r r e n c e of beds of en-
assemblages,
rolled trilobites also suggests a behavioral response to stressed
c o n d i t i o n s . Instances o f r a n d o m o r nearly o n e - t o - o n e ratios o f
orientations and
may each
be has
observed distinct
in
different
implications
trilobite
for
burial
conditions in which trilobites may have b u r r o w e d into the sedi-
c o n v e x - u p and - d o w n o r i e n t a t i o n s o c c u r primarily in heavily
m e n t and enrolled themselves, only to be buried in place and
b i o t u r b a t e d sediments, w h e r e , in addition, skeletal elements may
apparently unable to escape from m u d blanketing.
display lateral or edgewise o r i e n t a t i o n s . T h e m i x i n g of skeletal
Speyer and Brett ( 1 9 8 5 ) also recognized species-segregated
c o m p o n e n t s into sediment m a y a c c o u n t for the randomizing
clusters of fully articulated skeletons ( " b o d y clusters"; Plates 5 9 ,
of o r i e n t a t i o n . However, such o r i e n t a t i o n s are unstable under
102, 147, and 152) and m o l t e n s e m b l e s ( " m o l t clusters") involv-
hydrodynamic currents and h e n c e , are probably o n e of the better
ing at least three species of trilobites on single bedding planes in
indicators o f b i o t u r b a t i o n o r s o m e type o f protective c o n c e n t r a -
the Devonian Hamilton G r o u p . T h e s e trilobite clusters appear
tion trap on the substrate.
to represent preserved behavioral patterns. Using the a r g u m e n t
C o n c a v o - c o n v e x trilobite skeletal e l e m e n t s and whole bodies,
that the molt ensembles could not have been t r a n s p o r t e d any dis-
like
tance, Speyer and Brett inferred that these represented species-
c o n v e x - d o w n o r i e n t a t i o n s . For e x a m p l e , cephala and pygidial
segregated aggregations of trilobites that molted en mass. Speyer
shields in a preferred c o n v e x - u p position are p r o b a b l y most
and Brett further pointed out that synchronized m o l t i n g typically
typical
serves as a prelude to m a t i n g in m o d e r n peracarid crustaceans
Hesselbo 1987, Lask 1 9 9 3 ) showed that even gentle currents will
(e.g., marine isopods) and that several species also m a y t i m e their
affect trilobite r e m a i n s resting on the seafloor in such a way that
m a n y shells, c o m m o n l y display preferred c o n v e x - u p or
of c o n c e n t r a t e d
trilobite
beds.
Flume
studies
(e.g.,
reproductive cycles to a c o m m o n external stimuli, such as lunar
they flip to a hydrodynamically stable o r i e n t a t i o n , at which point
cycles. This analog suggests that trilobite clusters may represent
currents will glide over their s t r e a m l i n e d , c o n v e x - u p surfaces.
preserved mating "orgies."
T h e effect is particularly evident in areas of m u d d y substrate because o f the i m p e d a n c e (i.e., frictional d r a g ) .
Transport and Reorientation Trilobite skeletons can be sensitive indicators of hydrody-
Hence,
the
occurrence
of
abundant
convex-up
trilobite
cephala or pygidia on bedding planes m a y provide evidence for reworking of skeletal material under slightly current-agitated
namic conditions in the depositional e n v i r o n m e n t . U n d e r low-
conditions.
energy e n v i r o n m e n t s , typical of m u d r o c k s , o r g a n i s m remains
was processed by o n e or m o r e s t o r m - g e n e r a t e d current events.
may be buried in situ. O n e might use the a r g u m e n t that well-
Excellent examples are f o u n d in large assemblages of Flexicaly-
Beds of this sort represent skeletal material that
articulated fossils such as trilobites must not have been trans-
mene and Isotelus pygidial
and cephalic
ported and that their o c c u r r e n c e therefore indicates quiet water
o f h u m m o c k y laminated
calcisiltite beds, and
environments, but this inference must be m a d e cautiously. Aside
Pseudogygites
from obvious cases where this is not so (e.g., in which these skele-
in prep.).
from
the
Ordovician
shields on
the bases
pavements o f
Collingwood
Shale
(Brett,
tons are found at the bases of turbidites or even within skeletal
Beds of preferentially c o n c a v e - u p w a r d trilobite t a g m a are not
grainstone deposits), there is experimental evidence that if o r g a n -
as c o m m o n . In settings where the m o l t parts are suspended
isms are transported within the first few hours following death,
briefly and allowed to resettle from suspension, they will almost
their remains may stay articulated. Allison ( 1 9 8 6 ) d e m o n s t r a t e d
invariably settle c o n c a v e - u p w a r d (Lask 1 9 9 3 ) . Such reorienta-
in tumbling barrel e x p e r i m e n t s that a r t h r o p o d s such as s h r i m p ,
tion would o c c u r in areas very close to s t o r m wave-base in which
which provide possible analogs to trilobites, m a y be potentially
the
moved even tens of kilometers without disarticulating, provided
materials t e m p o r a r i l y o f f the b o t t o m and allow t h e m to free-fall
that this happens within the first few hours following their
b a c k to the substrate. Rapid burial following this stirring would
demise.
i n c o r p o r a t e such shells as a basal pavement of a possibly graded
Trilobite fossils also may provide evidence for reworking or
rather
gentle
storm-generated
waves
would
lift skeletal
m u d s t o n e layer. Excellent e x a m p l e s of this m o d e of burial are
transport of skeletons that would otherwise remain unsuspected.
seen
in
pavements of Triarthrus cranidia
in
Ordovician dark
It is not unusual in m a r i n e m u d r o c k s to observe c o n c e n t r a t i o n s
shales. C o u n t s of m a n y bedding planes from the Ordovician
of shells in localized pockets, typically associated with evidence
C o l l i n g w o o d Shale of O n t a r i o show a p r e d o m i n a n c e of concave-
for current scour. Such a c c u m u l a t i o n s c o m m o n l y display c o n v e x -
upward o r i e n t a t i o n s of the cranidia and of tiny ostracode valves,
36
TAPHONOMY
even on bedding planes in which larger skeletons are r a n d o m
slightly, by currents after death. As noted already, however, c u r -
o r c o n v e x - u p w a r d ( C . Brett, unpublished o b s e r v a t i o n , 1 9 9 8 ) . W e
rents usually have the effect of flipping concavo-convex elements
suggest that under certain c o n d i t i o n s only the smallest, light-
to a preferred convex-upward position. If trilobite remains were
weight skeletons were suspended and then resettled—typically
lifted into a current slightly and then resettled, they might still
concave-upward.
assume this position. Also, the generation of decay gasses within
Stacks of nested or shingled fossils apparently o c c u r where
the b o d y cavities of trilobites might give t h e m slight buoyancy
densely packed skeletal r e m a i n s were affected by oscillatory,
and make t h e m m o r e subject to the lifting and settling required
s t o r m - g e n e r a t e d waves or currents, and provide an indication
to invert the carcasses. All such inferences seem to point to a b r i e f
of deposition well within s t o r m wave-base. A similar m o d e of
interlude between m o r t a l i t y and final burial.
involves c o n c a v o - c o n v e x
A few o c c u r r e n c e s of p r e d o m i n a n t l y convex-upward out-
shells bundled together in nested groupings, either c o n v e x -
preservation
in
shell
accumulations
stretched trilobites are also k n o w n ; for example, groupings of
upward or -downward and often b o t h in the s a m e layers. T h e s e ,
Eldredgeops milleri f r o m the Silica Shale typically show this o r i -
too, seem to reflect the effects of settling and c o n c e n t r a t i o n of
e n t a t i o n . T h e s e are c o m m o n l y associated with perfectly enrolled
skeletal remains during t u r b u l e n c e events, and they are c o m -
trilobites and they m a y represent examples of m o r e nearly instan-
m o n l y associated with m i n o r s e d i m e n t grading. An intriguing
t a n e o u s burial with little disturbance.
example consists of masses of nested cephalic and pygidial shields
Speyer ( 1 9 8 7 ) and B r a n d t ( 1 9 8 5 ) also considered the possi-
of bumastid trilobites from Silurian b i o h e r m s , such as those in
bility of preferential orientation of enrolled trilobites but c a m e
the Silurian I r o n d e q u o i t L i m e s t o n e . T h e s e m a y represent molt
to different c o n c l u s i o n s .
parts that were c o n c e n t r a t e d in sheltered " p o c k e t s " or scours on
o r i e n t a t i o n s in s o m e mass occurrences of enrolled Flexicalymene
b i o h e r m surfaces.
meeki f r o m Upper Ordovician shales of the C i n c i n n a t i , O h i o ,
Brandt
reported essentially r a n d o m
A particularly intriguing a n d still e n i g m a t i c aspect of o r i e n t a -
region. In contrast, Speyer recognized preferred and species-
tion in trilobites is the p r e d o m i n a n t l y c o n c a v e - u p w a r d (dorsal
specific o r i e n t a t i o n s in Devonian enrolled trilobites from New
shield d o w n )
York.
o r i e n t a t i o n o f articulated trilobites. M a n y o c c -
Greenops
s p e c i m e n s were
commonly found
a
cluding
the
showed cephalon-lateral or -downward positions. Speyer sug-
Ordovician Trenton L i m e s t o n e of New York (Brett et al. 1 9 9 9 ) ,
gested that these findings recorded different modes of pre-enroll-
beds
m e n t behavior by the trilobites.
occurrences
of Dalmanites
(Taylor and
Brett
limulurus
of Ceraurus pleurexanthemus in
the
Silurian
from
Rochester
Shale
Eldredgeops rana
in
cephalon-up
mass
position, while
most
urrences of clusters of trilobites display this p h e n o m e n o n , in-
more commonly
1 9 9 6 ) , and aggregations of Eldredgeops rana
T h e preferred azimuthal ( c o m p a s s bearing) orientation o f
from the f a m o u s trilobite beds of the Lake Erie region (Speyer
elongate skeletons has been the subject of n u m e r o u s studies,
and Brett 1 9 8 5 ) . Several explanations for such o r i e n t a t i o n s have
including
been put forth. R a y m o n d ( 1 9 2 0 a ) believed that these represented
Kidwell a n d B o s e n c e 1 9 9 1 , for s u m m a r y ) . From such studies, it
both
observational
and
experimental
studies
(see
examples of trilobites buried in a life position, postulating that
has b e c o m e c o m m o n knowledge that elongate particles typically
these animals swam upside down, like horseshoe crabs, and per-
o r i e n t themselves selectively within a current. Elongate objects
ished in this position. However, m o s t later a u t h o r s have c o n s i d -
that do not roll, such as the trilobite carcasses m e n t i o n e d earlier,
ered this unlikely and argued for a p o s t m o r t e m reorientation of
c o m m o n l y will be aligned parallel to the direction of the current.
carcasses. O n c e again, if the c o n c a v o - c o n v e x carcasses of trilo-
Such may be the case with consistently aligned specimens of C.
bites were suspended and resettled freely, they would very likely
pleurexanthemus reported by Brett et al. ( 1 9 9 9 ) . T h e o c c u r r e n c e
assume this position. Several features associated with the dorsal-
of s o m e aligned trilobites in elongated "windrows," such as in the
downward trilobites m a y bear on this issue. F o r e x a m p l e , Speyer
Silurian Rochester Shale, may represent accumulation in very
and Brett ( 1 9 8 5 ) n o t e d that s o m e o f the " i n v e r t e d " trilobites were
m i n o r scours o r gutters (Whiteley and Smith 2 0 0 1 ) .
headless and thus, likely molted thoracopygidia. Such s p e c i m e n s obviously were not " s w i m m i n g , " and yet they are f o u n d alongside c o m p l e t e specimens that are also p r e d o m i n a n t l y c o n v e x - d o w n -
Fragmentation and Biased Preservation
ward. Even a m o n g these latter s p e c i m e n s there m a y be signs of
Trilobite skeletal elements are variably affected by biotic and
incipient disarticulation (e.g., c e p h a l o n or pygidium is rotated
abiotic destructive agents p o s t m o r t e m . T h e degree to which the
slightly away f r o m thorax; t h o r a c i c s e g m e n t s are very slightly
skeletons are affected is a function not only of the delicacy of their
pulled a p a r t ) . Such observations suggest that the trilobites repre-
original c o n s t r u c t i o n but also of residence t i m e on the seafloor.
sent carcasses that had u n d e r g o n e a very slight a m o u n t of decay
O n c e e l e m e n t s are disarticulated, they may be acted on by
prior to burial. In all of the a b o v e - m e n t i o n e d cases, there is also
various hydrodynamic processes that serve to sort, fragment,
a vaguely to strongly preferred long-axis o r i e n t a t i o n (e.g., see
abrade, or c o r r o d e t h e m . H y d r o d y n a m i c size sorting is u n c o m -
Brett et al. 1 9 9 9 ) . T h i s would also seem to imply that the trilo-
m o n in m o s t m u d r o c k s , for which generally low-background
bites reflect carcasses or m o l t s that were t r a n s p o r t e d , if only
e n v i r o n m e n t a l energies are characteristic. However, size sorting
FOSSIL
37
DIAGENESIS
may o c c u r on the scale of a single graded bed as a result of tur-
exposure t i m e , as well. Conversely, s o m e shells in condensed
bulent events that briefly suspend skeletal particles and sediment
deposits show few, if any, epibionts. M i c r o b o r i n g s of forms such
and allow them to resettle.
as endolithic algae may be recognized and may provide particu-
Biased ratios of skeletal parts, for which original p r o p o r t i o n s
larly useful indicators of exposure a n d , in s o m e cases, of relative
are known to have been o n e - t o - o n e , m a y provide evidence of
depth ( G o l u b i c et al. 1 9 7 5 ; Vogel et al. 1 9 8 7 ) . Endolithic algae of
sorting or, m o r e frequently in the case of m u d r o c k s , of preferen-
various sorts, for e x a m p l e , are c o n f i n e d to various p o r t i o n s of the
tial destruction. For e x a m p l e , it is a c o m m o n observation that
e u p h o t i c to upper dysphotic zone. Algal m i c r o b o r i n g s in Dicra-
certain portions of trilobite skeletons are preferentially preserved,
nurus s p e c i m e n s have been used to suggest that these trilobites
whereas the others tend to be f r a g m e n t e d . T h i s results in a selec-
lived in the e u p h o t i c zone. Kloc ( 1 9 9 7 ) suggested that abundant
tive bias that is not related to size but rather to relative robust-
encrusters on the cephalic spines of these trilobites may have
ness of the skeletal parts. For e x a m p l e , the rather robust pygidia
settled during the life of the trilobite and served as camouflage
of dalmanitids and asaphids are m o r e c o m m o n l y preserved intact
against visual predators.
than are cephala. Pygidia-cephala ratios up to t e n - t o - o n e were found in s o m e spectacular bedding plane pavements of Pseudogygites latimarginatus from the C o l l i n g w o o d Shale in O n t a r i o ( C . Brett, unpublished d a t a ) . Conversely, for the small trilobite Tri-
Fossil Diagenesis: Geochemical Processing of Potential Fossils
arthrus eatoni, from the same beds the small, fragile pygidia are
Early diagenetic p h e n o m e n a c o m p r i s e the physicochemical
very rare, and s o m e bedding planes show n o t h i n g but cranidia
processes that act on o r g a n i s m r e m a i n s primarily after burial.
("head shales"). Beds with approximately o n e - t o - o n e ratios of
Diagenetic features of fossils m a y provide i n f o r m a t i o n regarding
cephala and pygidia may indicate rather rapid and intact burial,
the g e o c h e m i s t r y o f b o t t o m waters and the upper " t a p h o n o m i -
whereas those that show a strongly biased ratio probably repre-
cally active z o n e " ( T A Z ) of the sediment c o l u m n . Diagenetic fea-
sent time-averaged a c c u m u l a t i o n s in which rather long exposure
tures of note include evidence for early dissolution, c o m p a c t i o n ,
times on the sea b o t t o m prevailed. D u r i n g these intervals, m i n o r
and mineralization of fossils.
physical disturbances, such as multiple s t o r m s or even biotic disturbances, fragmented the less rigid skeletal parts preferentially.
T h e relative t i m i n g of dissolution is c o m m o n l y recorded in skeletons. Trilobite exoskeletons were i m p r e g n a t e d with calcite and thus are relatively resistant to dissolution and are preservable;
Abrasion, Corrosion, and Encrustation
however, they still
may show evidence of early dissolution.
Trilobite skeletons that are dissolved prior to c o m p a c t i o n may
It is often difficult to distinguish between trilobite skeletons
leave no record but, in m a n y cases, m a y be preserved as plasti-
that have been physically abraded and those that have been
cally d e f o r m e d molds (Seilacher et al. 1 9 8 5 ) . Such preservation
corroded by biogeochemical processes. As a generalization most
would indicate u n d e r s a t u r a t i o n with respect to calcite in the
trilobite remains are too fragile to withstand p r o l o n g e d abrasion,
upper s e d i m e n t s and possibly low pH c o n d i t i o n s . Conversely,
and this fragility may a c c o u n t for the rarity of trilobite skeletons
many f o s s i l skeletons and their molds display mosaic fracture
in s o m e nearshore, sandy e n v i r o n m e n t s in which trace fossils indi-
patterns on their surfaces; these are particularly p r o m i n e n t on
cate that trilobites were c o m m o n . It is well k n o w n that clay-sized
large shields such as the cephala and pygidia of Isotelus species.
sediment is ineffective as an abrasive agent. H e n c e , truly abraded
Such skeletons r e m a i n e d hard in early phases of c o m p a c t i o n ,
fossils are rare in m u d r o c k s and, if f o u n d , might indicate a m u c h
which caused brittle fracture. (In rare instances, both plastically
m o r e c o m p l e x history to the deposit in which shells were trans-
d e f o r m e d " g h o s t e d " s p e c i m e n s and brittly fractured (or unfrac-
ported into a quieter water e n v i r o n m e n t by a turbulent event.
tured)
well-calcified
specimens
occur
on
the s a m e bedding
C o r r o d e d trilobite remains tend to o c c u r in offshore, low-
planes. Such associations have been used to suggest the presence
energy e n v i r o n m e n t s , and b i o e r o s i o n , likewise, tends to p r e d o m -
o f " s o f t - s h e l l e d " ( i m m e d i a t e l y p o s t m o l t ) and intermolt individ-
inate over physical abrasion in these offshore settings (Kidwell
uals (Speyer 1 9 8 7 ) . )
and Bosence 1 9 9 1 ; Parsons and Brett 1 9 9 1 ) . M a n y apparently
Early diagenetic minerals such as siderite, calcite, and pyrite
abraded shells in m u d r o c k s probably have been chemically etched
generally f o r m as a result of the action of anaerobic bacteria
or acted on by m i c r o b o r i n g o r g a n i s m s .
and are partly c o m p o s e d of their respiratory b y - p r o d u c t s (Allison
Even in life, trilobite exoskeletons may b e c o m e encrusted with epibiontic organisms, such as b r y o z o a n s and even b r a c h i o p o d s
1 9 8 8 a ) . T h e y m a y provide valuable i n f o r m a t i o n o n sediment g e o c h e m i s t r y and rates of burial.
(Tetreault 1992; Kloc 1993; Taylor and Brett 1 9 9 6 ) . P o s t m o r t e m
O n e o f the best u n d e r s t o o d o f the early diagenetic minerals
trilobite remains may be encrusted both externally and internally.
is pyrite, iron disulfide ( B e r n e r 1981a; Canfield and Raiswell
Internal encrustation provides an excellent indication that skele-
1 9 9 1 a , 1 9 9 1 b ) . Pyrite is c o m m o n in m a n y m a r i n e m u d r o c k s and
tons have lain disarticulated for a period of t i m e on the sea
is c o m m o n l y associated with fossil trilobite remains. T h e ferrous
b o t t o m . T h e extent o f encrustation may provide a n indicator o f
iron required in pyrite f o r m a t i o n is available in terrigenous
38
TAPHONOMY
sediments, and dissolved sulfate is a b u n d a n t in m a r i n e water (but
as a
not in fresh water). Under aerobic decay of organic matter, the
the m o s t c o m m o n trilobite in these beds, and s o m e specimens
m a j o r respiratory b y - p r o d u c t s o f o r g a n i s m s are water ( H 0 ) and
are partially pyritized and have a solid pyritic core and s o m e
c a r b o n dioxide ( C O , ) .
However, under a n a e r o b i c c o n d i t i o n s
coiled s p e c i m e n s form the nucleus of a pyrite nodule. These beds
particular types of bacteria, referred to as sulfate-reducing bacte-
appear to represent the rapid burial of organism bodies in an o t h -
ria, use sulfate ( S 0 ~ ~ ) as an oxygen d o n o r for metabolizing
erwise l o w - o r g a n i c sediment. T h e high c o n c e n t r a t i o n of pyritic
:
4
nucleus for over-pyrite precipitation.
Greenops grabaui is
organic matter, and p r o d u c e hydrogen sulfide ( H , S ) and bicar-
material a r o u n d burrows and fossils indicates nucleation around
b o n a t e ( H C 0 " ) as b y - p r o d u c t s . Iron r e d u c t i o n , mediated by a
local centers of a n a e r o b i c decay associated with buried organic
second group of a n a e r o b i c bacteria, can generate ferrous ions,
matter.
3
which in turn m a y react with the H , S generated by sulfate reduc-
A n a e r o b i c decay processes, including sulfate reduction, also
tion, to produce the precursors of pyrite (Canfield and Raiswell
generate H C O , " ( b i c a r b o n a t e ) , which may initiate the precipita-
1991 b ) . T h e presence of pyrite shows that the s e d i m e n t was anoxic
tion of calcite or siderite c o n c r e t i o n s around decaying organic
but does not necessarily d e m o n s t r a t e that the overlying water
matter. W h e r e trilobites or other fossils are enclosed within car-
c o l u m n was anoxic. Pyrite can f o r m either very early or relatively
b o n a t e c o n c r e t i o n s , they are nearly always t h r e e - d i m e n s i o n a l .
late in the burial history of a sediment ( H u d s o n 1 9 8 2 ; Brett and
T h i s proves that the c o n c r e t i o n s f o r m e d early and before organ-
Baird 1 9 8 6 ; Allison 1 9 8 8 a , 1 9 8 8 b ; Canfield a n d Raiswell 1 9 9 1 a ) .
ism remains could be c o m p a c t e d by overburden pressure.
Under c o n d i t i o n s o f high o r g a n i c - m a t t e r p r o d u c t i o n , anoxic
It is less c o m m o n for phosphatic nodules to form because
c o n d i t i o n s may e x t e n d into the water c o l u m n ; in such euxinic set-
p h o s p h o r u s is only present in very small quantities in seawater.
tings H , S is in excess, and any iron that is i n t r o d u c e d into the
However, the a n a e r o b i c decay of organic matter does liberate
system is pyritized as it is deposited. T h u s , pyrite tends to be
phosphate-bearing
distributed evenly in the s e d i m e n t as disseminated tiny crystal
can b e c o m e a d s o r b e d to ferrous hydroxides in the sediment.
aggregates called framboids; it is n o t c o n c e n t r a t e d a r o u n d any
As these ferrous hydroxides are buried and pass through the
o r g a n i s m remains that m a y settle into these settings. U n d e r oxic
anoxic-oxic
b o t t o m - w a t e r c o n d i t i o n s , however, o r g a n i c material is not dis-
liberates phosphates to pore-water. Dissolved phosphate m a y
tributed so u n i f o r m l y because m u c h of it is degraded aerobically
be released b a c k to the water c o l u m n if anoxia persists to the
at or near the s e d i m e n t - w a t e r interface, a n d the sediment b e -
s e d i m e n t - w a t e r interface. However, if a micro-oxidized zone
c o m e s anoxic b u t nonsulfidic, in Berner's ( 1 9 8 1 a ) terminology.
exists in the upper s e d i m e n t , then the phosphates may be repre-
T h u s , pyritization tends t o o c c u r m o r e locally within the n o n -
cipitated, especially a r o u n d phosphatic skeletal nuclei, such as
sulfidic s e d i m e n t , particularly in the vicinity of anaerobically
the
decaying organic matter. As a result of local sulfate r e d u c t i o n ,
(Swirydczuk et al. 1 9 8 1 ; B e r n e r 1981a; Allison 1 9 8 8 b ) . Trilobites
sulfide is liberated a r o u n d this d e c o m p o s i n g organic material.
are not u n c o m m o n l y phosphatized. Phosphatization of trilobites
Dissolved iron will react at the site of sulfate r e d u c t i o n so that
occurs primarily under c o n d i t i o n s of slow sedimentation. T h u s ,
pyrite is restricted to a n o x i c o r g a n i c - r i c h m i c r o e n v i r o n m e n t s
for e x a m p l e , in the M i d d l e Devonian Hamilton G r o u p , p h o s -
within a broadly dysoxic, l o w - o r g a n i c setting. Well-preserved
phatized internal m o l d s of enrolled trilobites may o c c u r at m i n o r
pyritized fossils, including trilobites tend to o c c u r in b i o t u r b a t e d
disconformities.
compounds
boundary,
chitinophosphatic
they
are
material
to
solution.
reduced.
forming
Also,
This
phosphate
process
arthropod
also
skeletons
gray m u d s t o n e s and thus are indicators of b o t t o m - w a t e r oxy-
If the s e d i m e n t a t i o n rate is high, then the time spent by any
genation (Brett and Baird 1 9 8 6 ; Allison and Brett 1 9 9 5 ) . T h e ear-
particular s e d i m e n t layer at this micro-oxidized interface will be
liest phases of pyrite tend to be fine-grained fillings of cavities,
low, and the p h o s p h o r u s c o n c e n t r a t i o n in pore-water will be
such as the interiors of enrolled trilobites. Later generations of
increased only slightly. Conversely if the sedimentation rate is
larger crystalline " o v e r p y r i t e " may nucleate on existing pyritic
low, then the t i m e spent at the interface will be high. T h u s , a large
cores. In this way, pyritic nodules m a y f o r m (Figure 3 . 2 ) . T h e r e are a n u m b e r of M i d d l e D e v o n i a n fossil beds in New
p r o p o r t i o n o f the a d s o r b e d p h o s p h o r u s c o m p o u n d s will b e c o n centrated at o n e layer in the sediment. Such c o n c e n t r a t i o n can
York wherein large a m o u n t s of pyrite are f o u n d associated with
increase pore-water levels of p h o s p h o r u s so that phosphate m i n -
the fossils ( D i c k and Brett 1 9 8 6 ) . T h e best k n o w n of these is the
erals can precipitate. T h e s e minerals may replace organic remains
Alden Pyrite Beds in the Ledyard Shale. T h e Ledyard Shale is
o r f o r m c o n c r e t i o n s . T h u s , the o c c u r r e n c e o f phosphatic fossil
a dark-gray, generally poorly fossiliferous shale. However, a
molds or c o n c r e t i o n s is nearly always an indicator of low rates of
h o r i z o n in the lower Ledyard in western New York is rich in pyrite
sedimentation.
nodules and pyritized m o l d s o f fossils o f all sorts. M o s t c o m m o n
O t h e r f o r m s o f diagenetic modification o f trilobite material
are the fossils that had calcific or aragonititic shells in life, such
are u n c o m m o n to absent in New York. T h e r e are a few references
as pelecypods, b r a c h i o p o d s , nautiloids, a m m o n o i d s , and trilo-
to silica replacement of exoskeletal material in trilobite protaspids
bites. T h e pyrite nodules have fossils at their core, which indicates
b u t n o observations o f phosphate replacement o r carbonized
that the original pyrite, f o r m e d f r o m the organic decay, acted
specimens.
FIGURE 3.2. Conditions for the formation of pyritized, w e l l - p r e s e r v e d fossils. A. Fauna on a b o t t o m with poor oxygenation. There is low diversity, a n d the b o t t o m is anoxic a short d i s t a n c e b e n e a t h the s u r f a c e . The plus s i g n s (+) indicate anoxic conditions. B. R a p i d burial by s e d i m e n t a n d the r e d u c e d o x y g e n levels of the water raise the anoxic level to the newly b u r i e d o r g a n i s m s . A n o x i c bacteria in m e t a b o l i z i n g o r g a n i c matter of the trilobites also r e d u c e the sulfate in the pore water, resulting in sulfide ions that then react with r e d u c e d ( F e ) iron in the sediment. In a s e d i 2+
ment rich in terrigenous material, the iron level is high e n o u g h to c a u s e iron sulfide precipitation at or c l o s e to the d e c a y i n g organic material. C. Pyrite a c c u m u l a t e s at the source of the o r g a n i c decay, a n d the fossil is c o v e r e d with pyrite at this nucleation site. A - C from Speyer a n d Brett (1991). R e p r o d u c e d with p e r m i s s i o n . D, A trilobite b u r i e d in a rapid burial event but in an o x y g e n a t e d s e d i m e n t so that only a minor a m o u n t of pyrite f o r m e d . E. A coiled trilobite buried under anoxic, low-organic ( e x c e p t at the site of trilobite d e c a y ) , a n d iron-rich c o n d i t i o n s , resulting in a total c o v e r i n g of pyrite, PRI 4 9 6 6 4 .
40
TAPHONOMY M o r e c o m p l e t e i n f o r m a t i o n on the processes of fossil preser-
plete typically outstretched, inverted individuals may occur, even
vation can be f o u n d in collective volumes edited by Briggs and
in clusters. U n d e r lower sedimentation the event signature c o m -
C r o w t h e r ( 1 9 9 0 ) ; Allison and Briggs ( 1 9 9 1 a , 1 9 9 1 b ) ; D o n o v a n
prises primarily intact molt e n s e m b l e s , with few if any fully artic-
( 1 9 9 1 ) ; Einsele, Ricken, and Seilacher ( 1 9 9 1 ) ; a n d M a r t i n ( 1 9 9 9 ) .
ulated carcasses. Finally, under low-energy, dysoxic (background) c o n d i t i o n s , intact molt parts and tagma are the rule; the event signature of this setting is distinctive in showing an abundance
Trilobite Taphofacies
o f enrolled, c o m m o n l y pyritized, s p e c i m e n s o f trilobites.
Various aspects of fossil preservation can be c o m b i n e d into
Each taphofacies records different types of i n f o r m a t i o n . T h e
the recognition and description of t a p h o n o m i c facies or t a p h o -
distinctive m o d e s of preservation provide i n f o r m a t i o n on sedi-
facies (Speyer and Brett 1 9 8 6 , 1 9 9 1 ) . Together with lithofacies,
m e n t a r y e n v i r o n m e n t s that c a n n o t be determined otherwise.
biofacies, and ichnofacies, taphofacies tend to vary predictably
F o r e x a m p l e , the e n r o l l m e n t of trilobites may reflect a response
with sedimentary e n v i r o n m e n t s , as shown by studies in m o d e r n
to toxic stimuli triggered by a stirring up of anoxic, sulfide-rich
marine settings (Parsons and Brett 1 9 9 1 ) . T h e m o d e s o f preser-
m u d s . Similarly, pyritization suggests burial in anoxic, low-
vation of fossils can provide i m p o r t a n t insights into a n u m b e r of
organic m u d s .
features o f m u d r o c k deposition, including ( 1 ) the s e d i m e n t a r y
In this way, the study of trilobite preservation has helped to
e n v i r o n m e n t (depth, t e m p e r a t u r e , salinity, oxygen level, substrate
provide a new tool in paleoenvironmental analysis. Development
consistency);
of predictive models relating preservation to depositional envi-
(2)
the
dynamics
of
sediment
accumulation,
average rates, as well as evidence for episodicity of s e d i m e n t a t i o n
r o n m e n t s and positions in sedimentary sequences, in turn, may
and erosion;
aid
(3)
the temporal scope o f individual m u d r o c k
units; and ( 4 ) the s e d i m e n t g e o c h e m i s t r y and early diagenetic
paleontologists
in
prospecting
for
new fossil
bonanzas,
including spectacular trilobite beds.
environments. T h e overall c o n d i t i o n of trilobite skeletons can be assessed qualitatively or quantitatively but certainly should be n o t e d in the
Trilobite Lagerstatten
field. Semiquantitative indices can be f o r m u l a t e d by d e t e r m i n i n g
Lagerstatten (derived from the G e r m a n mining term trans-
the p r o p o r t i o n of skeletal parts in different, arbitrarily defined
lated loosely as " m o t h e r lodes") are extraordinary fossil assem-
preservational states. In such cases, it is c o m m o n l y useful to
blages. Trilobite Lagerstatten include o b r u t i o n deposits, reflecting
determine a set of standards with which particular shells can be
a rapid s m o t h e r i n g of benthic faunas by sediment, yielding fully
c o m p a r e d and assigned to a category, in m u c h the s a m e way that
articulated remains and Konservat-Lagerstatten, in which even
grain shapes and roundness indices have long been assessed on
soft parts are preserved by a c o m b i n a t i o n of rapid burial, anaer-
the basis of standardized profiles by sedimentologists. Skeletal
o b i c decay, and early diagenetic mineralization (Seilacher et al.
condition is particularly valuable for recognizing qualitatively
1 9 8 5 ) . In this section we describe examples of trilobite Lager-
the differing relative extents of s e d i m e n t a r y time-averaging and
statten f r o m New York State to illustrate the general t a p h o n o m i c
therefore, may be related to burial rates.
concepts.
In their original f o r m u l a t i o n of the n o t i o n of taphofacies,
A f a m o u s trilobite site is Beecher's Trilobite Bed in the Upper
Speyer and Brett ( 1 9 8 6 ) used M i d d l e Devonian trilobites and
Ordovician Frankfort F o r m a t i o n just north of R o m e , New York.
their modes of preservation to exemplify the general m o d e l . T h e y
A 5 - m m - t h i c k , light-gray layer within a dark-gray m u d s t o n e
also subsequently emphasized the fact that fossils in a particular
contains an unusual collection of fossil material (Cisne 1 9 7 3 ) .
taphofacies may show b o t h b a c k g r o u n d and event t a p h o n o m i c
The
signatures. That is, there may be distinct aspects of preservation
appendages and s o m e internal organs replaced by pyrite. B e e c h e r
Triarthrus
eatoni
specimens
within
this
layer
have
their
of fossils under day-to-day c o n d i t i o n s or under episodic cata-
( 1 8 9 3 , 1 8 9 4 , 1 8 9 5 ) reported the discovery by Valiant and the
strophic burial c o n d i t i o n s in a given e n v i r o n m e n t (Speyer and
preparation o f specimens. Cisne ( 1 9 7 5 , 1981) prepared very-
Brett 1 9 8 8 ) . T h e well-known D e v o n i a n trilobite species E. rana
high-resolution radiographs of Beecher's specimens and was able
and Greenops species o c c u r in m a n y distinct associations repre-
to report on internal structures never seen before. W h i t t i n g t o n
senting different e n v i r o n m e n t s . Yet, these trilobites show very dis-
and A l m o n d ( 1 9 8 7 ) also e x a m i n e d specimens from the beds and
tinctive m o d e s o f c o m m o n
suggested that certain structural elements of the appendages were
preservation that are u n i q u e t o
particular o n s h o r e - o f f s h o r e positions and s e d i m e n t a t i o n c o n d i -
utilized in food t r a n s p o r t , a m o n g other things. Replacement of
tions. For example, in high-energy shallow water, m o s t trilobite
organic tissue in such high resolution by pyrite is very unusual.
remains are disarticulated, abraded f r a g m e n t s , although articu-
Briggs, Bottrell, and Raiswell ( 1 9 9 1 ) e x a m i n e d these trilobites and
lated, typically outstretched individuals may o c c u r occasionally,
concluded that this type of soft tissue replacement was due to
owing to pulses of burial. In low-energy, fully oxic settings,
bacterial decay in anoxic c o n d i t i o n s resulting in sulfide f o r m a -
mainly u n b r o k e n , t h o u g h typically disarticulated trilobite m a t e -
tion. T h e rocks represent deep-water turbidites, and anything
rial is preserved; under episodes of higher s e d i m e n t a t i o n , c o m -
buried probably would have been subject to anoxic conditions.
TRILOBITE
LAGERSTATTEN
41
T h e concentration of organic matter in the sediment was rela-
b i o t u r b a t e d . A possible explanation is that the subsequent thicker
tively low, and the c o n c e n t r a t i o n of iron in the pore-water rela-
beds were deposited a short t i m e after this o n e , essentially sealing
tively high. In this situation the sulfide produced at local decay
it and m a k i n g any decay anaerobic. T h e low level of iron in the
sites was precipitated as an iron m o n o s u l f i d e (a precursor to
sediment precluded significant pyrite f o r m a t i o n .
pyrite) at the site where it was f o r m e d , resulting in the nearly perfect
replacement
Edgecombe
of
soft
tissue
by
pyrite
(Briggs
T h i s f o r m of appendage preservation
is unique and was
and
only f o u n d when Walcott m a d e sections of the trilobites and
1 9 9 3 ) . Beecher's Trilobite Bed remains the most
observed t h e m by t r a n s m i t t e d light. T h e very fine calcite in-filling
productive source of preserved trilobite soft tissue k n o w n .
in a c a r b o n a t e m a t r i x is very difficult to see and evaluate by
Trilobites found on limestone bedding planes typically are
reflected light. T h i s difficulty raises the question as to whether
compressed for the same reasons that trilobites are c o m p r e s s e d
this m o d e of preservation is actually m o r e c o m m o n but gener-
on shale partings. T h e i r history is similar in that they were buried
ally unobserved.
by a calcareous m u d f l o w with little t r a n s p o r t . S o m e of the best
T h e Lower Silurian R o c h e s t e r Shale c o n t a i n s several horizons
trilobites, however, from the physical preservation standpoint, are
yielding n u m e r o u s trilobites. S p e c i m e n s of the large lichid genus
found within limestones. In s o m e instances, these trilobites were
Arctinurus, when articulated, are usually f o u n d right side up but
apparently caught up in a calcareous sediment flow, killed, p r o b -
flattened.
ably transported s o m e distance, and e n t o m b e d in the settled
always inverted and often o c c u r s in narrow, elongate " w i n d r o w s "
sediment. In such limestones the trilobites typically are r a n d o m l y
that may show s o m e evidence of a preferred o r i e n t a t i o n . T h e
oriented, with the bodies flexed in unusual postures, and retain
living Arctinurus a n i m a l s probably were buried in place by a
much
of
their
original
three-dimensional
character.
Dalmanites
limulurus,
from
several
layers,
is
almost
Large
heavy blanket of s e d i m e n t , which resulted in their death. T h e D.
numbers of Isotelus gigas have been taken from the limestones of
limulurus, however, were t r a n s p o r t e d s o m e w h a t before the burial
the Middle Ordovician Trenton G r o u p in central New York. T h e
process, aggregated, and c u r r e n t aligned in windrows. T h e fact
specimens from bedding planes are flattened and c o m m o n l y
that most are upside d o w n is less easily explained. Convex surface
upside down, while those from within the limestone retain their
down may be a preferred o r i e n t a t i o n d u r i n g transport in a
three-dimensional character.
c u r r e n t , or the living a n i m a l m a y have been t u m b l e d by the
In the Walcott-Rust Q u a r r y within the Trenton G r o u p , several
current and the u p s i d e - d o w n trilobites m a y not have been able
layers of the thinly bedded limestone have yielded excellent trilo-
to right themselves. Again, alignment probably indicates slight
bites (Brett et al. 1 9 9 7 , 1 9 9 9 ) . O n e thin micritic limestone yielded
transport of dead individuals by a c u r r e n t . Alternatively, it is pos-
large
sible that these trilobites swam upside down to escape the sedi-
numbers
of articulated
C.
pleurexanthemus,
Flexicalymene More
m e n t and were buried in this p o s i t i o n . A possible model for this
than 9 8 % of the trilobites on the base and those within the layer
latter behavior c o m e s from the living h o r s e s h o e c r a b , Limulus,
were upside down while 6 0 % o f those o n the upper surface,
which n o r m a l l y crawls a r o u n d the b o t t o m but m o r e rarely swims
including many that were still partially within the l i m e s t o n e , were
upside d o w n .
senaria, and
Meadowtownclla
trentonensis
(but
no
I.
gigas).
right side up. T h e t h i n n e r bed probably records a large popula-
T h e siltstones,
m u d s t o n e s , shales, and
limestones
Hamilton
yielded
of the
tion of trilobites caught in a current and transported far e n o u g h
Middle
to be mixed within the small a m o u n t of sediment involved. M o s t
n u m b e r s o f articulated s p e c i m e n s o f trilobites. O n e well-studied
of the trilobites were killed in the process. T h e larger m o r e robust
bed, the B r o w n s C r e e k B e d , a lime m u d s t o n e in the lower C e n -
ones may have managed to struggle to the surface and die there
terfield
or were covered and killed by a subsequent event. T h e observa-
Monodcchenella
tion that no /. gigas were involved suggests that they were large
b e d can be traced f r o m Centerfield, O n t a r i o C o u n t y , to East
Devonian
Limestone,
yields
macrocephala,
Group
superbly and
have
preserved
Pseudodechenella
enormous
Eldredgeops
rana,
rowi.
This
and strong enough to escape. Two other, thicker beds had no
Bethany, G e n e s e e C o u n t y , a distance of over 4 9 k m (31 miles).
trilobites on the base or top but well-preserved s p e c i m e n s inter-
T h e trilobites are f o u n d in r a n d o m o r i e n t a t i o n , indicating they
nal to the limestone. T h e s e internal trilobites, which included /.
were swept up, t u m b l e d , and t r a n s p o r t e d s o m e distance from
gigas, were randomly oriented.
their original position in the sudden event f o r m i n g this bed.
Within the thin bed, coiled and semicoiled C. pleurexanthemus and F. senaria were discovered by C. D. Walcott ( w h o also discovered
the
Burgess
was
Wanakah
and
the
Windom
rana. Especially notable are the " G r a b a u Trilobite B e d s " (lower
This unique m o d e of preservation is believed to result from
Creek and the S m o k e C r e e k Beds in the f o r m e r P e n n - D i x i e
anoxic
the
Q u a r r y at H a m b u r g , Erie C o u n t y . T h e s e trilobite beds yield
appendage, followed by calcite in-filling as the appendage m a t e -
t h r e e - d i m e n s i o n a l clusters of E. rana, which Speyer and Brett
precipitation
evidence
lower
W a n a k a h Shale) along the Lake Erie shore near Eighteenmile
calcite
there
the
of
induced
and
within
appendages preserved as calcite infillings (Walcott 1876, 1 8 7 7 b ) . bacterially
Shale),
Horizons
m e m b e r s of the H a m i l t o n G r o u p are especially productive of E.
within
rial decayed (Brett et al. 1997, 1 9 9 9 ) . T h e Ceraurus bed is a thin,
( 1 9 8 5 ) divided into molt and b o d y clusters. T h e clusters are
15 to 5 0 - m m bed that normally would be expected to be heavily
nearly intact, which suggests little t r a n s p o r t . T h e s e beds are the
42
TAPHONOMY
result of very rapid burial and death, with very little turbulence
of 5 to 20 individuals are k n o w n . T h e general preservation is
d u r i n g the burial event.
similar to that of the M u r d e r Creek Beds.
T h e E. rana s p e c i m e n s from the lower W a n a k a h M u r d e r Creek
In s u m m a r y , t a p h o n o m i c analysis is a m a j o r factor in under-
Beds are f o u n d tightly coiled, fully o u t s t r e t c h e d , and in various
standing trilobite deposits, as well as fossil deposits in general.
semicoiled configurations in between these two e x t r e m e s . Typi-
T h e understanding that c o m e s from such analyses not only is
cally, in the semicoiled s p e c i m e n s , the pygidium appears to be
intellectually rewarding but also provides a foundation for inter-
missing. On close e x a m i n a t i o n , however, the pygidium can always
preting the geological history of fossil sites. T h e r e is also a
be f o u n d inside the body cavity of the trilobite. It is suggested
predictive factor that c a n n o t be ignored. For example, storm
that the pygidium was displaced when the tightly coiled trilobite,
deposits over stable sea b o t t o m s , below storm wave-base, often
u p o n decay, partially o p e n e d and viscous m u d filled the n o w -
b u r y a living fauna, resulting in well-preserved articulated fossils
e m p t y cavity.
such as trilobites and crinoids. Fossil preservation provides
T h e trilobites in the f o r m e r P e n n - D i x i e Q u a r r y are mostly in
the interpretation of fossil deposits and
the S m o k e Creek Beds o f the W i n d o m M e m b e r n a m e d for their
a n c i e n t e n v i r o n m e n t s . T a p h o n o m y and its concepts are power-
outcrop on S m o k e Creek, Erie C o u n t y . Well-preserved individ-
ful tools for general fossil collectors as well as professional
ual trilobites are c o m m o n in these beds, and occasional clusters
paleontologists.
1
1
i m p o r t a n t clues in
The Penn-Dixie Quarry is a public fossil site operated by the Hamburg
Natural History Society (P.O. Box 7 7 2 , Hamburg, NY 1 4 0 7 5 ; 7 1 6 - 6 2 7 - 4 5 6 0 ) .
4
The Paleozoic Geology of New York
T h e Paleozoic strata of New York State are a classic repository of
they c a n n o t be u n d e r s t o o d in isolation from the region as a
fossils, including, at m a n y levels, trilobites and a host of o t h e r
whole. D u r i n g this interval, present-day New York lay generally
invertebrate and even vertebrate and plant fossils. For details on
in southern subtropical to w a r m t e m p e r a t e latitudes, with the
the paleoecology and fossils, we r e c o m m e n d the publications
equator
by Linsley ( 1 9 9 4 ) , Isachsen et al. ( 1 9 9 1 ) , Landing ( 1 9 8 8 ) , Shaw
C l i m a t e s ranged from hot and arid to warm and h u m i d during
( 1 9 6 8 ) , and the references therein.
this t i m e .
running
approximately
centrally
through
Laurentia.
T h e purposes of this chapter are twofold: first, to give the reader both s o m e general b a c k g r o u n d regarding the tectonic, climatic, and paleoenvironmental history of life on earth (Archean through Pleistocene, Figure 4 . 1 A ) , and m o r e specifically of New York State and adjacent ancestral North A m e r i c a during the early
Prelude to the Paleozoic: Late Proterozoic Collisions and the Grenville Orogeny T h e oldest rocks in New York, exposed in the Adirondack
to middle Paleozoic Era ( C a m b r i a n through D e v o n i a n , Figure
M o u n t a i n s and the
4 . I B ) , the time during which trilobites lived and were deposited
than 1 billion years old. T h e s e are crystalline rocks that were
Hudson
Highlands, are s o m e w h a t m o r e
in New York's sedimentary rocks; and second, to provide s o m e
m e t a m o r p h o s e d or altered from older rocks by e n o r m o u s heat
details on the stratigraphy, sedimentology, and paleoecology of
and pressure (Figure 4 . 2 ) . S o m e were originally igneous rocks,
the intervals from Early C a m b r i a n to Middle Devonian that have
f o r m e d from the c o o l i n g and crystallization o f m a g m a s , and
yielded abundant trilobite remains. To these ends, the discussion
others are s e d i m e n t a r y deposits, such as q u a r t z sandstones,
of each Paleozoic t i m e interval is subdivided into two p o r t i o n s :
limestones,
first, an overview of global and New York geological history, and
phosed) by recrystallization and under intense heat and pressure;
and
shales.
T h e s e were
transformed
(metamor-
second, details of the stratigraphy and sedimentary e n v i r o n m e n t s
for example, sandstones were altered to tough metaquart/ites,
of the trilobite-bearing intervals, listed chronologically. Such
limestones to m a r b l e s , and shales to m i c a - r i c h schists. By looking
general discussion of geology might seem out of place in a b o o k
at the types of minerals f o r m e d within these rocks by m e t a m o r -
devoted to trilobites; however, we believe that students of New
p h i s m , geologists can be certain that the rocks now exposed in
York's trilobites should be well aware of these b r o a d e r contexts
the Adirondacks were o n c e buried up to 25 km within Earth
in order to understand the e n v i r o n m e n t s and ecology ot these
during a great o r o g e n i c or m o u n t a i n - b u i l d i n g e p i s o d e — a b o u t a
remarkable ancient o r g a n i s m s .
billion years ago. T h i s event, the Grenville Orogeny, apparently resulted when the (present) eastern edge of ancestral North
Overview T h e Paleozoic deposits of New York record a p o r t i o n of the
America
was overridden
by a n o t h e r c o n t i n e n t , perhaps the
northwestern side ot" present-day South America. This e n o r m o u s collision helped to weld together a supercontinent known as
history of ancestral North America or Laurentia during s o m e
P r o t o p a n g e a or Rodinia (Figures 4.3 and 4 . 4 ) . For nearly half a
2 0 0 million years of geologic t i m e (Figure 4 . I B ) , and as such,
billion years, this s u p e r c o n t i n e n t held together, and the massive •13
A
FIGURE 4 . 1 . A. Time scale for Earth history a n d for life on Earth. The New York Paleozoic is from the early C a m b r i a n to the e n d of the D e v o n i a n . A d a p t e d from S. M. Stanley, Exploring Earth and Life through Time. New York: W. H. Freeman, 1989. B. G e n e r a l i z e d g e o l o g y a n d detailed scale for the Paleozoic rocks in New York. The vertical scale is linear a n d proportional to time; note that the d a t e s of b e g i n n i n g s a n d e n d s of periods are given in millions of years before present. The right c o l u m n s list the n a m e s of major unconformities a n d the n a m e s of Sloss s u p e r s e q u e n c e s . Most of the information is from a n u m b e r of s o u r c e s . The d a t e s for the C a m b r i a n are from D a v i d e k et al. (2000) a n d L a n d i n g et al. (1998a,b). The T in the C a m b r i a n time p e r i o d indicates where trilobites first a p p e a r in the fossil r e c o r d , 5 1 9 million years before present. The d a t e s in the E p o c h s are the most current. The d a t e s in the Stages h a v e yet to be r e c o n c i l e d in the literature.
45
OVERVIEW
B
Grenville m o u n t a i n belt, fully f o r m e d by a billion years ago, was
present eastern seaboard region (eastern Massachusetts, eastern
exposed in a life-less continental interior to the forces of weath-
Nova Scotia, eastern N e w f o u n d l a n d ) , Florida, California, and
ering and erosion. We know that by about 5 5 0 million years ago,
o t h e r marginal areas that were added later. Laurentia was isolated
an entire thickness of continental crust had b e e n removed and
as a separate c o n t i n e n t during rifting and o p e n i n g of new ocean
erosion had exposed the roots of the ancient Grenville M o u n t a i n s .
basins that began approximately 7 0 0 to 6 0 0 million years ago
Laurentia is the term geologists apply to the ancestral Paleo-
during the interval of t i m e referred to as late P r o t e r o z o i c (or N e o -
zoic core of N o r t h A m e r i c a , lacking certain areas such as the
p r o t e r o z o i c ) . Notably, on the present east side of Laurentia,
THE
46
PALEOZOIC
GEOLOGY
OF
NEW
YORK
FIGURE 4 . 2 . P r e c a m b r i a n Grenville ( 1 . 0 BP) m e t a m o r p h i c / i g n e o u s rocks. Note the granitic pegmatite (a) and smaller folded dikes (b) cutting g n e i s s . Rte. 12, near Alexandria Bay, J e f f e r s o n County.
rifting or fracturing of the crust began over 6 0 0 million years
Initially, a large v o l u m e of sediment that eroded from the
a g o — s o m e 4 0 0 million years after the Grenville Orogeny. T h e
ancient Grenville terrane was shed o f f the old weathered craton
fractures and faults ultimately tore the s u p e r c o n t i n e n t of Rodinia
and into the n a r r o w Iapetus basin. However, as the sea level c o n -
apart. Evidently, ancestral S o u t h A m e r i c a , which had collided
tinued to rise during the latest Proterozoic and Early C a m b r i a n ,
with p r o t o - L a u r e n t i a to f o r m the Grenville M o u n t a i n s , n o w
the old land was finally flooded and the source of sediments cut
pulled back away. A new o c e a n , the Iapetus, or P r o t o - a t l a n t i c ,
o f f (Figure 4 . 5 A ) . T h e entire eastern b o r d e r of Laurentia had
began to open along a line that would pass through present-day
b e c o m e a passive continental edge; a sea with a very broad c o n -
central
tinental shelf ultimately extended from the area of central New
New
Carolinas, a
England bit
east
and of the
southward Blue
Ridge
to
the
east-central
Mountains
in
the
Appalachian chain (these m o u n t a i n s f o r m e d later).
England and the m i d Atlantic states region westward to the present Mississippi Valley during the C a m b r i a n to Early O r d o v i -
At the b e g i n n i n g of the Paleozoic Era, a b o u t 5 4 5 million years
cian (Figure 4 . 5 A , B ) . As spreading ensued in the Iapetus m i d o -
ago, a narrow, but widening Iapetus O c e a n lay slightly to the east
cean rift, the ocean basin grew wider, at least up to the Early
of present-day New York State (Figure 4 . 5 A ) . East of the present
Ordovician t i m e , about 4 8 0 million years ago, before the process
Berkshire ( M a s s a c h u s e t t s ) and Green ( V e r m o n t ) M o u n t a i n s lay
began to reverse and the Iapetus basin began to shrink and
the edge of Laurentia, w h i c h , m u c h like the present eastern edge
ultimately close.
of modern North A m e r i c a , f o r m e d a continental shelf bordered eastward (southward at that t i m e ) by a relatively steep d r o p - o f f along a continental slope into the deep water of the Iapetus Ocean.
Cambrian Period T h e C a m b r i a n Period, as it is now dated, spans approximately
Seawater was displaced upward o u t of the Iapetus O c e a n ,
54 million years from about 5 4 3 to 4 8 9 million years before
partly because of the e x p a n d i n g m i d o c e a n ridge ( o r spreading
present. Yet this was o n e of the most significant times in the
center, an area of hot upwelling m a g m a ) . T h i s seawater spread
history of life, for it was during this time that nearly all phyla or
out o n t o the c r a t o n of Laurentia and caused a m a j o r rise of the
m a j o r groups of a n i m a l s appeared in the rock record.
shoreline (transgression) up o n t o the old weathered r e m n a n t s of the Grenville rocks.
T h e onset of the C a m b r i a n was marked by a time of lowdiversity fossil assemblages typified by "small shelly" skeletons
FIGURE 4.3. M a p s h o w i n g the extent of the Grenville belt in eastern North A m e r i c a . Rocks in this area were d e f o r m e d a n d m e t a m o r p h o s e d about 1 billion years a g o . The d o t t e d pattern s h o w s regions w h e r e Grenville rocks are buried b e n e a t h y o u n g e r strata; the slanted lines indicate areas w h e r e Grenville rocks are e x p o s e d ; a n d cross-hatching shows areas where the Grenville rocks are d e f o r m e d by later o r o g e n i e s . M o d e r n A d i r o n d a c k Mountains lie just to the east of the Frontenac A r c h ( l a b e l e d ) . From Isachson et al. (1991). Printed with permission of the New York State M u s e u m , Albany, N.Y.
•18
THE
PALEOZOIC
GEOLOGY
OF
NEW
YORK
FIGURE 4 . 4 . P a l e o m a g n e t i c reconstruction of the supercontinent Rodinia as it existed in the late Proterozoic, a b o u t 7 0 0 million years a g o . Note the position of Laurentia, near the center. The dark belt shows the position of the Grenville O r o g e n i c belt. After Dalziel (1997), r e p r o d u c e d with permission.
including a variety of calcareous p h o s p h a t i c tubes, rods, and
divided
plates. O n l y in later Early C a m b r i a n t i m e ( - 5 1 9 millon years ago)
a u t o c h t h o n o u s ( n o n t r a n s p o r t e d ) rocks, which are in the site
into two great packages of strata
(Figure 4 . 7 ) . T h e
did trilobites first appear as body fossils, although fossil track-
of original deposition, a shallow-shelf e n v i r o n m e n t , and the
ways (traces of walking on the b o t t o m ) and resting pits (Ruso-
Taconic
phycus) suggest that unpreserved s o f t - b o d i e d trilobites, or similar
shales (now often slates) that were originally deposited to the east
a r t h r o p o d s , o c c u r r e d earlier.
of New York in o c e a n i c e n v i r o n m e n t s . T h e Taconic rocks were
T h e C a m b r i a n rocks of New York State are generally s u b -
allochthonous
transported
westward
(displaced)
80km
rocks,
during
the
mostly deep-water
Middle
Ordovician
CAMBRIAN
PERIOD
49
FIGURE 4 . 5 . Position of the Laurentian plate and neighboring Baltica and Avalonia terranes from the Cambrian to the Devonian. Areas of e x p o s e d land are dark, a r e a s c o v e r e d by water a r e white, a n d New York is highlighted in black. A. Middle Cambrian as most of what is the United S t a t e s w a s beginning to be flooded by shallow s e a s resulting in c a r b o n a t e depositions in the Late C a m brian (note Taconic island a r c s ) . B. Middle Ordovician with almost c o m p l e t e flooding of Laurentia and the beginning of the Taconic Orogeny with collision in the s o u t h e a s t . C. Silurian, with Baltica colliding with Laurentia from the e a s t . Also note the smaller "islands of Avalonia." D. Middle Devonian, with the A c a d i a n O r o g e n y fully d e v e l o p e d d u e to collision of the Avalon t e r r a n e s ; note heavy sedimentation from the e a s t e r n mountains. Modified from Witzke ( 1 9 9 0 ) . R e p r o d u c e d with permission.
Taconic Orogeny (a m a j o r m o u n t a i n - b u i l d i n g event) along m a j o r
continental shelf. Following the interval of rifting that produced
thrust faults. T h i s d e f o r m a t i o n apparently was related to the
the Protoatlantic or Iapetus O c e a n , the continental shelf, which
convergence of an island arc c o m p l e x with eastern Laurentia ( o r
is represented by the region f r o m New England westward into
ancestral North A m e r i c a ) .
central New York, u n d e r w e n t rapid subsidence due to cooling. L i m e m u d and silt probably were produced by o r g a n i s m s such as
Cambrian Autochthon T h e a u t o c h t h o n o u s rocks o f the central Appalachians, which
algae, the growth of which was evidently able to keep up with the rate of subsidence so that these shelf e n v i r o n m e n t s remained in very shallow water, above the n o r m a l wave-base. C a m b r i a n to
include basal sandstone and limestone or d o l o s t o n e , represent
Early
extensive shallow, subtropical platform seas, s o m e t i m e s referred
carbonates—the
Ordovician
rocks
to as the Great American Tidal Flat (Figures 4 . 5 , 4 . 7 ) . In s o u t h -
by major unconformities.
Sauk
form
a
package
Sequence—bounded
of sandstones above
and
and below
eastern New York and southern Pennsylvania these beds may be
D u r i n g C a m b r i a n and Early O r d o v i c i a n t i m e , eastern North
up to 5 km thick and were mainly deposited in very shallow, tide-
A m e r i c a lay in a subtropical position, perhaps 25° south of
influenced e n v i r o n m e n t s . T h e i m m e n s e thickness is a c c o u n t e d
the p a l e o e q u a t o r (Figure 4 . 5 ) . Today this zone is known as the
for by active subsidence, or downward sinking, of the C a m b r i a n
"subtropical desert b e l t " because it is here that s o m e of the driest
FIGURE 4.6. C a m b r i a n rocks in New York. A. U p p e r C a m b r i a n P o t s d a m S a n d s t o n e at Ausable
Chasm
near
Plattsburg,
Clinton
County.
B.
Tilted
Precambrian-Cambrian
(Lippalian) nonconformity. A p p r o x i m a t e l y 500 million years of n o n d e p o s i t i o n a n d erosion s e p a r a t e dark a m p h i b o l i t e s (a) ( m e t a m o r p h i c rocks) from the light gray, U p p e r C a m b r i a n Little Falls Formation (b). Cut a l o n g U.S. Rte. 5S near Fonda, M o n t g o m e r y County.
CAMBRIAN
51
PERIOD
conditions on Earth develop. T h e r e is s o m e evidence that ances-
stone of the M o h a w k Valley, the Hoyt L i m e s t o n e of the Saratoga
tral North America in the C a m b r i a n was relatively arid in the
area, the Whitehall F o r m a t i o n of Lake G e o r g e , and the Theresa
(present) eastern regions. T h e r e is also evidence for the buildup
D o l o s t o n e of the Saint Lawrence region are carbonates: lime-
of slight hypersalinity in the C a m b r i a n waters. C a m b r i a n d o l o -
stones, and d o l o s t o n e s , f o r m e d very late in the C a m b r i a n or in
stones contain vugs and s o m e t i m e s m o l d s of evaporite crystals
earliest O r d o v i c i a n t i m e (Figures 4.7 and 4 . 9 ) . T h e y display an
such as gypsum, anhydrite, and halite. T h e o c c u r r e n c e of o o i d s
a b u n d a n c e o f stromatolites. S o m e o f the most f a m o u s stromato-
(small spherical, concentrically ringed c a l c i u m c a r b o n a t e grains)
lites in the n o r t h e a s t e r n part of North A m e r i c a o c c u r in the
and stromatolites ( m o u n d l i k e structures f o r m e d o f sediment
Petrified G a r d e n s within the Hoyt Limestones near Saratoga
trapped by cyanobacteria or blue-green algae) also is typical of
Springs, New York (Figure 4 . 9 ) .
hypersaline (elevated-salinity) waters in the subtropics today.
T h e Little Falls F o r m a t i o n stromatolitic dolostones (up to
Whatever the case, m u c h of the C a m b r i a n seafloor in the New
30 m thick) are only very sparsely fossiliferous. O n l y a single free
York area was relatively low in shelly o r g a n i s m s .
c h e e k of the trilobite Elvinia has been f o u n d from the C a m b r i a n
T h e broad continental shelf of the C a m b r i a n sea was bordered
Little Falls F o r m a t i o n , but this is sufficient to bracket its age
to the east, in what are today central V e r m o n t , western Massa-
within the s e c o n d to last or F r a n c o n i a n Stage of the C a m b r i a n .
chusetts, and C o n n e c t i c u t , by an abrupt slope into deeper water.
( T h e Little Falls is noted for its characteristic vugs or cavities that
In the region flanking the continental shelf of N o r t h A m e r i c a ,
contain beautiful, doubly t e r m i n a t e d q u a r t z crystals referred to
sediments accumulated very gradually (Figures 4.7 and 4 . 9 ) .
as H e r k i m e r D i a m o n d s ; such crystals f o r m e d m u c h later during deep burial of the Little Falls sediments.) Just why most Upper C a m b r i a n c a r b o n a t e s are so p o o r in b o d y fossils, including trilo-
Cambrian Trilobite-Bearing Autochthonous Rocks T h e lowest and shallowest-water deposit of the C a m b r i a n
bites, but rich in stromatolites is as yet unclear. It may be that
a u t o c h t h o n o u s rocks belong to the Potsdam S a n d s t o n e or equiv-
the seas were s o m e w h a t hypersaline. However, the Hoyt L i m e -
alent sandy Little Falls F o r m a t i o n (Figures 4.6 and 4 . 8 ) . T h e
stone c o n t a i n s not only the f a m o u s stromatolites but also beds of
Potsdam Formation consists of up to 140 m of clean quartz a r e n -
oolitic l i m e s t o n e and s o m e fossiliferous limestone
ites or quartzose sandstones that display c r o s s - s t r a t i f i c a t i o n ,
sclerites (skeletal pieces) of trilobites b e l o n g i n g to a n u m b e r
ripple marks, and other features indicative of deposition in
of Late C a m b r i a n species are f o u n d ; Ludvigsen and Westrop
shallow-wave
and
sometimes
tide-dominated
environments
(Figure 4 . 8 ) . T h e Potsdam sediments represent sand eroded from deeply weathered areas of the N o r t h A m e r i c a n c r a t o n . The
presence
of
herringbone
cross-stratification
in which
( 1 9 8 3 ) recently described these species f r o m the Hoyt and Galway limestones. T h e s e trilobites are associated with o t h e r fossils, including
(inclined
b r a c h i o p o d s , m o l l u s c a n f r a g m e n t s , and even plates of the world's
bedding formed by alternate migration of ripples in opposite
oldest c h i t o n s . T h i s diverse assemblage indicates relatively favor-
directions) is an indication of the oscillatory currents associated
able, n o r m a l - s a l i n i t y c o n d i t i o n s in the area of Saratoga Springs
with tidal action. S o m e p o r t i o n s display large-scale trough cross-
during this p e r i o d . However, relative high energy due to wave
bedding and may represent w i n d - f o r m e d sand dunes in coastal
action and slow deposition prevented the easily disarticulated
areas.
trilobites f r o m being preserved whole. T h e trilobites o f the
The Potsdam is generally sparse in body fossils, although trace
Galway and Hoyt are:
fossils (burrows, tracks, and trails) of a variety of forms are present, including the bizarre and huge (by C a m b r i a n standards)
GALWAY
Climactichnites. T h i s elongate trail up to 3 0 c m ( o r m o r e than a
Calocephalites
foot) wide closely resembles m a r k s m a d e by a tractor tire in soft
Dellea
sand. This trace is found in flat-bedded sands that may represent
Drabia cf.
FORMATION
cf.
C.
minimus
saratogensis D.
menusa
Cameraspis Drabia
cf.
Elvinia
convexa D.
curtoccipita
granulata
the upper foreshore or beach. Just what large o r g a n i s m in the C a m b r i a n was able to c o m e out into very shallow water is quite
HOYT
unclear. Traces may have been preserved by the sun drying ini-
Delicti?
tially wet sands of coastal areas. Yochelson and F e d o n k i n ( 1 9 9 3 )
Keithiella
argued that it might have been a large gastropod-like mollusk. In
Pie tho pelt is sara togensis
Prosaukia
hartti
slightly m o r e offshore Potsdam f a d e s , trace fossils such as verti-
Prosaukici
Saratogia
(Saratogia)
cal
shafts
(Skolithos)
and
U - s h a p e d burrows
(Diplocraterion)
LIMESTONE
landingi depressa tribulis
Hoytaspis Pletlwpeltis
speciosa granulosa calcifera
are
quite abundant. Lingulid b r a c h i o p o d s and small fragments of trilobites also have been o b t a i n e d in a few levels, but in general body fossils are rare.
Cambrian of the Taconic Allochthon Initially, a substantial a m o u n t of silt, sands, and muds was
T h e higher beds of the Upper C a m b r i a n and those straddling
swept from the Grenville b a s e m e n t of Laurentia, which had been
the Ordovician b o u n d a r y are c a r b o n a t e s . T h e Little Falls D o l o -
exposed to weathering and erosion for over 4 0 0 million years.
FIGURE 4.7. N e w York in the C a m b r i a n . A. New York in the U p p e r C a m b r i a n , s h o w i n g the c a r b o n a t e bank over m u c h of the state. B. Cross section of the plate m o v e m e n t d u r i n g the U p p e r C a m b r i a n . C. Stratigraphic chart of the C a m b r i a n e x p o sures in N e w York. From Isachsen et al. (1991). Printed with p e r m i s s i o n of the New York State M u s e u m , Albany, N.Y.
CAMBRIAN
53
PERIOD
FIGURE 4 . 8 . C l o s e - u p of Upper Cambrian P o t s d a m S a n d s t o n e showing s e t s of c r o s s - b e d d e d quartz-rich s a n d s t o n e . Cut along Street, Whitehall, Washington County.
T h e coarser sands derived from this erosion a c c u m u l a t e d in
ing redox (oxidation states) c o n d i t i o n s on the seafloor. At certain
nearshore areas to form the Potsdam S a n d s t o n e facies (Figures
intervals, the b o t t o m water seems to have been better oxygenated,
4.8 to 4 . 1 0 ) .
However, substantially larger a m o u n t s of fine-
grained sediment (clay minerals) were either carried in dilute sus-
leading to the d e v e l o p m e n t of reddish or green slates with little o r n o a c c u m u l a t e d organic m a t t e r (Figure 4 . 1 0 ) .
pensions to offshore areas, where they settled o u t as h e m i p e l a g i c " r a i n " of m u d , or perhaps were blown offshore d u r i n g dust storms, eventually to settle out and a c c u m u l a t e as deep-water deposits.
Cambrian Trilobite-Bearing Allochthonous Rocks T h e green and purple Early C a m b r i a n shales or slates of the T a c o n i c M o u n t a i n s (up to 6 0 0 m t h i c k ) are c o m m o n l y quarried
T h e rocks that were deposited in the continental slope and rise
as roofing slates in eastern New York and V e r m o n t , but these
belt are no longer found in their area of original a c c u m u l a t i o n .
quarries are not m a j o r fossil localities. In general, it appears that
Rather, they are found as a series of thrust sheets that lie east of
relatively few o r g a n i s m s
the Hudson River Valley in present eastern New York, referred to
C a m b r i a n t i m e , and m o s t slates are b a r r e n , even as life was just
inhabited
the deep sea
during the
as the Taconic M o u n t a i n s (see under O r d o v i c i a n ) . T h e earliest,
bursting forth in shallow-shelf settings. Very few fossils have been
pretrilobite, portion of the C a m b r i a n is poorly recorded in New
f o u n d within these beds, although an unusual b r a n c h i n g trace
York. However, s o m e of the thick, m u d d y sandstones and silt-
fossil, Oldhamia, is a b u n d a n t in s o m e of the purple shales low in
stones of the high Taconic M o u n t a i n s may represent this t i m e
the T a c o n i c succession. T h e s e very small trace fossils may repre-
interval.
sent s o m e of the earliest deep-water grazing animals. A few local-
T h e low or western p o r t i o n s of the Taconics display well-
ities
have
yielded
trilobites.
The
Lower
Cambrian
Middle
preserved successions of C a m b r i a n - E a r l y Ordovician strata that
Granville or Nassau F o r m a t i o n , in the vicinity of Troy, Rensselaer.
comprise a series of alternating green to purple or black slaty
County,
shales (Truthville, Browns Pond, Middle Granville). T h e alterna-
asaphoides as well as a fairly c o m p l e t e growth series of this trilo-
tion between purple, green, and black m u d r o c k s indicates differ-
bite. T h e Lower C a m b r i a n trilobites found are:
has
yielded
articulated
remains
of
Elliptocephala
FIGURE 4 . 9 . U p p e r C a m b r i a n limestones. A. U p p e r C a m b r i a n limestone, Whitehall Formation. Note the darker gray oolitic limestone (a) in s h a r p c o n t a c t with fine-grained " r i b b o n limestone" (b). Also note the small stromatolite (c) a t t a c h e d to the U p p e r c o n t a c t of the oolitic limestone. Warner Hill Cuarry, Whitehall, W a s h i n g t o n County. B. D o m a l stromatolites ( " c r y p t o z o a n " ) in u p p e r C a m b r i a n Hoyt Formation. C r y p t o z o a n l e d g e , Petrified G a r d e n s R o a d , Lester Park, S a r a t o g a County.
CAMBRIAN
PERIOD
55 nant or when productivity in the surface waters was increased, allowing the a c c u m u l a t i o n of organic matter. T h e s e intervals also contain beds of l i m e s t o n e breccia or angular c o n g l o m e r a t e s that represent debris flows broken f r o m the edge of the continental shelf that avalanched d o w n into deeper water. Actually, a rather diverse fauna o f trilobites has b e e n o b t a i n e d f r o m s o m e o f the l i m e s t o n e breccias o r c o n g l o m e r a t e s . T h e dark facies include the Lower C a m b r i a n Browns Pond F o r m a t i o n and the Lower to Upper C a m b r i a n Hatch Hill Form a t i o n (a 1 5 0 - m interval of dark slaty shales) (Figures 4 . 6 C and 4 . 1 1 ) . B o t h units c o n t a i n n u m e r o u s interbeds of several types, including ripple cross-stratified sandstones that probably record turbidites. T u r b i d i t y c u r r e n t s (masses o f suspended sediment that flow d o w n s l o p e u n d e r the influence of gravity) swept finegrained siliciclastic silt and sand o f f of shelf regions into the deeper water. A n o t h e r c o m m o n type o f b e d consists o f lightgray weathering b a n d s of very fine-grained l i m e s t o n e . Careful e x a m i n a t i o n of s o m e of these beds reveals that they contain very fine l a m i n a t i o n s or even c r o s s - l a m i n a t i o n s and display sharp bases. T h e r e f o r e , they have been inferred to have been deposited as relatively dilute turbidites of suspended c a r b o n a t e silt and m u d that were e x p o r t e d into the deeper water f r o m the carb o n a t e b a n k near the top of the slope. Such deposits rarely contain fossils, t h o u g h trilobite fragments are k n o w n from s o m e turbidites. A n o t h e r type o f a c c u m u l a t i o n within the Taconic dark
shale
Perhaps
consists
the
"Schodack
best
of brecciated
known
Limestone"
of these
found
near
(fragmented) is
the
limestones.
Lower
Castleton
Cambrian
Cutoff
in
the
Hudson Valley. Relatively few o r g a n i s m s could actually live in the low-oxygen deeper waters.
But the remains of shallow-water
o r g a n i s m s were abruptly t r a n s p o r t e d into s o m e o f these envir o n m e n t s as debris flows from the shallow-shelf regions above where these a n i m a l s lived. T h e fossiliferous breccia beds are a key to the stratigraphy of the lower part of the T a c o n i c rocks. T h e s e thin limestone-clast c o n g l o m e r a t e s contain a sandy m a t r i x that yields a b u n d a n t fossils of a variety of trilobites, including agnostids, b r a c h i o p o d s , and s o m e of the world's oldest bivalves. T h e s e fossils, particularly the trilobites, are invaluable for dating the succession. A few Lower C a m b r i a n a l l o c h t h o n o u s rocks are rich with fragFIGURE 4 . 1 0 . Simplified stratigraphy of the C a m b r i a n -
mentary, small trilobites, particularly agnostids and eodiscids.
Ordovician rocks in the Taconic allochthon. A d a p t e d from
Rasetti ( 1 9 4 6 , 1 9 5 2 , 1 9 6 6 a , b , 1967) with T h e o k r i t o f f ( 1 9 6 7 )
Landing ( 1 9 8 8 ) , with permission.
reported the following: LOWER
MIDDLE
GRANVILLE
Calodiscus lobotus
Elliptocephala
Fordaspis nana
Kootcnia
Serrodiscus
CAMBRIAN
Acidiscus
FORMATION
asaphoides
fordi
speciosus
birdi
Acimctopus Analox
Acidiscus
bilobatus obtusa
Bathydiscus
Analox Atops
dolichometopus
Bolboparia
hexacanthus bipunctata trilineatus elongata
Bolboparia
superba
Calodiscus
T h e dark shaly intervals with m o r e n u m e r o u s interbeds rep-
Calodiscus
fissifrons
Calodiscus
lobatus
agnostoides
resent a period of time either when the b o t t o m was m o r e stag-
Calodiscus
meeki
Calodiscus
occipitalis
THE
56
PALEOZOIC
GEOLOGY
OF
NEW
YORK
FIGURE 4 . 1 1 . Lower C a m b r i a n allochthonous b e d s of the low Taconic Mountains. Thin-striped white b e d s in the lower view are lime turbidites. A thin b e d of b r o k e n or b r e c c i a t e d limestone (a) o c c u r s (to the left of the h a m m e r ) . U p p e r b e d s are b l a c k shaly H a t c h Hill Formation ( b ) . Rte. 9 south of H u d s o n , C o l u m b i a County.
Calodiscus
reticulatus
Calodiscus
schucherti
Olenoidcs
Calodiscus
theokritofft
Calodiscus
walcotti
Pagetia
Chelediscus
chathamensis
Elliptoccphala
Eoagnostus
acrorhachis
Fordaspis
Hyolithellus
micans
"Kochiella"
Kootenia Oodiscus Pagetia
longispinus binodosus
bigra nulosa
Pagetides
amplifrons
fitchi
Seopagetina
punctulatus taconica
Oodiscus Pagetia
connexa
Pagetides
elegans
leiopygus
Pagetides
minutus
Pagetides
rupestris
Peronopsis
evansi
Rimouskia
cf.
P.
primigenea
Serrodiscus
speciosus
Serrodiscus
subclovatus
Stigmadiscus
Prozacanthoides Serrodiscus
typica
stenometopus
clytioides
Ptychagnostus
gibbus
Plethomctopus
knopfi
punctuosus
UPPER
Prosaukia
CAMBRIAN
briarcliffcnsis
subgranulatus
Pagetides Peronopsis
Ptychagnostus
Pagetia
nana
Leptochilodiscus
fordi
Litometopus
asaphoides
stockportensis erratica
Serrodiscus
Ordovician Period Unlike the C a m b r i a n , the Ordovician Period was a relatively long interval, s p a n n i n g about from 4 8 9 to 4 3 8 million years eatoni
griswoldi
ago. D u r i n g this long span, North America c o n t i n u e d to straddle the paleoequator, and New York lay in the southern subtropics
spinulosis
(Figure 4 . 5 ) . Early Ordovician saw a c o n t i n u a t i o n of the passive
Stigmadiscus
gibbosus
Great A m e r i c a n Tidal Flat e n v i r o n m e n t . But during the Middle
Weymouthia
nobilis
O r d o v i c i a n t i m e the eastern edge of Laurentia began to enc o u n t e r an offshore volcanic island arc ( c h a i n ) (Figure 4 . 1 2 ) .
MIDDLE
Ultimately this collision pushed (thrust) a great mass of deep-sea
CAMBRIAN
Baltagnostus
angustilobus
Baltagnostus
Bathyuriscus
eboracensis
Bolaspidella
Corynexochides?
expansus
Hypagnostus
stockportensis fisheri parvifrons
sediments up o n t o the present eastern edge of Laurentia, creating the Taconic M o u n t a i n s and causing the continental edge to collapse into a f o r e l a n d basin.
ORDOVICIAN
57
PERIOD
FIGURE 4.12. Stratigraphic chart of the O r d o v i c i a n e x p o s u r e s in N e w York.
M o d i f i e d from I s a c h s o n et al. (1991).
Printed with
permission of the New York State M u s e u m , Albany, N.Y
Early Ordovician
the C a n a d i a n Series. S o u t h of the Adirondacks, in the central M o h a w k River Valley, the Lower O r d o v i c i a n rocks are relatively
T h e Lower Ordovician interval in eastern N o r t h A m e r i c a is locally referred to as the Canadian Series (Figures 4 . 1 2 and 4 . 1 3 ) .
thin ( a b o u t 35 to 5 0 m ) a n d are assigned to a single f o r m a t i o n , the Tribes Hill (Figure 4 . 1 2 ) .
This interval, s o m e 20 million years in d u r a t i o n , is set o f f f r o m
T h e Lower O r d o v i c i a n ( C a n a d i a n ) Series rocks c r o p out in a
the higher Ordovician rocks in New York State and in most parts
roughly c o n c e n t r i c belt a r o u n d the A d i r o n d a c k region. To a large
of North America by the m a j o r K n o x U n c o n f o r m i t y , a 2 5 - to 30
extent, this o u t c r o p belt is controlled by the rather recent (late
million-year gap in the geological record. Moreover, the type of
C e n o z o i c ) uplift of the Adirondacks. However, it should be noted
sediments and the style of stratigraphy c h a n g e markedly between
that C a n a d i a n or Lower O r d o v i c i a n rocks are thin to absent in a
the Lower Ordovician and the late M i d d l e O r d o v i c i a n rocks that
belt r u n n i n g n o r t h w a r d f r o m Utica, New York, to an area north
overlie the unconformity. Lower O r d o v i c i a n rocks contain a
of Watertown.
sparse and rather poorly d o c u m e n t e d fossil fauna, d o m i n a t e d by
A m e r i c a was covered by shallow seas d u r i n g the Early O r d o v i -
certain small mollusks, such as various gastropods and nautiloid
cian, a low peninsular area of land extended eastward o f f the
cephalopods and rare trilobites. T h i s restricted Early O r d o v i c i a n
C a n a d i a n Shield roughly in the area of the T h o u s a n d Islands and
fauna suggests that somewhat unusual, perhaps slightly hyper-
Adirondacks of the present day. T h i s peninsula probably has
saline, conditions c o n t i n u e d in the N o r t h A m e r i c a n interior seas
n o t h i n g to do with the present expression of the Adirondacks. It
This
suggests
that
although
most
o f North
during this interval ot geologic time.
represents an a n c i e n t arch that was present in the c o n t i n e n t and
T h e Lower Ordovician strata
in New York generally are
probably developed during the t i m e of rifting of North America
assigned to the upper part of the B e e k m a n t o w n G r o u p that takes
f r o m a n o t h e r c o n t i n e n t in the late Proterozoic (late P r e c a m -
its name from an area near Lake C h a m p l a i n . In the o u t c r o p belt
brian). This
the Lower Ordovician rocks are best exposed and most c o m p l e t e
Ordovician c a r b o n a t e s thickened regularly to the south away
in areas around Lake C h a m p l a i n and southward to a b o u t Lake
f r o m this area. I s o p a c h m a p s ( m a p s showing variations in thick-
region
is referred to as the Frontenac Arch. T h e
formations,
ness of a particular rock unit) for the Lower Ordovician reveal a
each b o u n d e d by a m i n o r u n c o n f o r m i t y , are represented in
pattern c o m p a r a b l e to that seen in the C a m b r i a n in which the
George.
In
this
vicinity
at
least
four
distinct
FIGURE 4.13. New York d u r i n g the Early a n d early M i d d l e O r d o v i c i a n . A. Early O r d o v i c i a n , 495 million years before present. B. Early O r d o v i c i a n , 475 million years before present C. Cross section of plate m o v e m e n t d u r i n g the Early Ordovician s h o w i n g the Taconic Orogeny. D. C h a z y a n time. E. Black River times. F. Cross section of the plates d u r i n g the Middle O r d o v i c i a n . From I s a c h s o n et al. (1991). Printed with p e r m i s s i o n of the N e w York State M u s e u m , Albany, NY
FIGURE 4.13.
Continued.
THE
60
PALEOZOIC
GEOLOGY
OF
NEW
YORK
rocks thicken dramatically southward from the a p p r o x i m a t e zero
Ordovician were exposed to the a t m o s p h e r e for s o m e 25 to 30
line ( p i n c h - o u t of the Lower O r d o v i c i a n strata) that coincides
million years and b e c a m e deeply eroded. Because they had a
approximately with
relatively low siliciclastic c o n t e n t when exposed to rainwater, they
the
present
position
of the s o u t h
Lake
O n t a r i o shoreline. O r d o v i c i a n or C a n a d i a n age strata attain a
mainly underwent solution, although s o m e thin residues of sili-
thickness of nearly 1500 m in the subsurface near the New York
ciclastic m u d may have been developed on the karstic or solution
State-Pennsylvania b o r d e r and thicken still m o r e into central
surface. M a j o r solution features, such as sink holes and collapse
Pennsylvania, where they may exceed 3 0 0 0 m in thickness. T h e
b r e c c i a s , developed at this t i m e owing to karstification or disso-
rocks also thicken eastward from the area of the eastern A d i r o n -
lution and cave f o r m a t i o n and collapse of the older Ordovician
dacks into New England where they are represented as m e t a -
and C a m b r i a n c a r b o n a t e s . In places, the u n c o n f o r m a b l e surface
morphosed
has a relief of up to tens of meters.
carbonates
M a r b l e , about
1000m
(marble), thick.
including
North
the
o f the
Stockbridge
F r o n t e n a c Arch,
relatively thin sandy d o l o s t o n e s of the u p p e r m o s t Theresa and
Lower O r d o v i c i a n Strata and Trilobites
Ogdensburg f o r m a t i o n s represent the Lower O r d o v i c i a n c a r b o n -
T h e oldest o f the Lower Ordovician Canadian Series o f rocks
ates. T h e high sand c o n t e n t in these c a r b o n a t e s and in s o m e thin
are represented by the Whitehall F o r m a t i o n (Figures 4 . 1 2 and
intervals within the Lower O r d o v i c i a n of the M o h a w k Valley sug-
4 . 1 3 ) that is well exposed in the region n o r t h of Lake George near
gests that s o m e t e r r i g e n o u s sediment c o n t i n u e d to be swept f r o m
the town of Whitehall. T h i s f o r m a t i o n technically spans the
the now deeply weathered and eroding highlands of the F r o n -
C a m b r i a n - O r d o v i c i a n b o u n d a r y , but an upper unit within the
tenac Arch into adjacent shallow seas.
Whitehall appears to be set o f f by a m i n o r u n c o n f o r m i t y that
T h e upper units of the Lower O r d o v i c i a n , particularly the Fort
o c c u r s close to that b o u n d a r y . An erosion surface and overlying
Cassin F o r m a t i o n (Figures 4 . 1 2 a n d 4 . 1 3 ) , are restricted to areas
sandstone
east of the Adirondack M o u n t a i n s and the C h a m p l a i n Valley. No
appears to be a signature of a drop and initial rise in sea level.
and
siltstone
unit,
the
Winchell
Creek
Member,
trace of these units is f o u n d in the M o h a w k Valley farther west.
T h i s may represent a widespread regression that occurred near
T h i s suggests that toward the e n d of the Early O r d o v i c i a n ,
the end of the C a m b r i a n but still within the overall Sauk Super-
the area of deposition was restricted to a relatively narrow,
s e q u e n c e . T h e W i n c h e l l Creek Siltstone is overlain by somewhat
n o r t h - s o u t h trending basin lying close to the present eastern
fossiliferous limestone that has yielded occasional fragments of
New York State line and into New E n g l a n d . T h i s m a j o r change
trilobites as well as o t h e r m a r i n e fauna, suggesting partially
from widespread
normal marine conditions.
shallow seas over m u c h
of eastern
North
America to a n a r r o w eastern basin is n o t fully u n d e r s t o o d . In
T h e next higher and s o m e w h a t better-known interval, the
part, it may reflect m a j o r regression (shallowing or lowering of
Tribes Hill F o r m a t i o n (0 to 3 0 m ) , in the M o h a w k Valley, again
sea level) associated with the end of the Sauk S u p e r s e q u e n c e
c o m m e n c e s with a sandy or silty c a r b o n a t e in New York State, the
(Figure 4 . 1 3 B ) . A n o t h e r factor m a y b e tectonic d i s t u r b a n c e o f
Palatine Bridge M e m b e r . Again, this silt and sandstone unit over-
the eastern m a r g i n of N o r t h A m e r i c a . Such a d i s t u r b a n c e m a y
lies an u n c o n f o r m i t y that may represent an interval of m i n o r sea-
have produced a s u b s i d i n g ( d e e p e n i n g due to crust d e f o r m a t i o n )
level drop. T h e Palatine Bridge is overlain by b u r r o w - m o t t l e d and
trough in the region east of New York and New E n g l a n d , while at
s o m e w h a t fossiliferous W o l f Hollow M e m b e r that represents
the same t i m e the f o r m e r shelf area to the west was uplifted in a
shallow m a r i n e shelf deposition. T h e most fossiliferous unit
broadly upwarped archlike feature. It is notable that the Fort
within the Tribes Hill is the Fonda M e m b e r . T h e Fonda is a fossil-
Cassin F o r m a t i o n bears a n u m b e r of faults that are truncated by
rich limestone. S o m e beds display reddish to greenish c o l o r due
the overlying u n c o n f o r m i t y . T h e s e faults m u s t have o c c u r r e d
to the presence of iron mineralization, especially the clay mineral
following the deposition
glauconite. This latter mineral is believed to form in open marine
of the Fort
Cassin
Formation
but
before the deposition of the overlying M i d d l e O r d o v i c i a n strata.
e n v i r o n m e n t s during times of relative sediment starvation that
Evidently, the eastern edge of the c o n t i n e n t was u n d e r g o i n g s o m e
enables
stresses, perhaps associated with an initial e n c o u n t e r of eastern
decaying organic matter, especially fecal pellets. T h e glauconite
North A m e r i c a with a trench and the d e v e l o p m e n t of an offshore
granules of the Fonda M e m b e r are also associated with hashy,
volcanic island arc c o m p l e x . S o m e geologists have argued that
finely broken d o w n , skeletal remains of many types of organisms.
mineral
precipitate to b e c o m e concentrated
around
m i n o r volcanic ash input was already c o m i n g into the basin
Particularly prevalent are several species of small gastropods,
during the t i m e of the late C a n a d i a n E p o c h .
c e p h a l o p o d s , and a bivalve-like organism referred to as a ribeiroid
Early
Ordovician
deposition
was
terminated
throughout
rostroconch.
The
rostroconchs
represent
a
nonhinged
bivalve
eastern North A m e r i c a by a m a j o r fall in sea level, and an e r o -
mollusk that possibly was ancestral to the bivalves. T h e Fonda
sional u n c o n f o r m i t y , c o m m o n l y referred to as the Knox Uncon-
M e m b e r also contains rare, disarticulated fragments of trilobites.
formity,
middle
Finally the upper p o r t i o n of the Tribes Hill is represented
p o r t i o n of the O r d o v i c i a n , in places such as M o h a w k Valley, the
was
initiated
(Figure
again by stromatolitic dolostones assigned to the C h u c t u n u n d a
older c a r b o n a t e s that
C r e e k M e m b e r . T h i s unit c o n t a i n s large, but very poorly pre-
had
4.13A-D).
been
During
deposited
the
d u r i n g the
Early
ORDOVICIAN
PERIOD
61
served stromatolites referred to as " h i p p o b a c k s " where they c r o p
assigned to the Poulteney or D e e p Kill F o r m a t i o n (Figure 4 . 1 0 ) .
out, especially along C a n a j o h a r i e Creek, M o n t g o m e r y County.
Although p r e d o m i n a n t l y green, s o m e thin dark shale partings
T h e two higher packages of p r e d o m i n a n t l y d o l o m i t i c but
o c c u r within these strata, particularly in the vicinity of Deep Kill,
mollusk-containing carbonates, the Rochdale and the Fort Cassin
a small creek near Melrose, Rensselaer County. T h e s e Deep Kill
F o r m a t i o n , make up the remainder of the B e e k m a n t o w n G r o u p
black beds have yielded a highly diverse and well-preserved
of Lower Ordovician in New York (Figure 4 . 1 2 ) . T h e Fort Cassin
assemblage of graptoloid graptolites, typically preserved as silvery
shows a repeat of the same pattern observed in the lower units,
c a r b o n i z e d r e m n a n t s on the dark slaty bedding planes. T h e s e
particularly the Tribes Hill; that is, it c o m m e n c e s with a wide-
graptolites have been used to correlate the D e e p Kill rocks with
spread silt and sandstone unit, the Ward Siltstone M e m b e r , and
p o r t i o n s of the Lower O r d o v i c i a n in o t h e r parts of the world.
this in turn is overlain by a fossiliferous c o n d e n s e d limestone that
However, the bulk of the p r e d o m i n a n t l y green Poulteney or Deep
may record m a x i m u m m a r i n e flooding o f the c r a t o n o n a n o t h e r
Kill F o r m a t i o n is sparsely fossiliferous. In the Taconic allochthon
higher cycle. T h e upper part of the Fort Cassin, the B r i d p o r t
succession as on the c r a t o n , the Sauk U n c o n f o r m i t y or the Knox
Member, consists mostly of t h i n - b e d d e d to massive stromatolitic
U n c o n f o r m i t y appears to be present as a gap or break in sedi-
dolostones and records the final regression in the late part of the
m e n t a t i o n . Just why this should be so in deeper water is unclear.
Canadian Series. Brett and Westrop ( 1 9 9 6 ) recently reviewed
In fact, o n e might anticipate the o c c u r r e n c e of an increase in the
the trilobites from the Fort Cassin F o r m a t i o n .
influx of s e d i m e n t s , at least if shales were uplifted and exposed
Trilobites reported, in total, from the Lower O r d o v i c i a n are
to erosion d u r i n g the long span of the K n o x U n c o n f o r m i t y . However, the c o n t i n e n t a l slope and rise area of New England
as follows:
appear to have been relatively sediment starved during this interAcidiphorus
whittingtoni
Bathyurus
Bathyurus?
perkinsi
Bathyurellus
Bellefontia
gyracanthus
Bellefontia
Benthamaspis
striata
Clelandia
caudatus
Hystricurus Hystricurus
cf.
platypus
able volcanic ash beds that were being implaced within the
seelyi
world's oldest r a d i o l a r i a n - b a s e d , deep-sea silica o o z e deposits.
Iapetus O c e a n . Also, s o m e cherty beds represent s o m e of the
sp.
Hystricurus
H.
Hystricurus
T h e Poulteney is overlain, probably with an u n c o n f o r m i t y , by the
cassinensis
Grinnellaspis
conicus
val of t i m e . Siliceous layers within the Poulteney represent p r o b -
(?)
Bolbocephalus Eoharpes
parabola
Strigigenalis
levis
cf.
G.
marginiata
crotalifrons ellipticus
the east edge of Laurentia began u n d e r t h r u s t i n g the rest of
Isotcloidcs
canalis
Isoteloides
Isoteloidcs
whitfieldi
Paraplethopeltis
cf.
Shumardia
In the Early to M i d d l e O r d o v i c i a n , the Iapetus O c e a n no longer c o n t i n u e d to widen, and a p o r t i o n of seafloor attached to
oculilunatus
Robergiella
Indian River red slates. Trilobites are not present in these deposits.
R.
brevilingua
pusilla
Symphysurina
sp.
Symphysurus
convexus
peri
the seafloor (Figures 4 . 5 B and 4 . 1 3 C ) . T h i s a c t i o n produced a sp.
deep trench on the Iapetus O c e a n floor and a so-called sub-
Scotoharpes
cassinensis
duction zone, wherein the western plate or slab was forced d o w n -
Strigigenalis
cassinensis
ward beneath the eastern or overriding slab. T h i s interaction
Symphysurina
cf.
S.
woosteri
p r o d u c e d a n offshore island a r c — t h e Taconic o r A m m o n o o s u c Arc, a chain of volcanoes that f o r m e d on the overriding plate of p r o t o - A t l a n t i c seafloor. T h e m a g m a s (melted rock) were gener-
Allochthonous Rocks of Early Ordovician Age Lower Ordovician a l l o c h t h o n o u s rocks exposed in the western
ated by frictional heating and partial melting of the d o w n g o i n g slab and b r o k e through to the surface, f o r m i n g the volcanic chain.
part of the Taconic M o u n t a i n s closely resemble those of the
As s u b d u c t i o n of the p r o t o - A t l a n t i c seafloor c o n t i n u e d , the
Upper C a m b r i a n . Lowest Ordovician strata are represented by
eastern edge of Laurentia itself eventually was b r o u g h t close to
beds of the upper Hatch Hill F o r m a t i o n , as indicated by the pres-
the s u b d u c t i o n zone. Ultimately, the s e d i m e n t s that bordered
ence of distinctive index fossils including c o n o d o n t s . Graptolites
Laurentia along the c o n t i n e n t a l slope and deep o c e a n floor were
first b e c o m e c o m m o n in the strata of the Taconic M o u n t a i n s in
scraped o f f f r o m the d o w n g o i n g slab, f o r m i n g a s o m e w h a t c h a o t -
the earliest Ordovician, where they are represented by dendroid
ically d e f o r m e d series of slabs of s t r a t a — a n accretionary wedge.
graptolites such as the genus Dictyonema. Hatch Hill Shales, n o w
T h i s mass would b e c o m e the rocks of the T a c o n i c Allochthon (a
m e t a m o r p h o s e d in places to slates, c o n t i n u e upward from the
mass of rock ultimately displaced s o m e 80 km west of its site
Cambrian to the Ordovician. T h i n c a r b o n a t e s , sandstones, and
o f origin into the area o f present-day eastern New York). T h e
carbonate breccias, o c c u r r i n g at or near this level as well, repre-
Taconic a l l o c h t h o n was thrust or pushed up o n t o the c o n t i n e n -
sent sediments that were washed o f f the shallow p l a t f o r m of
tal shelf of Laurentia, or o n e m i g h t say that the east edge of the
North America at the end of the C a m b r i a n and earliest O r d o v i -
continental shelf was subducted beneath this mass of deformed
cian. T h e Hatch Hill dark shales give way upward in the strati-
sediments. T h e latter area also was being compressed and thrust
graphic succession to olive-greenish, slaty shales with s o m e thin
westward by collision of the A m m o n o o s u c Arc with the accre-
limestones but no debris flow breccias. T h e s e strata have been
tionary wedge and Laurentia.
THE
62
Middle Ordovician
PALEOZOIC
GEOLOGY
OF
T h e K n o x U n c o n f o r m i t y is manifested as the sharp upper units. T h e Knox U n c o n f o r m i t y , with a relief of up to several meters, due to karstification, is o n e of N o r t h America's m a j o r stratal breaks and f o r m s the subdivision
between two huge
packages of strata, referred to as Sloss supersequences: the Sauk Supersequence below and the base of the Creek phase of the T i p p e c a n o e Supersequence above (Sloss 1 9 6 3 ; Figure 4 . 1 ) .
o f trilobites: Acanthoparypha? Apianurus Bumastoides
(Basiliella)
gardenensis
Calyptaulax
annulata triacantheis granulosa
the Knox U n c o n f o r m i t y in New York are the sandstones and car-
Eobronteus Gabriceraurus
(Figure 4 . 1 3 D ) . T h e s e strata are o f middle M i d d l e Ordovician
Glaphurus
or Chazyan age (the Llandeilo Series of the British t e r m i n o l o g y )
Hemiarges
(Figures 4 . 1 3 D , 4 . 1 4 , 4 . 1 5 ) . T h e Chazy G r o u p strata evidently
Hibbertia Illaenus
Carrikia
globosus setoni
Ceraurinella
latipyga (Nieszkowskia)
Cyrtometopinid
dintoncnsis
bonates of the Lake C h a m p l a i n area, assigned to the Chazy G r o u p
comes
Bumastoides
Chcirurus
prima
Dinieropyge
accumulated in a relatively narrow, restricted trough or foreland
Bumastoides
aplatus
Bumastoides
Cybeloides
T h e oldest Middle O r d o v i c i a n strata to a c c u m u l a t e above
minganensis
Basilicas
whittingtoni
Ccratocephala
Middle O r d o v i c i a n C h a z y G r o u p
Amphilichas
sp. narrawayi
Ceraurus
hudsoni pustulosus turneri
amiculus
valcourensis crassicauda
antiquatus
Glaphurina
lamottensis
Heliotneroides Hibbertia Hyboaspis
depressa
Isotelus
angusticaudum
Isotelus Isotelus
and V e r m o n t . Laterally equivalent s e d i m e n t s of the Y o u n g m a n
Kawina?
and C a r m e n f o r m a t i o n s o f western V e r m o n t consist o f thin-
Kawina
bedded,
shales
Lonchodomas
halli
Nanillaenus?
that represent m o r e basinal a c c u m u l a t i o n s (Figure 4 . 1 3 D ) . T h e
Nanillaenus?
raymondi
Nieszko wskia
localized nature of the Chazy basin probably reflects additional
Nieszkowskia?
salyrus
Nileoides
subsidence o f the o u t e r p o r t i o n o f the continental margin o f
Otarion
Laurentia, w h i c h , d u r i n g the Middle O r d o v i c i a n , was b e g i n n i n g
Physemataspis
to e n c o u n t e r the trench or s u b d u c t i o n zone associated with the
Platillacnus
collision of a volcanic island arc.
Proetus
and
interbedded
dark
beta giganteus chazyensis vulcanus
spinicaudatum insularis limbatus
referred
to
as
the
Day
including lingulid b r a Point
Formation.
These
canalis
Isotelus
harrisi
Kawina?
sp.
Lonchodomas
chaziensis punctatus billi) lgs i perkinsi
Paraceraurus
ruedemanni
Platillacnus
erastusi
Pliomerops
canadensis
approximus Remopleurides
Pseudosphaerexochus
canadensis
vulcanus
siliciclastic sands apparently were recycled from older sandstones
Sphaercxochus
that had been e r o d e d d u r i n g the long span of the K n o x U n -
Thaleops
conformity. T h e y were deposited in shallow subtidal to inter-
Uromystrum
tidal e n v i r o n m e n t s . T h e overlying beds of the C r o w n
Vogdesia
Point
Isotelus
Pseudosphaerexochus?
clelandi
Chazy strata typically c o m m e n c e with c r o s s - b e d d e d sandstones c o n t a i n i n g sparse fossils, b u t
akacephala
sp.
o n c e extended farther to the n o r t h and east into central Q u e b e c
limestones
sp.
Eoharpes
sp.
mars
sp.
Dolichoharpes
basin that existed in present-day n o r t h e a s t e r n New York State and
chiopods,
YORK
trilobite fauna of the Chazy (Shaw 1968) includes over 60 species
contact o f the B e e k m a n t o w n G r o u p o r the underlying C a m b r i a n
ribbon-like
NEW
parvus
arctura brevispinum bearsi
Sphaerocoryphe Thaleops Uromystrum Vogdesia?
goodnovi
longispina minor obtusus
and Valcour in the Chazy G r o u p are c a r b o n a t e s that display generally deepening upward trends. C o a r s e - g r a i n e d limestones representing c r o s s - b e d d e d shoals of crinoidal and o t h e r skeletal
Middle Ordovician Allochthonous Rocks
debris occur low in the C r o w n Point F o r m a t i o n but are inter-
As previously m e n t i o n e d , the A m m o n o o s u c island arc, lay
bedded with and succeeded by n o d u l a r to wavy-bedded fine-
o u t b o a r d of N o r t h A m e r i c a in the proto-Atlantic in the region
grained limestones c o n t a i n i n g a b u n d a n t fossil fragments. Locally,
that today would be o c c u p i e d by central New England. T h e old
within
were
c r a t o n i c edge of Laurentia m a y have been uplifted to the east of
developed o n skeletal shoals (Figure 4 . 1 4 ) . T h e s e were c o m -
the Chazy trough. T h i s relatively narrow trough area lay to the
posed of sponges and b r y o z o a n s , with s o m e primitive rugose and
east, between the old continental margin (present-day central
tabulate corals. A small patch reef or b i o h e r m in a cow pasture
V e r m o n t and Massachusetts) and an accretionary prism, consist-
the
Crown
Point,
small
bioherms
or
reeflets
on Isle L a m o t t e , V e r m o n t , is often said to be the world's oldest
ing of the s e d i m e n t a r y rocks and s o m e o c e a n - f l o o r volcanics,
coral reef, although most of these b i o h e r m s were not c o m p o s e d
which were being o b d u c t e d o f f of the d o w n g o i n g o c e a n i c - f l o o r
o f corals.
plates (Figure 4 . 1 3 B , part E, F ) . T h e u p p e r m o s t or youngest sed-
Trilobites as disarticulated e l e m e n t s are rather c o m m o n and
iments of the Taconic a l l o c h t h o n succession accumulated in this
highly diverse in s o m e of the Chazy n o d u l a r limestone beds. T h e
trench during the Middle Ordovician and are approximately the
ORDOVICIAN
PERIOD
63
FIGURE 4 . 1 4 . Details of the stratigraphy of the Chazy Group in northeastern New York. From Shaw (1968), reproduced with permission.
same age as the Chazy carbonates of New York State. T h e s e strata
developed d u r i n g the l o n g - t e r m exposure of the craton during
belong to the Normanskill C r o u p and have been subdivided into
development of the K n o x U n c o n f o r m i t y . However, it also reflects
three m a j o r f o r m a t i o n s (Figure 4 . 1 0 ) . T h e lowest is a very dis-
a high degree of oxygenation of the b o t t o m waters during this
tinctive brick-red shale or slate referred to as the Indian River For-
t i m e . T h i s might be associated with the elevation of the seafloor
mation. This slate is exposed in m a n y places in the low Taconics
due to buckling and arching as the c o n t i n e n t edge of Laurentia
and has been quarried extensively tor roofing slates in the region
was driven into a t r e n c h .
of the New Y o r k - V e r m o n t border. T h e s e red slates lack b o d y
Overlying s e d i m e n t s o f the M o u n t M e r i n o F o r m a t i o n are
fossils but contain small trace fossils apparently m a d e by deep-
mostly dark gray or greenish gray siliceous shale. T h e y are also
sea burrowing organisms. T h e red coloration of the Indian River
noted for thin, r i b b o n - l i k e beds of distinctive light green cherts.
Slate is unusual and makes it particularly attractive as building
T h e s e cherts perhaps represent a c c u m u l a t i o n s o f radiolarian
stones. It reflects oxidized iron c o n c e n t r a t i o n within the m u d d y
(silicaceous microfossil)
sediments. However, the source of these iron e n r i c h m e n t s is still
seafloor. B o d y fossils are, again, extremely rare within the M o u n t
poorly understood. It might reflect highly weathered soil that
M e r i n o F o r m a t i o n . No trilobites are known.
skeletal
oozes on
the proto-Atlantic
THE
64
PALEOZOIC
GEOLOGY
OF
NEW
YORK
T h e highest and thickest p o r t i o n of the Normanskill G r o u p is
water intervals of the Black River deposition. T h e r e are no really
c o m p o s e d of the Austin Glen F o r m a t i o n ( c o m m o n l y referred to
g o o d m o d e r n analogs of such vast tidal flats with which to
as graywacke or m u d - r i c h s a n d s t o n e ) . In places, the Austin Glen
c o m p a r e the Black River depositional e n v i r o n m e n t s . Probably the
is enriched in small fragments of m e t a m o r p h i c rocks, including
closest would be s o m e of the extensive platform of the Bahama
slates
metamorphosed
Banks of today. Presumably, the Black River sediments a c c u m u -
c h u n k s o f the original proto-Atlantic seafloor s e d i m e n t s . T h e
lated in subtropical e n v i r o n m e n t s , approximately 25° south of
Austin Glen is noted for its thick sandstone deposits that are
the e q u a t o r (Figure 4 . 7 ) . However, s o m e workers have argued that
interpreted as t u r b i d i t e s , the product of deposition f r o m basin-
the overlying Trenton sediments may actually have developed
ward-flowing masses of suspended
under c o o l e r t e m p e r a t e rather than warm subtropical conditions.
that
probably
represent
uplifted
and
sediment and water that
moved due to gravity (Figure 4 . 1 5 ) . It is evident f r o m distinctive
Perhaps a climatic fluctuation did characterize the transition
flute and groove-casts on the undersides of m a n y of these thick
from the Black River to the Trenton.
turbidite beds that the sediment source now was f r o m the east,
T h e b r o a d , flat-shelf c o n d i t i o n s of the Black River deposition
probably o f f the erosion of the rising a c c r e t i o n a r y prism and vol-
indicate tectonic quiescence. However, the presence of thick b e n -
canic island arc. T h e s e rocks c o m p r i s e the highest p o r t i o n of the
t o n i t e s (volcanic ash beds) within the Black River strata indicates
Taconic strata, and they were thrust westward s o m e 80 km to their
that volcanism was o c c u r r i n g , perhaps in the A m m o n o o s u c Arc
present resting position, which approximates the position of the
at this t i m e .
modern Hudson River Valley. T h e Austin Glen does c o n t a i n o c c a -
T h e main b o d y of the Black River G r o u p consists of the
s i o n a l f o s s i l s i n places, including s o m e m a r i n e benthic fauna o f
Lowville F o r m a t i o n , also called Gull River F o r m a t i o n in O n t a r i o .
b r a c h i o p o d s , bryozoan scraps, and very rare trilobites.
T h i s is distinct, very pale gray weathering, massive micritic limestone. In past times it was c o m m o n l y referred to as the "bird's eye
Black River G r o u p
l i m e s t o n e " because of dark calcite spar-filled vugs. T h i s term
T h e rocks of the Chazy G r o u p in the C h a m p l a i n region are
originally referred to burrows of vertically excavating w o r m s ,
c o n f o r m a b l y overlain by a n o t h e r series of typically pale gray,
Phytopsis (Figure 4 . 1 6 ) . However, the term "bird's eye structure"
rather pure limestones referred to as a part of the Black River
has c o m e to refer to smaller vug- or pit-filling areas of dark calcite
G r o u p (Figures 4 . 1 3 E , 4 . 1 6 , a n d 4 . 1 7 A ) . T h e Black River G r o u p ,
also c o m m o n in the Lowville or Gull River F o r m a t i o n of the
n a m e d for exposures in the Black River Valley near W a t e r t o w n ,
Black River G r o u p . T h e s e are thought to represent originally
Jefferson C o u n t y , is a very widespread and distinctive package of
gas b u b b l e holes that developed from desiccation of c a r b o n a t e
shallow m a r i n e limestones that ranges up to 70 m thick (Figures
sediments in a tidal flat or perhaps from the decay of algal or
4 . 1 3 E , 4 . 1 6 , and 4 . 1 7 ) . Unlike the restricted Chazy, however, the
bacterial filaments within the sediments. Black River limestones
Black River limestones e x t e n d over most of central and western
contain
New York State and into the m i d c o n t i n e n t . In places, the Black
example, desiccation crack polygons are beautifully displayed on
evidence of extremely shallow-water deposition.
For
River c a r b o n a t e s rest directly on the K n o x U n c o n f o r m i t y and
m a n y bedding planes (Figure 4 . 1 6 A ) . Stromatolites also may be
c o m p r i s e the basal p o r t i o n of the Creek phase of the T i p p e c a n o e
c o m m o n locally, as are flat pebble conglomerates. T h e unit tends
Supersequence. In o t h e r areas to the northwest, the Black River
to be rather sparsely fossiliferous, although in places it contains
G r o u p rests directly on C a m b r i a n or even the Grenville (billion
spectacular, large nautiloid and e n d o c e r i d cephalopods. Scattered
year old)
unit,
trilobite fragments have been found within these rocks, but they
referred
basement to
as
rocks.
Pamelia
In these
Formation
in
regions, the basal
Shadow Lake
are not particularly c o m m o n or well preserved. Nonetheless,
F o r m a t i o n in O n t a r i o , consists of greenish gray to reddish m u d -
New York
or
the laterally equivalent Gull River Formation in O n t a r i o has
stones and sandstones, typically as p o o r l y sorted thin layers.
yielded beautifully articulated remains of the trilobite genus
W h e r e it rests on P r e c a m b r i a n b a s e m e n t , the S h a d o w Lake
Bathyurus
may incorporate cobbles o f quartzite o r other m e t a m o r p h i c
evidently ranged into relatively shallow water, as their remains
rocks. T h i s unit is n o r m a l l y unfossiliferous and is interpreted as
are associated with "bird's e y e " structures and other features
and
the
large
asaphid
Isotelus
sp.
These
trilobites
extremely shallow m a r i n e or n o n m a r i n e fluvial s e d i m e n t . H o w -
c o m m o n l y taken to indicate the upper subtidal to intertidal
ever, a few very scrappy r e m a i n s of fossils, including trilobites,
zone (Figure 4 . 1 6 B ) . Perhaps these represent carcasses that were
have been discovered from c a r b o n a t e beds in the S h a d o w Lake
stranded within the inner lagoon or tidal m u d flat e n v i r o n m e n t
F o r m a t i o n in O n t a r i o .
and buried intact.
T h e carbonates of the Black River G r o u p were deposited at a
Slightly darker gray limestones within the Black River G r o u p ,
t i m e when shallow, rather m o n o t o n o u s c o n d i t i o n s existed over
particularly near the top in what has been referred to as the House
very broad tracts of seafloor. Judging from the persistence of fea-
Creek and
tures, such as beds of m u d c r a c k s over h u n d r e d s of square miles
of the distinctive coral Tetradium, so called because of the four-
(Figure 4 . 1 6 A ) , it appears that vast areas were exposed and wetted
fold s y m m e t r y of its corallites and septa. T h e s e strata reflect
periodically by particularly high tides, at least d u r i n g shallow-
shallow-shell" lagoonal e n v i r o n m e n t s .
Watertown formations,
may contain
abundant
thickets
ORDOVICIAN
PERIOD
65
FIGURE 4.15. Flute casts on Austin Glen g r a y w a c k e ("dinosaur leather"). Basal surface of a vertically d i p p i n g b e d of M i d d l e O r d o v i c i a n sandstone turbidites shows sole marks i n c l u d i n g groove casts (a), ripple-like s c o u r s , a n d smaller flute casts (b). These features clearly indicate deposition from a turbidity current. A l l o c h t h o n o u s b e d s of low Taconics Austin Glen Formation. Cut in Rte. 9W near C o x a c k i e , G r e e n e County.
In a few locations, trilobites are f o u n d in the Lowville but
Basilicus ulrichi
Basilicus?
mostly in the upper or Watertown F o r m a t i o n (Young 1943a,
Basiliella barrandei
Bathyurus
1943b; D e M o t t 1 9 8 7 ) . T h e trilobites reported from the Black
Bathyurus johnsoni (Lowville)
Bumastoides
River are as follows':
vetustus extans
*Bumastoides milleri (Lowville)
Ceraurinella
Eoharpes pustulosus
b'ailleana
Illaenus latiaxiatiis
Isotelus
Isotelus simplex
Pterygometopus
tinguished by early authors. The trilobites with an asterisk may be early
Raymondites longispinus
*Raymonditcs
Trenton.
Thaleops ovata
The base of the Trenton and the top of the black River were not well dis-
(Lowville)
billingsi scofieldi indeterminata homalonotoides schmidti spiniger
FIGURE 4.16. Black River limestones. A. D e s i c c a t i o n c r a c k p o l y g o n s on the u p p e r surface of a b e d of fine-grained limestone. Lowville Formation, Black River G r o u p , East C a n a d a Creek near I n g h a m Mills in Fulton County. B. Beds of laminated limestone (intertidal environment) with vertical burrows (a) (Phytopsis). Note the s h a r p contact (b) with the dark gray limestone ( d e e p e r subtidal). East C a n a d a Creek at I n g h a m Mills in Fulton County.
FIGURE 4.17. M i d d l e O r d o v i c i a n Black River G r o u p . A. Overview of Black River G r o u p ( M i d d l e Ordovician) at East C a n a d a Creek, near I n g h a m Mills in Fulton County. Note the c h a n n e l in the u p p e r left a n d the sharp contact (arrow) of light gray vertically b u r r o w e d limestone over dark gray fossiliferous limestone. B. Sharp c o n t a c t (arrow) between the M i d d l e O r d o v i c i a n Watertown Limestone (Black River G r o u p ) a n d t h i n - b e d d e d N a p a n e e Limestone (Trenton G r o u p ) , Sugar River, Lewis County (north of Booneville, O n e i d a County).
G8
T H E
G E O L O G Y
O F
N E W
Y O R K
River erosion surface in m a n y localities, as the Napanee or A m s -
Trenton Group (General) A m o n g the most fossiliferous c a r b o n a t e rocks within the New York O r d o v i c i a n section are those o f the 3 0 - t o
P A L E O Z O I C
terdam F o r m a t i o n (Figure 4 . 1 2 ) .
130-m-thick
Trenton G r o u p (Figures 4 . 1 2 and 4 . 1 8 t o 4 . 2 3 ) . T h e Trenton rocks
Lower T r e n t o n G r o u p S h e l f Facies and Trilobites
record a m a j o r change f r o m shallow c a r b o n a t e shelf into a deep
T h e N a p a n e e F o r m a t i o n (Figure 4 . 1 8 ) ( 0 t o 1 0 m ) consists
foreland basin due to the thrusting of the T a c o n i c a l l o c t h o n .
o f t h i n - b e d d e d , b r o w n i s h c a r b o n a t e m u d s t o n e s o r calcisiltites
T h i s interval, which is classically exposed at Trenton Falls on West
interbedded with dark brownish gray shales that reflect a sig-
Canada Creek (Figures 4.21 to 4 . 2 3 ) , is a very widespread, highly
nificant increase in the a m o u n t of siliciclastic m u d input into
fossiliferous and c o m p l e x c a r b o n a t e succession.
the depositional system relative to the underlying clean car-
T h e lower portion of the Trenton rests sharply on the u n d e r -
b o n a t e s o f the Black River G r o u p . T h e Napanee carbonates
lying Black River G r o u p with an erosion surface that represents
are highly fossiliferous and are characterized particularly by
a regional u n c o n f o r m i t y . H e n c e , the base of the Trenton repre-
the b r a c h i o p o d Triplesia, but also a high diversity of other b r a -
sents a lesser s e q u e n c e b o u n d a r y within the T i p p e c a n o e Creek
c h i o p o d , b r y o z o a n , mollusk, and trilobite fossils. T h e s e beds
Supersequence (Figure 4 . 1 ) . Presumably, at the t e r m i n a t i o n of
are
Black River G r o u p deposition, seas were briefly withdrawn from
Flexicalymene
m u c h of eastern N o r t h A m e r i c a . However, the overlying Trenton
Napanee has been a source of debate. S o m e have argued that
strata appear to represent relatively deeper-water facies. All of the
the
Trenton G r o u p beds are highly fossiliferous.
lagoon system inboard of and protected by a c a r b o n a t e shoal.
some
of the
muddy
lowest
trilobites.
that
The
carbonates
contain
depositional
record
the
abundant
remains
environment
deposition
of a
of
of the shallow
Apparently, the t r a n s g r e s s i o n or deepening that began the
However, the widespread nature of these beds and the resem-
Trenton s e d i m e n t a t i o n was a strong or rapid o n e , of perhaps a
b l a n c e of their lithology and fauna to those of s o m e of the indis-
few tens of t h o u s a n d s of years, such that offshore l i m e m u d s and
putably deeper-water upper Trenton beds suggest a very different
silts were the first deposits that a c c u m u l a t e d above the top Black
interpretation.
FIGURE 4.18. C l o s e - u p of s h a r p Black River/Trenton ( W a t e r t o w n - N a p a n e e ) c o n t a c t . Note the massive limestone of the Watert o w n (a) a n d the thinly b e d d e d b r o w n limestone a n d shale of the N a p a n e e (b). East C a n a d a Creek near I n g h a m Mills in Fulton
ORDOVICIAN
PERIOD
69
T h e Napanee Limestone is abruptly overlain by a very thin interval
(0.5 m)
of beds
that
display a
shallow-water
River-like lithology with the return of Phytopsis and
Black
Tetradium
finds
Erratencrinurus
vigilans
and
Hemiargespaulianus,
two
trilo-
bites rarely f o u n d , if at all, in the higher units of the Trenton. Calyptaulax
callicephalus
occasionally
found
in
the
higher
rugose corals. These beds give way upward to the distinctive Kings
Trenton is c o m m o n in the Kings Falls. T h e trilobites notably
Falls Limestone, which contains a m i x t u r e of skeletal g r a i n s t o n e s
a b u n d a n t are s p e c i m e n s of Flexicalymene senaria and in the Sugar
that represent high-energy shoal e n v i r o n m e n t s and are c o m p o s e d
River
primarily
and
Cryptolithus lorettensis; the latter two fossils are considered to be
crinoids (Figure 4 . 1 8 ) . T h e grainstones are interbedded with
an index of the Sugar River ( S h o r e h a m i a n age). T h e y disappear
thinner-bedded limestones and dark gray shales that may repre-
f r o m the stratigraphic sections above the Sugar River in New
sent shallow, subtidal deposits that a c c u m u l a t e d o u t b o a r d from
York, but a related species reappears considerably higher, in the
the shoals. In turn, the Kings Falls c a r b o n a t e beds pass grada-
Lorraine G r o u p .
of
the
fragmentary
remains
of brachiopods
specimens
of the
trinucleids
Cryptolithus
tessellatus
and
tionally upward into the thicker Sugar River F o r m a t i o n ( 1 5 to
In the west, the upper Sugar River F o r m a t i o n includes a
25 m ) , which comprises, for the most part, wavy-bedded, rather
b u n d l e o f t h i n - b e d d e d micrites o r lutites ( f i n e - g r a i n e d , light
massive, slightly shaly wackestones and p a c k s t o n e s .
gray weathering l i m e s t o n e s ) , or R a t h b u n M e m b e r , that alternate
T h e Sugar River and Kings Falls units thin eastward to the
with dark gray shales capped by m o r e skeletal grainstones. T h e s e
vicinity of C a n a j o h a r i e , M o n t g o m e r y C o u n t y , where they are
seem to represent distal s t o r m deposits or lime m u d turbidites
locally pinched out near the crest of an apparent arch feature
that were a c c u m u l a t e d from shallower-shelf regions from the
on the Ordovician seafloor referred to as the Canajoharie Arch
n o r t h and west. T h e R a t h b u n appears to pass eastward into dark
(Figures 4.12 and 4 . 2 0 ) . A sharp discontinuity in this area sepa-
brownish gray shales in the vicinity of Little Falls, H e r k i m e r
rates the Sugar River, or Glen Falls L i m e s t o n e , the eastern New
C o u n t y . T h e y are typically rich in small, r a m o s e b r y o z o a n s and
York equivalent of the Sugar River, from the overlying dark gray
also m a y c a r r y a b u n d a n t trilobites of a n u m b e r of species, but
calcareous shales. T h e s e beds are particularly noted for their diverse assemblages of bryozoans, b r a c h i o p o d s , and trilobites. In the Kings Falls, o n e
particularly
dominated
by
Flexicalymene
sp.
Cryptolithus
lorettensis.
FIGURE 4 . 1 9 . C l o s e - u p of storm b e d s in the Middle Ordovician Kings Falls Limestone (Trenton Group). Note the c h a n n e l e d b a s e of b e d (arrow) midway up the h a m m e r handle. East C a n a d a C r e e k near Ingham Mills in Fulton County.
tes-
sellatus is mostly replaced in the R a t h b u n by a variant called C.
FIGURE 4 . 2 0 . M a p s of New York d u r i n g late M i d d l e O r d o v i c i a n times. A. Kirkfieldian d u r i n g deposition of the lower Trenton. B. S h e r m a n i a n d u r i n g the d e p o s i t i o n of the m i d d l e Trenton. From Isachson et al. (1991). Printed with permission of the N e w York State M u s e u m , Albany, N.Y
ORDOVICIAN
71
PERIOD
Trilobites of the lower T r e n t o n , up through the Sugar River are as follows:
C o m m o n l y the beds display a basal surface that may be overlain by a thin (millimeters thick) layer of shell and trilobite hash. U p p e r parts of the beds m a y be laminated but typically show disruptions due to b u r r o w i n g . C o n t a c t s with the overlying c h o c o -
Amphilichas
trentonensis
Bathyuropsis
schucherti
Bumastoides
porrectus
Bumastoides
trentonensis
late-brown shales may also display well-preserved fossils, such as
callicephalas
Calyptaulax
eboraceous
articulated
pleurexanthemus
Cryptolithus
lorettensis
ticularly c o m m o n in beds of the Poland near Trenton Falls. Large
C.
s p e c i m e n s of Isotelus gigas are also locally a b u n d a n t , and a bed
Calyptaulax Ceraurus Cryptolithus
tesscllaius
Cyphoproetus
cf.
Encrinuroides
cybeleformis
Encrinuroides
Eomonorachus
convexus
Erratencrinurus
Flexicalymene
senaria
Gabriceraurus
Gravicalymene
Hemiarges
wilsonae
/.
Isotelus
gigas
Isotelus
latus
near the top of the Poland at Trenton Falls apparently yielded the
vigilans
m a j o r i t y of the well-preserved, black, articulated I. gigas specim e n s that are f o u n d in m u s e u m s a r o u n d the world. T h i s is a
dentatus
relatively coarse bed of skeletal debris, a p p r o x i m a t e l y 30 cm thick,
paulianus
Illaenus Isotelus
served, mostly inverted, on the base of this b e d . T h e s e evidently
latidorsatus
represent o r g a n i s m s that were caught up and buried in a graded
jacobus
Plaiylichas ingalli
Triarthrus
is par-
which fine upward into c a r b o n a t e silt and trilobites were pre-
conradi
Raymondites
T h e trilobite Flexicalymene senaria
trentonensis
magnotuberculata Illaenus cf.
trilobites.
debris bed deposited by s t o r m waves.
inconsuetus
Sceptaspis
bebryx
Various features of the Poland beds strongly suggest that they
beckii
represent s t o r m - d e p o s i t e d c a r b o n a t e s t r a n s p o r t e d to offshore, n o r m a l l y quiet water e n v i r o n m e n t s . A relatively diverse fauna
M i d d l e and Upper T r e n t o n G r o u p
o c c u r s in these beds. In addition to trilobites, varied b r a c h i -
T h e middle p o r t i o n of the Trenton has been referred to as the Denley or Denmark Formation ( 1 9 3 7 , 1943,
(Figures 4 . 2 1
and 4 . 2 2 ) .
Kay
o p o d s and b r y o z o a n s , especially small s p e c i m e n s of Prasopora, are a b u n d a n t within s o m e layers in the Poland. F o r a listing of
1968) subdivided the middle Trenton into two
the trilobites of the Denley F o r m a t i o n , see the publications by
m e m b e r s : the Poland and the Russia, b o t h defined in the T r e n t o n
Titus ( 1 9 8 2 , 1 9 8 6 ) and D e l o ( 1 9 3 4 ) for discussions o f the trilo-
Falls area. M o r e recently, the middle Trenton units of the Poland
bites and their c o m m u n i t i e s .
and Russia M e m b e r s were grouped into the D e n l e y L i m e s t o n e . T h e s e units
Toward the top of the Poland interval occurs a series of thin
represent a transition f r o m d e e p - s h e l f to m o r e
b e n t o n i t e beds. W e a t h e r i n g o f these beds typically f o r m s n o t c h e s
shallow-shelf encrinal limestones. T h e y pass eastward in dark
in o u t c r o p s , such as those seen near the top of the classic
gray and black shales, the Flat C r e e k F o r m a t i o n , that signify c o l -
S h e r m a n Falls section at Trenton Falls, H e r k i m e r C o u n t y (Figure
lapse of the eastern Laurentia shelf area into a foreland basin as
4 . 2 2 ) . T h e top of the Poland is b o u n d e d by a distinctive wide-
the Taconic O r o g e n y set in.
spread interval including two key b e n t o n i t e s . T h e s e two beds,
In the area around Middleville, H e r k i m e r C o u n t y (in par-
separated by 0 . 5 - to 1-m n o d u l a r l i m e s t o n e , f o r m the base of the
ticular, outcrops on City B r o o k ) , the thin beds of the R a t h b u n
Russia M e m b e r (Figure 4 . 2 3 ) . T h e Russia, overall a b o u t 2 3 m
Member
fossiliferous
thick, is divisible into a series of small-scale coarsening upward
limestone about 1 m thick that marks the base of the Denley
cycles. Each cycle c o m m e n c e s with dark shale and thin tabular,
F o r m a t i o n . T h i s interval, the " C i t y B r o o k b e d " o f the Poland
very-fine-grained micritic limestones similar to those seen in
M e m b e r , is exceptionally rich in fossil nautiloid and e n d o c e r i d -
parts o f the Poland. T h e tops o f these cycles are c o m p o s e d o f
cephalopods. T h e City B r o o k bed carries the distinctive coiled
m o r e thickly bedded, nodular, fossil-rich l i m e s t o n e . A particu-
nautiloid
are
capped
Trocholites
by
nodular,
larly distinctive package of fine, evenly b e d d e d , light gray weath-
particularly
ering limestones o c c u r s near the top of the Russia in areas around
c o m m o n . T h i s interval is also rich in the remains of Flexicalymene,
Trenton Falls (Figure 4 . 2 3 ) . Like the Poland, the Russia M e m b e r
mostly disarticulated, but s o m e of which are preserved as enrolled
yields a b u n d a n t and c o m m o n l y well-preserved fossils. Certain
individuals. T h i s c e p h a l o p o d - and trilobite-rich bed is considered
bedding planes again display completely articulated trilobites
to represent a condensed h o r i z o n , that is, a b e d that records a
and crinoids, indicating very rapid pulses of burial. As with the
considerable a m o u n t of t i m e in a relatively thin interval. Such
P o l a n d , it is t h o u g h t that m a n y of these thin limestones repre-
cephalopod beds are typical of times of relatively rapid deepen-
sent distal w a s h o f f of c a r b o n a t e silt sands and m u d s following
ing when siliciclastic sediments appear to b e c o m e trapped in
times when s t o r m s disrupted the shallow shelf. S t o r m s resus-
nearshore areas and carbonates are not produced at a high rate.
pended finer c a r b o n a t e silts and m u d s and incorporated them
although
was
distinctive
the
Trocholites bed,
and
a
the
previously latter
fossil
referred is
not
to
as
T h e remainder o f the Poland M e m b e r consists o f thin- t o
into basin-flowing g r a d i e n t c u r r e n t s . T h e s e currents carried the
m e d i u m - b e d d e d , fine-grained limestones with c h o c o l a t e - b r o w n
sediments d o w n a gently sloping shelf or r a m p to the southeast.
shale partings (Figure 4 . 2 2 ) . M a n y of the limestones are rather
Nodular beds within the Russia represent s o m e w h a t shallower
fossiliferous and c o m p o s e d mainly of c a r b o n a t e silt (calcisiltite).
water e n v i r o n m e n t s that were disturbed b o t h by w i n n o w i n g
FIGURE 4 . 2 1 . M i d d l e Trenton at Trenton Falls. A. C o n t a c t of the Poland (a) a n d Russia (b) M e m b e r s of the Denley Formation ( M i d d l e O r d o v i c i a n Trenton G r o u p ) , m a r k e d by a d e e p re-entrant (arrow at b a s e of s h a d o w ) of Kayahoora bentonite b e d s . West C a n a d a Creek a b o v e S h e r m a n Falls, Trenton Falls, O n e i d a County. B. Overview of the M i d d l e O r d o v i c i a n Trenton G r o u p , s h o w i n g the Russia M e m b e r (a) of the Denley Formation a n d the Rust Formation (b). U p p e r H i g h Falls, Trenton Falls on West C a n a d a Creek, O n e i d a County.
ORDOVICIAN
73
PERIOD
FIGURE 4.22. M i d d l e O r d o v i c i a n Trenton G r o u p limestone (mainly Poland M e m b e r of the Denley Formation) at S h e r m a n Falls, West C a n a d a Creek, Trenton Falls, O n e i d a County. Note the d e e p l y r e c e s s e d n o t c h e s (arrow) m a r k i n g the positions of bentonites (volcanic ash b e d s ) near the t o p of the falls.
currents and by b u r r o w i n g o r g a n i s m s that flourished in the
Ash
somewhat better-oxygenated shelf e n v i r o n m e n t s . T h e sediment
the base of a thin shaly interval at the b o t t o m of the Rust
(Figure
4.23A).
This
critical
marker
bed
occurs
at
shutoff associated with the t o p s of t h e cycles allowed develop-
Formation.
m e n t of s o m e thin skeletal hash limestone deposits that capped
Approximately 17 m above the base of the Rust is an interval
the cycles. Abrupt deepening above these caps is recorded by
up to 3 m thick in which the beds are heavily c o n t o r t e d and
a
d e f o r m e d (Figure 4 . 2 3 B ) . In places, individual beds appear to
shift
back
to
dark
shales
and
then
to
very-fine-grained
limestones.
have b e e n o v e r t u r n e d u p o n o n e a n o t h e r and doubled along
Overlying the Denley F o r m a t i o n , near Trenton Falls, is the Rust
F o r m a t i o n , approximately 30 m of rather nodular, fos-
r e c u m b e n t folds. In o t h e r areas such as near the spillway of the power d a m
within Trenton G o r g e , these d e f o r m e d
intervals
siliferous, poorly bedded, t h i n - b e d d e d limestone (Figure 4 . 2 3 ) .
appear to be confined to lenticular channel-like features. T h e s e
T h e Rust
begins abruptly with a very distinctive b e n t o n i t e
beds record an interval of s l u m p i n g of sediment on the Trenton
bed exposed in the upper High Falls of the Trenton G o r g e on West
seafloor that may have b e e n the result of tectonic steepening
Canada Creek in H e r k i m e r C o u n t y , referred to as the High Falls
o f the c a r b o n a t e r a m p . T h i s d e f o r m a t i o n together with the
FIGURE 4.23. Trenton at Trenton Falls. A. View of U p p e r H i g h Falls s h o w i n g c o n t a c t of the t h i n - b e d d e d , light gray w e a t h e r i n g Russia Member, Denley Formation (a), overlain by the darker Rust Formation (b). C o n t a c t is m a r k e d by a re-entrant (arrow) at the position of the H i g h Falls bentonite. Trenton Falls on West C a n a d a Creek, O n e i d a County. B. C o n t o r t e d b e d s (a) in the m i d d l e portion of the Rust Formation. Thin b e d s near the b a s e (b) are the Walcott-Rust Quarry b e d s . Spillway at the p o w e r d a m within the Trenton Falls G o r g e , Herkimer County.
ORDOVICIAN
PERIOD
7b
abundant thin bentonite (or volcanic ash) layers in the D e n l e y -
fine structure of appendages was well preserved by very early
Rust interval signals the onset of m a j o r tectonic activity in the
calcite infilling. T h e s e fossils were the first trilobites for which
Taconic belt east of the present Hudson River Valley. T h e s e
appendage structure was carefully analyzed and d o c u m e n t e d
channel-like
features are still
poorly u n d e r s t o o d
but
might
(Walcott 1 8 7 5 a , b , c , d ) . T h i s s a m e bed yielded abundant speci-
represent m i n o r bypass channels eroded into the underlying c a r -
m e n s of C. pleurexanthemus, m o s t (at least 9 5 % ) in an inverted
bonate muds and then infilled with n o d u l a r shaly limestones,
position, along the lower surface and in c o n t a c t with the brown-
which in turn were slumped during a t i m e of tectonic distur-
ish gray interbedded shale. T h i s parting also has produced nearly
bance.
c o m p l e t e crinoids
There
is
no
doubt
that
this
slumping
deformation
( w i t h o u t the holdfasts)
and several other
occurred in relatively soft, wet sediments on the seafloor. T h e
species of whole trilobites. T h e q u a r r y limestone beds appear to
upturned edges of the folded beds in the middle part of the
represent distal deposits of s e d i m e n t s that were resuspended,
Rust are truncated by an erosion surface and overlapped by a bed
probably during s t o r m s in shallower water areas, and imported
of crinoidal grainstones that c o n t a i n s a breccia of pebbles derived
onto
from the underlying d e f o r m e d beds. T h e highest beds of the
Typical c o n d i t i o n s in this area were low-energy, soft, muddy
a
southeastward-sloping c a r b o n a t e
ramp
environment.
Rust Formation are t h i n - b e d d e d , coarser-grained crinoidal lime-
substrate settings that s e e m e d to favor a variety of delicate
stones that appear to b e c o m e s o m e w h a t thicker and coarser
b r y o z o a n s , c r i n o i d s , cystoids, and at least 21 reported species of
upward, perhaps in transition to the overlying massive crinoidal
trilobites, including the following:
grainstones of the Steuben F o r m a t i o n . T h e Rust limestones c o n t a i n very a b u n d a n t trilobite and
Achatella
achates
Amphilichas
crinoid debris, bryozoans, b r a c h i o p o d s , and particularly n a u -
Amphilichas
tiloid cephalopods. T h e beds tend to be rather poorly defined
Bumastoides
and c o m m o n l y s o m e w h a t a m a l g a m a t e d (bundled together) in
Bumastoides
porrectus
Calyptaulax
the lower portion of the Rust.
Calyptaulax
eboraceous
Ceraurus
A package of t h i n - b e d d e d
limestones referred to as
cornutus
conifrons
Amphilichas
decemsegmentus
inaequalis
Bumastoides
holei callicephalus pleurexanthemus
the
Diacanthaspis
parvula
Flexicalymene
VValcott-Rust Q u a r r y beds, o c c u r r i n g a b o u t 12 m above the base
Gabriceraurus
dentatus
Gerasaphus
of the Rust F o r m a t i o n s , has yielded exquisitely preserved fossils
Hypodicranotus
(Figure 4 . 2 3 B ) . T h i s t h i n - b e d d e d and rather fine-grained inter-
Isotelus
val in the Rust F o r m a t i o n sharply overlies burrowed fossiliferous
Nanillaenus
limestone that caps a shallowing cycle within the lower Rust.
Sphaerocoryphe
striatulus
Isotelus
walcotti
senaria ulrichiana
gigas
Meadowtownella
americanus
trentonensis
Proetid sp.
robusta
T h e presence of m o r e distal, deeper-water facies suggests that this interval represents a time of m i n o r deepening within the basin
T h e strata above the q u a r r y beds interval including the upper dis-
due to either tectonics or sea-level rise. T h e W a l c o t t - R u s t Q u a r r y
turbed zone c o m p r i s e t h i n - b e d d e d , very nodular, shaly fossili-
beds interval exposed near the f o r m e r Rust farm estate was dis-
m e n i d ferous wackestones and packstones that are particularly
covered and initially excavated by Charles Walcott. Collections
rich
from this site were sold to the M u s e u m of C o m p a r a t i v e Z o o l o g y
Platystrophia. Nautiloids are also c o m m o n , but trilobites are less
at Harvard in the 1870s and were listed in a paper by Delo ( 1 9 3 4 ) .
a b u n d a n t than below and are typically fragmentary. T h e s e beds
in
strophomenid
brachiopods
such
as
Rafinesquina
and
T h e Walcott-Rust Q u a r r y beds consist of sharply based a n d , in
record heavily b u r r o w e d sediments deposited in shallow, s t o r m -
s o m e cases, graded layers of fossil debris and c a r b o n a t e silt or
wave influenced e n v i r o n m e n t s .
m u d . Fossils o c c u r both on bedding planes and within s o m e of
T h e Rust grades up into the Steuben L i m e s t o n e , a heavy
the very-fine-grained beds. Remains of these o r g a n i s m s were
bedded
caught up within the turbid flows that carried the c a r b o n a t e
w i n n o w e d c r i n o i d remains and indicates a p e r i o d of shallow
m u d . Carcasses or living o r g a n i s m s may have been transported
water above wave-base. Although trilobites have been reported
crinoidal
grainstone.
The
Steuben
is
composed
of
a short distance locally before being deposited and very rapidly
from the c o m p r e s s e d shale between the beds of the S t e u b e n , they
covered by the settling c a r b o n a t e m u d s .
are u n c o m m o n .
However, in s o m e
instances, organism remains were buried exactly in situ as evidenced by upright cystoids and b r y o z o a n s encased within a l i m e -
In
the
Poland,
H e r k i m e r C o u n t y , area the entire
upper
Trenton also appears to u n d e r g o a rapid t h i n n i n g and passage
stone bed. Most of the carbonates represent individual c a r b o n a t e
into gray shales and calcilutites to the southeast. T h e Rust beds
event beds, either gradient current deposits or turbidites that fol-
and the underlying Russia are b o t h very c o n d e n s e d and thin
lowed turbulent scouring and resuspension in upslope areas. T h e
in the vicinity of Middleville,
original quarry site was re-excavated and the layers e x a m i n e d for
entire Rust M e m b e r , over 27 m at Trenton Falls, is thinned to just
trilobites in s o m e recent work (Brett et al. 1 9 9 9 ) . In particular, a
under 2 m.
bed at the base of the quarried interval yielded a series of partially enrolled F.
senaria and C. pleurexanthemus, in which
the
H e r k i m e r C o u n t y , where the
North and west of the M o h a w k Valley, the Hillier Limestone overlies the Steuben L i m e s t o n e in a generally deepening upward
76
THE
PALEOZOIC
GEOLOGY
OF
NEW
YORK
progression. T h e Hillier consists of shaly, nodular limestone with
upward with a series of thin ash beds into the overlying Dolgeville
abundant
Formation.
cephalopods
and
a
few
Flexicalymene specimens.
Lag
deposits between the Hillier and the overlying black shales ( D e e r
To the east, in the Hudson Valley area, the black Flat Creek or
River F o r m a t i o n ) with bored p h o s p h a t e pebbles c o n t a i n the trilo-
C a n a j o h a r i e shales grade into a thick succession (over 5 0 0 m ) of
bites
dark gray shales and turbidite sandstones referred to as the Snake
Ceraurus
sp.,
Flexicalymene
sp.,
and
Pseudogygites
latimar-
ginatus. T h e latter, a c o m m o n trilobite from the equivalent beds
Hill
in O n t a r i o , is only k n o w n f r o m the Hillier and the base of the
rapidly in a trough that lay just west ( m o d e r n directions) of the
Deer River in New York.
Formation
(Figure
4.20B).
These
sediments
accumulated
rising Taconic thrust mass. T h i s is evident not only because the
Trilobites are c o m m o n and diverse in the upper Trenton strata
thrust sheets lie o n , or in, the Snake Hill Shale, but also because
from Middleville northwest into O n t a r i o . W i t h i n the Trenton
the Snake Hill includes pebbles and even huge blocks of rocks
above the Sugar River are f o u n d the following:
derived from the old shelf edge to the east. T h e s e include limestone blocks that c o n t a i n disarticulated trilobite parts, evidently
Achatella
achates
Amphilichas
cornutus
Bumastoidcs
decemsegmcntus
Bumastoides Calyptaulax Ceraurus
inaequalis
lapsed as debris flows into the Snake Hill Shales.
Bumastoidcs
eboraceous
Ceraurinus
vigilans dentatus
Gravicalymene
magnotuberculata
gigas
Kawina
" b u l l d o z e d " by the i n c o m i n g Taconic thrust sheets and then col-
Amphilichas Calyptaulax
Erratenerinurus
Isotelus
conifrons
porrectus pleurcxanthemus
Gabriceraurus
Amphilichas
T h e eastern equivalent of the highest Trenton beds is a very
callicephalus marginal us
Little
Falls
exit,
called
the
Dolgeville
F o r m a t i o n . Dolgeville consists o f thin ( 5 t o 2 0 m m ) platy beds o f very-fine-grained limestone alternating with black shale (Figure
ulrichiana
4 . 2 4 A ) . T h e black and light bed striping are a very striking m o t i f
Gerasaphus Hypodicranotus
striatulus
walcotti
Lonchodomas
Proetid sp.
Pseudogygites Triarthrus
in weathered o u t c r o p s . T h e thin limestones are believed to be turbidites of l i m e m u d washed o f f the shallow platform (Rust to Steuben depositional area) to the west. T h e shaly intervals of the Dolgeville
americanus
contain
Triarthrus
beckii.
T h e top of the Dolgeville beds shows deformation presumably due to s l u m p i n g on the seatloor prior to deposition of the over-
latimarginatus Triarthrus
the
senaria
Nanillaenus
robusta
near
parvula
Meadowtownella trentonensis
Sphaerocoryphe
Thruway,
Flexicalymene
hastatus
Irenlonensis
distinctive rock unit, well exposed along the New York State
Diacanthaspis
Isotelus (Pseudosphaerexochus)
holei
bcckii
lying black shale. All of this suggests strong tectonic influence of the T a c o n i c O r o g e n y in the foreland basin at this time (Figures 4 . 2 0 B and 4 . 2 5 ) . T h e Dolgeville F o r m a t i o n and Steuben Lime-
catoni
stone are abruptly overlain by black shales of the Indian Castle Eastern T r e n t o n Equivalents a n d Utica Shale T h e eastward fate o f the upper p o r t i o n s o f the Trenton G r o u p remains s o m e w h a t u n c e r t a i n , pending correlation o f b e n t o n i t e
( U t i c a ) F o r m a t i o n (Figure 4 . 2 4 B ) . T h i s implies abrupt deepening over m u c h of New York State due to an episode of tectonic Subsidence.
beds within the section. T h e Poland and Russia M e m b e r s appear
On the basis of the m e t a b e n t o n i t e s and graptolites, it can be
to undergo a transition into p r e d o m i n a n t l y dark gray to black
shown that the Indian Castle Shale is a westwardly thinning
platy shales in the vicinity of Caroga Creek, Fulton C o u n t y , and
wedge of rock with progressively higher units onlapping the
St. Johnsville, M o n t g o m e r y C o u n t y . However, p o r t i o n s of the
Trenton U n c o n f o r m i t y in a westwardly direction. T h e s e black
Rust appear to e x t e n d eastward as a calcareous z o n e , " t h e W i n -
shale beds appear c o n f o r m a b l e with the underlying Dolgeville
tergreen Flats b e d s " that c a r r y Prasopora and c a l y m e n i d trilobite
in eastern M o h a w k Valley. Finally, near Middleville, H e r k i m e r
material at least as far east as Flat C r e e k near Sprakers, M o n t -
Count}', black shales of the u p p e r Utica b e d s rest with sharp dis-
g o m e r y County. D a r k Flat C r e e k ( C a n a j o h a r i e ) Shales, p a r t i c u -
c o n f o r m a b l e c o n t a c t on a c o r r o s i o n surface in the upper Steuben
larly those overlying the W i n t e r g r e e n Flats Prasopora b e d , contain
Limestone.
an a b u n d a n c e of the trilobite
Triarthrus beckii. T h e shales also
T h e dark Flat Creek and Snake Hill shales that are laterally
b e c o m e quite rich in graptolite fossils that are dated as near the
equivalent to the upper Trenton are sparsely fossiliferous and rep-
top of the Corynoides americanus graptolite zone. A dark gray or
resent deeper-water e n v i r o n m e n t s with high rates of sediment
black shale bed overlies the W i n t e r g r e e n beds in the vicinity of
accumulation.
C a n a j o h a r i e and appears to represent a m a j o r deepening within
sparingly.
Graptolites and
Triarthrus specimens are found
the section. T h i s interval may c o i n c i d e with the W a l c o t t - R u s t
In the H u d s o n Valley the Snake Hill Shale also contains debris
Q u a r r y beds in the Trenton Falls area. It is the zone that carries
flows and b l o c k s of exotic material that were thrust in from the
the m o s t a b u n d a n t Triarthrus beckii in the central and eastern
eastern area. O n e such mass is the Rysedorph C o n g l o m e r a t e .
M o h a w k Valley region. T h i s dark gray shale, newly t e r m e d the
Two miles southeast of Rensselaer, New York, Rysedorph Hill
"Valley B r o o k S h a l e " (Brett and Baird, in press), appears to pass
is underlain by an unusual c o n g l o m e r a t e , the Rysedorph C o n -
FIGURE 4 . 2 4 . Middle Ordovician Dolgeville and Utica rocks. A. Middle Ordovician Dolgeville Formation sharply overlain by Indian Castle (Utica Group) black shale (upper white arrow at the contact). Note the thin "ribbon limestones" and black shale, and the small folds (lower white arrow) in the Dolgeville beds. New York State Thruway, 1.6km (1 mile) west of the Little Falls exit, Herkimer County. B. Black Indian Castle (Utica Group) shale, with light weathering limestone beds. East Canada Creek, Dolgeville, Herkimer County.
78
THE
PALEOZOIC
GEOLOGY
OF
NEW
YORK
g l o m e r a t e . Various pebbles within this unit c o n t a i n fossils f r o m
center of the Utica trough, which extended eastward toward New
C a m b r i a n t o U p p e r O r d o v i c i a n . S o m e o f the M i d d l e Ordovician
England.
trilobites o b t a i n e d by R u e d e m a n n ( 1 9 0 1 ) are n o t f o u n d elsewhere in New York, but s o m e are k n o w n in Virginia. M o s t of
Lorraine Group
these remains are fragmentary, and the actual species assign-
Strata of the late p o r t i o n of the Ordovician Period in New
m e n t s may not be correct in all cases. T h e trilobites include the
York State reflect the filling and overfilling of the Taconic fore-
following:
land basin. In eastern and east-central New York State these strata above the Schenectady F o r m a t i o n have been removed by a s u b -
Achatella
achates
Calyptaalax
callicephalus
Ceraurus Cybele
Calyptaulax Ceraurus
pleurexanthemus
Isotelus
ulrichiana
Lonchodomas Otarion? Thaleops
Tretaspis
ovata
Tretaspis
the F r a n k f o r t ) , and Pulaski
americanus
m e m b e r s , and the Oswego and
Q u e e n s t o n f o r m a t i o n s (Figures 4 . 1 2 and 4 . 2 5 ) . T h e Frankfort-
linguatus
Sphaerocoryphe
tumidus
G r o u p with its Frankfort, W h e t s t o n e G u l f (western equivalent of
maximus
Remopleurides
matutinum
Remopleurides
filling phases of the Taconic Basin are recorded in the Lorraine
senaria
Nanillaenus
hastatus
M o h a w k Valley region and northwestern New York State, the
lunatus
Flexicalymene
Gerasaphus
Silurian and Early Devonian deposits. However, in the western
sp.
Eobronteus
sp.
sequent period of uplift and erosion prior to onlapping of
eboraceous
Pulaski succession c o m m e n c e s with dark gray shales and shows
major
a general c o a r s e n i n g - u p w a r d trend through siltstones and sand-
reticulata
stones. Parallel with this is a change from anoxic to fully oxy-
diademata
genated seafloor c o n d i t i o n s reflected in increasingly diverse fossil assemblages. Overall, the seafloor was shallower due to fill-in or
Late Ordovician
p r o g r a d a t i o n of the sediments from the east.
Utica S h a l e - S c h e n e c t a d y F o r m a t i o n
L o r r a i n e G r o u p E n v i r o n m e n t s a n d Trilobites
D u r i n g later Middle O r d o v i c i a n , c a r b o n a t e deposition ceased
T h e Frankfort F o r m a t i o n , o f the Lorraine G r o u p , consists o f
and was abruptly followed by deposition of widespread, black
a series of dark gray to black shales with interbedded, thin, silty
Utica Shale facies (Figure 4 . 2 5 ) . T h e eastern p o r t i o n o f the
turbidites and o n e significant package (Hasenclever) of fine-
basin was b e g i n n i n g to receive increased a m o u n t s of coarse
grained, m u d d y sandstones. T h i s package appears to represent a
siliciclastic sediments, primarily siltstone and sandstone turbidite
m i n o r sea-level lowstand or a drop in the relative sea level that
beds,
caused progradation or westward migration of the coarser silici-
Schenectady
eroding
Taconic
Formation,
Mountains
which
during
occur
the
closer
same
time
to
the
interval
(Figure 4 . 2 5 B ) .
clastic silt and sand facies over m u c h of central New York. This b u n d l e , however, is overlain by a return to dark gray shales,
T h r o u g h o u t m u c h o f the M o h a w k Valley, alternating c a r b o n -
referred to as Moyer Creek M e m b e r in the Utica region, and Deer
ates and shales of the Dolgeville F o r m a t i o n ( p r o b a b l e equivalents
River dark shales to the northwest. T h e s e upper Frankfort shales
of the u p p e r m o s t Rust or Steuben limestones) are abruptly over-
are notable in the area in R o m e , New York, for yielding extraor-
lain by a black, rusty weathering l a m i n a t e d shale, Utica (Indian
dinarily preserved fossils. T h e s e are displayed in thin beds of silty
Castle), with a b u n d a n t thin (1 to 2 cm thick) clay layers that r e p -
shale referred to as Beecher's Trilobite Bed that is exposed along
resent m e t a b e n t o n i t e s .
S i x M i l e Creek n o r t h of R o m e . T h e s e beds have yielded spectac-
These black shales are referred to herein as the Indian Castle Formation of the Utica Shale G r o u p .
T h e trilobite
Triarthrus
ularly
pyritized
Triarthrus
eatoni
specimens
with
preserved
appendages, in significant n u m b e r s in o n e thin band. T h e s e trilo-
of
bites were buried very rapidly by a thin, silty turbidite, probably
O n t a r i o , is found in the upper Indian Castle Shale i m m e d i a t e l y
c o m i n g out of the then-developed deltaic regions to the east. Very
spinosus, above
normally the
seen
Steuben
in
the
Limestone
Collingwood near
Holland
Formation Patent,
Oneida
early infilling of the appendages by fine-grained pyrite preserved these fossils exceptionally well. T h e y have been the subject of
County. Throughout
the
Mohawk
Valley
region,
black
shales
of
n u m e r o u s a n a t o m i c a l and t a p h o n o m i c studies. T h e similarly
varying age ranging up to 3 0 0 m thick constitute a relatively
pyritized
m o n o t o n o u s facies c o m p o s e d largely of black laminated g r a p -
beecheri are present but relatively rare.
tolite-rich shales. O n l y certain beds of the Indian Castle c o n tain o t h e r fossils, such as highly flattened o r t h o c o n i c nautiloids and pear
the trilobite to
have
Triarthrus eatoni.
been
particularly
Again, these trilobites a p adapted
to
the
trilobites
Exceptionally
Cryptolithus
well-pyritized
bellulus
cephalopod
arthrus s p e c i m e n s are also k n o w n
and
Cornuproetus?
remains
and
from the Frankfort
Tri-
(Deer
River) shales in the vicinity of Constableville, Lewis County, and
low-energy,
W h e t s t o n e Gulf, Lewis C o u n t y , in northwestern New York State.
low-oxygen e n v i r o n m e n t s represented by the Utica Shale deposi-
T h e s e beds show increasingly thick, silty and sandy beds that rep-
tion. Waters were probably relatively deep and stagnant in the
resent distal or m o r e proximal turbidites or storm deposits; these
FIGURE 4 . 2 5 . New York maps during the late Middle Ordovician and the Upper Ordovician. A. Late Middle Ordovician. B. Late Ordovician. C. Cross section of the plate movement during Late Ordovician. From Isachson et al. (1991). Printed with permission of the New York State Museum, Albany, N.Y.
80
THE
PALEOZOIC
GEOLOGY
OF
NEW
YORK
beds are interbedded with m e d i u m to b r o w n i s h gray shales that
(Figure 4 . 2 8 ) . Locally, bundles o f f i n e - g r a i n e d sandstone, s o m e o f
may represent b a c k g r o u n d c o n d i t i o n s . T h e s t o r m beds typically
t h e m c o n t a i n i n g a n i m a l burrows, are also present within the
display a hash or lag of shelly material that includes b r a c h i o p o d s ,
Q u e e n s t o n , for e x a m p l e , at Rochester. However, the presence of
bivalves, and c r i n o i d stems on their bases. T h e s e coarser skeletal
small, limey nodules, referred to as calcretes, in certain horizons
debris layers grade upward into fine grainstones and siltstones
suggests the development of s o i l - f o r m i n g features within the
that c o m m o n l y display h u m m o c k y c r o s s - l a m i n a t i o n ; these low-
Q u e e n s t o n F o r m a t i o n . T h e presence o f calcretes implies long
m o u n d e d l a m i n a t i o n s , typically c o n v e x - u p w a r d , are thought to
periods of aerial exposure during the deposition of these muds.
be f o r m e d by the interference of s t o r m waves and currents in rel-
We visualize the Q u e e n s t o n as representing a broad alluvial flood-
atively shallow waters. Tops of s a n d s t o n e beds also display fea-
plain of possibly m e a n d e r i n g streams that originated in the
tures such as interference and w a v e - f o r m e d ripple marks that
Taconic Highlands and spread sheetlike masses of sediment over
further indicate a relatively shallow-water origin for these beds.
a wide tract of the Appalachian Basin. Red coloration, together
T h e Pulaski beds are d o m i n a t e d by b r a c h i o p o d s , c l a m s , and a few
with the calcretes, indicates subaerial exposure for m u c h of the
species of crinoids. However, trilobites are k n o w n at certain levels
unit in New York State, at least. However, to the west, the Q u e e n -
and
ston interfingers with the gray calcareous m u d s t o n e and even
include
Cryptolithus
bellulus,
as
well
as
some
Flexicalymene
species and Isotelus species, which m a y o c c u r in thin, skeletal hash
brachiopod-
beds. T h e s e beds represent s o m e o f the highest o c c u r r e n c e s o f
H a m i l t o n , O n t a r i o . Still farther west, the Q u e e n s t o n appears to
and
bryozoan-rich
limestones
northwest
of
these trilobites. In New York the overlying beds of the Oswego
b e equivalent t o the R i c h m o n d G r o u p o f the C i n c i n n a t i , O h i o ,
and Q u e e n s t o n f o r m a t i o n s represent n e a r s h o r e o r n o n m a r i n e
area, part of the section n o t e d for exceptionally well-preserved
fades inappropriate for the preservation of trilobites, although
fossils, including a b u n d a n t trilobites of the genera Flexicalymene,
these genera persisted, with a b u n d a n t Flexicalymene and Isotelus
Isotelus, and others. H e n c e , these organisms lived on in the later
remains c o m m o n in the R i c h m o n d G r o u p in southwestern O h i o .
part of the O r d o v i c i a n in the m i d c o n t i n e n t , while New York State
Cryptolithus and
developed into a delta plain and tidal fiat c o m p l e x .
Isotelus and the entire trinucleid
and
asaphid
trilobite groups to which they b e l o n g appear to have b e c o m e extinct in the Late O r d o v i c i a n crisis, which affected m o r e than 2 0 % o f m a r i n e families.
T h e latest
p o r t i o n o f the Ordovician
Period
(Gamachian
Stage) is not recorded in New York, or in most of the m i d c o n t i nent of North A m e r i c a . T h e Q u e e n s t o n is terminated by a m a j o r
T h e reported trilobites of the U p p e r O r d o v i c i a n are as follows:
erosional
u n c o n f o r m i t y (referred
to as
the Cherokee
Unconfor-
mity; Figures 4 . 1 and 4 . 2 6 ) . T h i s erosion surface increases in magCalyincnc?
conradi
Ceraurinus
marginatus
Calyptaulax Ceraurus
cf.
beecheri
Cryptolithus
Flexicalymene
granulosa
Flexicalymene
stegops
Odontopleura Proetus
ceralepta
spurlocki
Triarthrus
eatoni
Triarthrus
spinosus
Isotelus
nitude to the east along the Q u e e n s t o n clastic wedge. Ultimately, it cuts down through the entire thickness of the Q u e e n s t o n (over 4 0 0 m) and through the underlying Pulaski, Frankfort, and upper
bellulus
parts of the Utica or Schenectady f o r m a t i o n s (Figures 4 . 1 2 and
meeki
pulaskiensis
4 . 2 6 ) . T h i s suggests that several things were happening in the Late
hudsonica
O r d o v i c i a n . First, there was a period of uplift in the eastern
Otarion? Pseudogygites Triarthrus
callicephalus
sp.
Cornuproetus? Homotelus
C.
p o r t i o n s of the old Q u e e n s t o n delta. T h i s might reflect renewed
latimarginatus
o r o g e n i c or m o u n t a i n - b u i l d i n g activity at the very end of the
glaber
O r d o v i c i a n or Early Silurian, or possibly isostatic forces (buoya n c y forces) that caused the eastern areas, formerly part of the m o u n t a i n belt, to b o b upward in m u c h the same way that land
Upper O r d o v i c i a n O s w e g o a n d Q u e e n s t o n F o r m a t i o n s T h e record of benthic life in the highest part of the O r d o v i c i a n
r e b o u n d s after removal of the weight of a glacier (Figure 4 . 2 5 B ) .
deposits in New York is sparse. T h e Pulaski gives way upward to
T h i s " p o p - u p " p h e n o m e n o n or isostatic r e b o u n d would be the
coarser g r a y - a n d - m a r o o n - m o t t l e d sandstones o f the Oswego For-
result of erosional redistribution of sediments from the old thrust
m a t i o n . Here, a b u n d a n t wave m a r k s , desiccation cracks, and other
belt of the Taconic M o u n t a i n s and out into a wider tract of the
features toward the top indicate shallowing of the O r d o v i c i a n
interior of North A m e r i c a . A second event that may have c o n -
seafloor effectively to sea level. T h i s shallowing was p r o d u c e d by
tributed to the development of a m a j o r widespread u n c o n f o r m i t y
the rapid in-filling of clastic s e d i m e n t s that were being shed out of
or erosion surface at the end of the Ordovician was the develop-
the n o w eroding Taconic Highlands to the east. T h e effect of this
m e n t of extensive continental glaciers in the region that today
was the o u t w a r d - b u i l d i n g or progradation of the so-called Q u e e n -
is
ston delta c o m p l e x into the Appalachian foreland basin.
have locked up a considerable a m o u n t of water as glacial ice and
Saharan Africa.
These
Late Ordovician
ice sheets would
along
thereby caused a d r o p in sea level from the Late Ordovician on
the s o u t h shore o f Lake O n t a r i o f r o m O s w e g o C o u n t y westward
the o r d e r of perhaps 100 m. T h i s is reflected in successions of rock
t o the Niagara G o r g e , consists o f m a r o o n - r e d blocky m u d s t o n e
worldwide as a m a j o r erosion surface associated with regression
with occasional streaks of greenish gray siltstone and sandstone
of seas. T h i s t i m e also corresponds to o n e of the great mass
T h e overlying Q u e e n s t o n
F o r m a t i o n , well
exposed
SILURIAN
81
PERIOD
FIGURE 4 . 2 6 . Ordovician/Silurian unconformities. A. Ordovician/Silurian unconformity. Upper Ordovician, black Frankfort S h a l e (a) is sharply overlain by Lower Silurian, light gray O n e i d a c o n g l o m e r a t e and s a n d s t o n e (b), which g r a d e s upward into gray Sauquoit S h a l e . Cut along Rte. 1 7 1 , Frankfort G o r g e of Moyer Creek, Frankfort, Herkimer County. B. Angular C h e r o k e e unconformity (arrow) b e t w e e n Ordovician Austin Glen Formation ( b ) (nearly vertical b e d s on right side) and gently dipping u p p e r m o s t Silurian Rondout Formation (a). Note that the unconformity, p r o d u c e d by erosion of the folded Taconic terrane, w a s later rotated to 45 d e g r e e s during the Devonian A c a d i a n Orogeny. Exit from Rte. 23 to 2 3 B , Catskill, G r e e n e County.
extinctions in Earth's history and o n e that decimated m a n y trilo-
recovery from m a j o r Late O r d o v i c i a n extinction and ended with
bites, such as the genux Isotelus.
m i n o r extinction before the D e v o n i a n . T h e r e is really not a m a j o r S i l u r i a n - D e v o n i a n break in m o s t areas. T h e Silurian marks a
Silurian Period
return to g r e e n h o u s e c o n d i t i o n s following the Late Ordovician glaciation, but there appears to have been s o m e lingering glacia-
T h e Silurian Period was a relatively short span ( 4 3 8 to 4 0 8
tion in the paleo-Andes M o u n t a i n s of S o u t h A m e r i c a through the
million years ago) (Figures 4.1 and 4 . 2 7 ) that c o m m e n c e d with
Early Silurian. Middle and Late Silurian climates appear to have
THE
82
PALEOZOIC
GEOLOGY
OF
NEW
YORK
FIGURE 4.27. Stratigraphic chart of the Silurian rocks in New York. The n u m b e r s are millions of years before present. M o d i f i e d from Brett et al. (1985).
been quite equable worldwide, with lots of shallow c a r b o n a t e
(Figures 4.5 and 4 . 2 6 ) . E t t e n s o h n and Brett ( 1 9 9 8 ) recognized
deposition and s o m e return to stagnation and black shale devel-
a third and final tectophase of Taconic O r o g e n y in the Early
o p m e n t in deeper basins.
Silurian; this m o u n t a i n - b u i l d i n g activity produced a new clastic
T h e eastern side of Laurentia, n o w a b o u t 25 to 30° south latitude, experienced the f i n a l r u m b l e s o f the Taconic O r o g e n y
wedge, the T u s c a r o r a - M e d i n a f o r m a t i o n s , and pulse of westward subsidence.
SILURIAN
83
PERIOD
Baltica (Ancestral Europe) had m o v e d northward through the
( M i d d l e t o n et al. 1 9 8 7 ) . T h e s e s e d i m e n t s appear to represent
Ordovician and now collided with n o r t h e r n Laurentia to f o r m
a reworking of older O r d o v i c i a n siliciclastics, as the Whirlpool
the widespread Caledonian
created
locally contains very m i n o r thin seams of greenish gray shale that
Euramerica ( o r " O l d Red C o n t i n e n t " ) straddling the p a l e o e q u a -
not o n l y c o n t a i n early Silurian a c r i t a r c h s , small microfossils
tor (Figure 4 . 5 ) . At the same t i m e the m i c r o c o n t i n e n t of Avalo-
representing p r o b a b l e algal resting cysts, but also acritarchs
nia was converging on (present-day) eastern N o r t h A m e r i c a ,
reworked f r o m the O r d o v i c i a n .
making initial
New
Trilobites evidently were present in s o m e of these environm e n t s , but their b o d y fossils are rare. T h e upper p o r t i o n of the
appears to have been s o m e orogenesis during the M i d d l e Silurian
W h i r l p o o l S a n d s t o n e is distinctly m a r i n e and locally shows h u m -
in
m o c k y c r o s s - l a m i n a t e d s a n d s t o n e beds with m i n o r fossil debris.
North
in
the
America
northern
Europe and
England region in the Late Silurian or Fail}' Devonian. T h e r e eastern
contact
O r o g e n y in
(Laurentia),
M a r i t i m e and
termed
the
Salinic
Orogeny, as evidenced by a renewed pulse of subsidence and west-
R e m n a n t s of starfish and c r i n o i d s have been f o u n d in the upper
ward migration of the foreland basin.
layer of the W h i r l p o o l in O n t a r i o , and farther west the Whirlpool
Sea level rose in the Early Silurian, following the lowest sea
grades laterally into or is overlain by the M a n i t o u l i n D o l o s t o n e ,
level at the end of a regression that p r o d u c e d the C h e r o k e e
a rather massive
Unconformity
locally c o n t a i n s a b u n d a n t b r a c h i o p o d s and m i n o r disarticulated
in
the
latest
Ordovician.
The
Silurian-Early
Devonian bundle of strata forms the Tutelo ( s e c o n d )
Phase
fine-grained
d o l o m i t i c c a r b o n a t e . T h i s unit
trilobite material. T h e probably equivalent massive Tuscarora
of Sloss's T i p p e c a n o e S u p e r s e q u e n c e . Sea level was quite high
Sandstone of Pennsylvania is generally devoid of body fossils but
throughout the mid Silurian but was beginning to fall in the later
contains
Silurian. Smaller-scale (third and fourth order) sequences were
resting traces o f trilobites.
developed owing to s h o r t e r - d u r a t i o n rises and falls of sea level during the Silurian in eastern N o r t h A m e r i c a .
abundant
traces
including
Rusophycus,
the
probable
In Niagara C o u n t y and westward into O n t a r i o the W h i r l p o o l S a n d s t o n e is overlain by dark gray to m e d i u m greenish gray
Although the Silurian is a relatively shorter interval of time
shales and with s o m e thin q u a r t z - r i c h sandstones. In western
than the Ordovician or the D e v o n i a n , Silurian rocks in New York
New York this Power Glen Shale tends to be sparsely fossiliferous.
State are highly varied, indicating a b r o a d range of depositional
O n l y m i n o r trace fossils, including Rmophycus, are f o u n d on the
environments. T h e Silurian can be subdivided into five m a j o r
base of sandstones at L o c k p o r t . At Niagara the unit is s o m e w h a t
divisions that are generally construed as groups (Figure 4 . 2 7 ) . In
m o r e fossiliferous, yielding a small fauna including snails, small
ascending order these are the (1) M e d i n a G r o u p : predominately
twiglike b r y o z o a n s , nautiloid c e p h a l o p o d s , and a single species
shales and sandstones of early Silurian age; ( 2 ) C l i n t o n G r o u p : a
of small c r i n o i d . Westward in O n t a r i o the unit b e c o m e s even
heterogeneous group c o m p o s e d o f shales, m i n o r sandstones, and
m o r e fossiliferous, and
shell-rich carbonates with m i n o r h e m a t i t e beds; ( 3 ) L o c k p o r t
a b u n d a n t remains o f several species o f b r y o z o a n s , b r a c h i o p o d s ,
G r o u p : predominantly d o l o m i t i c c a r b o n a t e s in western New
c r i n o i d s , and starfish. Trilobites tend to be u n c o m m o n in these
York, grading eastward into dark gray or greenish shales and thin
beds, but a few r e m a i n s of c a l y m e n i d s and dalmanitids have been
stromatolitic limestones; (4) Salina G r o u p : Upper Silurian shales,
collected.
near H a m i l t o n , O n t a r i o , it contains
carbonates, and evaporites; and ( 5 ) Bertie and R o n d o u t G r o u p s :
T h e Power Glen Shale and laterally equivalent C a b o t Head
dolomitic limestones and dolostones with s o m e shales and m i n o r
Shales appear to have a c c u m u l a t e d in shallow offshore shelf or
evaporites.
pro-deltaic settings of early M e d i n a . D a r k gray and greenish colo r a t i o n , as well as an a b u n d a n c e of pyrite at s o m e levels, suggests a c c u m u l a t i o n under low-oxygen, quiet water c o n d i t i o n s . Perhaps
Early Silurian
the i n n e r - s h e l f m u d s were deposited in a sheltered, muddy
Medina G r o u p
lagoon. However, s o m e w h a t m o r e o f f s h o r e sediments in the
T h e Early Silurian Medina S a n d s t o n e of New York and its equivalents, the Tuscarora sandstones and c o n g l o m e r a t e s of the
vicinity o f H a m i l t o n , O n t a r i o , d o show a b u n d a n t storm deposits, indicating deposition by s t o r m waves and currents.
central Appalachian region, represent a renewed influx of silici-
T h e higher beds of the M e d i n a G r o u p in western New York,
clastic sediments following the m a j o r e n d - O r d o v i c i a n u n c o n f o r -
assigned to the G r i m s b y F o r m a t i o n , are primarily reddish and
mity or period of erosion. T h e M e d i n a G r o u p is c o n f i n e d to
white-mottled, quartz-rich,
western and west-central New York, pinching out eastward in the
stones interbedded with green and red m u d s t o n e s . Generally,
vicinity o f O n e i d a , O n e i d a C o u n t y (Figure 4 . 2 8 ) . T h e M e d i n a i s
these sediments are sparsely fossiliferous, particularly east of
fine-grained
sandstones and silt-
characterized by quartz-rich sandstones and shales, m a n y of
L o c k p o r t , New York. But in the west they may contain abundant
which are red.
shells of lingulid b r a c h i o p o d s , a
In western New York and O n t a r i o the basal unit is a quartzose
few species of clams, and
even b r y o z o a n s at Niagara G o r g e . T h e presence of abundant
sandstone with trough c r o s s - b e d d i n g , the W h i r l p o o l S a n d s t o n e ,
b u r r o w i n g with m a n y p r i m a r y sedimentary structures, such as
which
wave and current ripple m a r k s , m i n o r trough and h u m m o c k y
may
represent,
in
part,
nonmarine
stream
deposits
84
THE
PALEOZOIC
GEOLOGY
OF
NEW
YORK
FIGURE 4.28. A. Ordovician-Silurian (Cherokee) unconformity (arrow). U p p e r O r d o v i c i a n , dark m a r o o n Q u e e n s t o n Shale (a) is sharply overlain by light gray Whirlpool S a n d s t o n e (Lower Silurian) (b). In turn, the Whirlpool g r a d e s a b r u p t l y u p w a r d into gray Power Glen Shale ( M e d i n a Group) (c). Cut on West J a c k s o n Street, Lockport, Niagara County. B. O r d o v i c i a n Lower Silurian s u c c e s s i o n . U n c o n f o r m i t y (arrow) at the t o p of the U p p e r O r d o v i c i a n Queenston Shale (a) is overlain by Lower Silurian Medina G r o u p (b). The white b a n d at the top of the M e d i n a is the K o d a k Sandstone (c). This unit is overlain, in turn, by green M a p l e w o o d Shale (d) of the Clinton G r o u p . G e n e s e e River G o r g e . Rochester, Monroe County.
cross-stratification, rare desiccation c r a c k s , evidence o f c h a n n e l -
upward to h u m m o c k y cross-laminated sandstone beds that may
ing and soft sediment d e f o r m a t i o n in the f o r m of load casts
be capped by thin lag beds of bryozoans and / inguhi brachiopod
and b a l l - a n d - p i l l o w - s t r u c t u r e s all point to deposition in very
shell hash representing m i n o r deepening events. In Rochester,
shallow waters associated with outer- to inner-tidal flats. W i t h i n
New York, s o m e w h a t different, fining upward cycles seem to have
the M e d i n a , o n e m a y recognize small-scale, c o a r s e n i n g , upward
s h a r p - b a s e d , channel-fill sandstones at the b o t t o m s , which are
cycles in western New York that range from m a r o o n m u d s t o n e s
often extensively burrowed by large wormlike organisms that pro-
SILURIAN duced
8b
PERIOD Daedalus, apparently as
ironstones are c o m m o n l y associated with phosphatic nodules and
feeding burrows in muddy sands. T h e upper reddish m u d s t o n e s
structures called Arthrophycus or
flooding surfaces (surfaces with very low sedimentation rates
in higher parts of the cycles apparently reflect upper tidal-flat
during deepening episodes).
conditions, where low-energy c o n d i t i o n s prevailed, allowing fine-
T h e lowest part of the C l i n t o n G r o u p c o m p r i s e s greenish gray
grained muds to drop out of suspension. All of these sediments
shales, the Neahga or M a p l e w o o d of western New York, overlain
apparently accumulated from c o n t i n u e d erosion and deposition
In- thin limestones. T h e Neahga is a highly fissile, very soft, chippy
o f f m o u n t a i n o u s highlands to the s o u t h - s o u t h e a s t . At present, it
shale that like the Power Glen seems to represent nearshore but
appears that a late phase of the Taconic O r o g e n y o c c u r r i n g within
dysaerobic m u d d y b o t t o m c o n d i t i o n s . T h e M a p l e w o o d contains
the early part of the Silurian m a y have uplifted or re-uplifted
very a b u n d a n t microfossils of acritarchs, algal resting cysts, and
these source terrains.
s o m e early plant spores but is almost devoid of macrofossils. A
Trilobites are extremely rare b o d y fossils in the upper sand-
very meager fauna, including o n e species o f calymenid, o n e
stones of the Medina G r o u p . Two persistent, often light gray or
odontopleurid
pinkish sandstones, the T h o r o l d and the Kodak, contain a b u n -
o b t a i n e d f r o m a basal p h o s p h a t e - r i c h layer that immediately
dant trace fossils but very few body fossils. Near Rochester the
overlies the u n c o n f o r m i t y at the b o t t o m of the M a p l e w o o d .
Kodak bears remains of beautifully preserved traces of trilobite
However, in general, the c o n d i t i o n s appear to have been unfa-
trilobite,
and
a
few
brachiopods,
has
been
activity, although as yet no b o d y fossils have been f o u n d . T h e s e
vorable for benthic or b o t t o m - d w e l l i n g life during deposition of
include the coffee b e a n - s h a p e d Rusophycus as well as delicately
the M a p l e w o o d m u d s .
detailed scratch marks produced by trilobites walking on the tips
T h e M a p l e w o o d grades eastward into a sandy, hematite-rich
of their claws. T h e fact that slabs of sandstone from Glen Edith
c o n g l o m e r a t e that is generally lacking in fossils. However, it
near Irondequoit Bay, M o n r o e County, display trilobite trackways
grades upward and laterally to the west into t h i n - b e d d e d lime-
superimposed on m u d cracks suggests that these trilobites m a y
stones and fossil-rich h e m a t i t e beds. T h e s e limestones, the Rey-
have lived in a very shallow water area that was subject to peri-
nales F o r m a t i o n , are rich in b r a c h i o p o d s , particularly the large
odic exposure and desiccation.
robust Pentamerus, which appears to have f o r m e d shell banks
Toward the end of Medina deposition, uplift of a b r o a d arch-
in shallow shoal areas on the seafloor. To the west, these b r a -
like feature, the Algonquin axis or Findley Arch, to the west of
c h i o p o d - r i c h shoals and crinoidal grainstones pass laterally into
New York State elevated parts of the M e d i n a seafloor above sea
m o r e nodular, shaly, and probably m o r e offshore limestones.
level and caused erosion to bevel down through the highest beds
T h e s e facies c o n t a i n smaller b r a c h i o p o d s , a b u n d a n t and diverse
of the Medina, producing a widespread u n c o n f o r m i t y that sepa-
b r y o z o a n s , c r i n o i d s , and a few trilobites. Particularly notable in
rates this unit from the overlying Clinton G r o u p (Figure 4 . 2 8 B ) .
these beds are a b u n d a n t cephala and pygidia of the trilobite
T h e T u s c a r o r a - S h a w a n g u n k - M e d i n a clastic wedge consists of quartz-rich sandstones overlying the angular T a c o n i c U n c o n f o r mity in the Appalachians, which seem to record renewed uplift
Encrinurus. Reported f r o m
the Reynales are the following species of
trilobites:
and sedimentation. In the Niagara region, these show a transgressive succession from n o n m a r i n e and nearshore sandstones, to
Bumastus sp.
deeper marine gray shales, followed by a shallowing (regression)
Encrinurus cf. E.
into tidal flat and n o n m a r i n e red beds.
Scutellum
Calymene rayhesli
sp.
Eophacops trisulcatus
niagarensis
Green and purplish shales of the overlying S o d u s F o r m a t i o n
Middle Silurian
in west-central New York represent a return to shallow water Lower C l i n t o n G r o u p
m u d d y c o n d i t i o n s similar to but s o m e w h a t different from those
Middle Silurian (late Llandovery series) m i x e d shales, c a r -
of the M a p l e w o o d . T h e s e m u d d y seafloors supported a modest
bonates, and ironstones of the lower C l i n t o n G r o u p record the
diversity of b r a c h i o p o d s d o m i n a t e d by the small atrypid Eocoelia,
wearing down of Taconic m o u n t a i n o u s source terranes to the east
small bryozoans, a few c r i n o i d ossicles, the conical fossil Tenta-
and a shift back to o p e n m a r i n e sediments. O n e distinctive
culites, and scattered s p e c i m e n s of small tabulate corals. A few
feature is the widespread " C l i n t o n iron o r e s " ; these f a m o u s fossil-
trilobites make up the r e m a i n d e r of the fauna. Both Diacalymene
rich and oolitic hematites have been m i n e d extensively f r o m
rostrata and Eophacops trisulcatus have been extracted from these
near B i r m i n g h a m , Alabama, to central New York for m a k i n g steel
beds. Here c o n d i t i o n s alternated between dysoxic and s o m e -
and red paint oxides. T h e s e beds probably were associated
what better oxygenated, as indicated by the green and purple
with enrichment of iron in sediments due to deep weathering of
b a n d i n g typical
laconic uplifts. Hematite coatings on grains precipitated under
temporarily hospitable to b e n t h i c assemblages of a very shallow-
conditions
in
water restricted type. T h e s e m u d s apparently represent accu-
muddy sediments, during times of sediment starvation. Hence,
m u l a t i o n s in a b r o a d shallow-water lagoonal setting inboard of
of
fluctuating
oxidizing-reducing
conditions
of the
unit, and
the seafloor was at least
THE
86
PALEOZOIC
GEOLOGY
OF
NEW
YORK
offshore, b r a c h i o p o d - r i c h shoals, which at that t i m e were as far
underlying middle and to the west of the lower Clinton units. It
west as O h i o .
is again a particularly oolitic-rich ironstone that contains a b u n -
T h e Wolcott F o r m a t i o n represents a return to Reynales-like
dant, small (1 to 2 m m ) spheroidal bodies of hematite that appear
c o n d i t i o n s in which a b u n d a n t p e n t a m e r i d b r a c h i o p o d s o n c e
to have f o r m e d by accretion of i r o n - r i c h coatings on grains that
again flourished and f o r m e d shell banks in west-central New
were at least intermittently rolled on the sea b o t t o m . This unit
York. However, an u n c o n f o r m i t y has removed western p o r t i o n s
probably represents a deposit accumulated during sea-level deep-
of the Wolcott so it is not possible to trace these facies westward
ening in which the fine-grained portion was winnowed away,
past Wayne C o u n t y into offshore n o d u l a r sediments c o m p a r a b l e
associated with an episode of a very widespread rise in sea level that o c c u r r e d globally during mid Silurian times. It is well dated
to those seen in the western Reynales. T h i n hematite bands, usually only a few c e n t i m e t e r s thick but traceable over distances of tens of kilometers, are f o u n d at several
by ostracods and sawblade-like graptolites that o c c u r in the interbedded shales.
levels within the lower part of the C l i n t o n G r o u p . As n o t e d , these
Beds of black-green shale of the mid Silurian Williamson
appear to represent periods of m a r i n e s e d i m e n t starvation in a
F o r m a t i o n and its eastern equivalent Willowvale Shale carry an
shoal margin e n v i r o n m e n t in which ferrous iron b e c a m e c o n -
a b u n d a n t fossil assemblage. Fossils typically are poorly preserved
centrated in pore-waters and precipitated out as coatings and
in the shales but they may include over a hundred species of
ooidlike grains on the seafloor. Reworking of these grains into
b r a c h i o p o d s , b r y o z o a n s , the small " b u t t o n " coral Palacocyclus,
windrows or piles c o n c e n t r a t e d
the iron oxide c o m p o n e n t s ,
crinoids, and
trilobites.
Both
Dalmanites and
Calymene species
making them lean iron ores. T h e s e ores are mostly "fossil ores,"
are relatively a b u n d a n t in the Willowvale of central New York.
meaning that they contain a b u n d a n t f r a g m e n t a r y fossils, espe-
To the west, the m e d i u m gray shales give way laterally to green
cially c r i n o i d ossicles, bits of b r y o z o a n s , and shell fragments.
satiny-smooth
Again trilobites, especially c a l y m e n i d s , are present but only as
s o m e t i m e s sandy shales that yield a great a b u n d a n c e locally of the
highly c o m m i n u t e d debris.
sawblade
T h e middle part of the C l i n t o n is represented by the Sauquoit Shales
and
their
eastern
reddish
sandstone
equivalent
the
clay
shales,
monograptid
alternating graptolite
with
black-laminated,
Monograptus
clintonensis,
which enables correlation of the Willowvale and Williamson Shales with o t h e r deposits globally that
represent this mid
Otsquago F o r m a t i o n in the Utica area and eastward. T h e s e units
Silurian highstand or sea-level rise event. T h e Williamson Shales
may be in excess of 30 m thick, and e x p a n d dramatically to the
in
southwest into a succession, up to 140 m thick, of purplish green
f o s s i l s , although s o m e beds carry small b r a c h i o p o d s and rare
the
Rochester
area
generally contain
few o t h e r
benthic
shales referred to as Rose Hill in Pennsylvania and M a r y l a n d . T h e
s p e c i m e n s of the trilobite Calymene have been f o u n d . Overall the
Sauquoit Shale from the middle part of the C l i n t o n G r o u p rep-
W i l l i a m s o n - W i l l o w v a l e Shales probably record the deepest-water
resents c o n d i t i o n s quite similar to those of the Sodus and M a p l e -
interval during deposition of the Silurian in New York State.
wood m u d s , and like t h e m it tends to c a r r y a restricted fauna,
B o t t o m c o n d i t i o n s range from anoxic or having very low oxygen,
although the trilobite Calymene is relatively c o m m o n and the
near the basin center, to fully oxygenated conditions favorable to
genus Dalmanites has also been reported f r o m these shales.
life in central New York Willowvale sections.
Upper C l i n t o n G r o u p
Sandstone represent s o m e w h a t shallower water c o n d i t i o n s in
T h e overlying
Rockway D o l o s t o n e and equivalent
Dawes
A very widespread u n c o n f o r m i t y separates the middle C l i n t o n
which r h y t h m i c limestones or calcareous sandstones a c c u m u -
and lower C l i n t o n shales from the overlying upper part of the
lated, alternating with shales. Again, fossils tend to be sparse
Clinton G r o u p . D u r i n g this medial part of Silurian t i m e , m o r e
because of p o o r preservation in these beds. But a few species of
dramatic arching of the sea b o t t o m in the vicinity of the Algo-
brachiopods,
nquin Arch (near H a m i l t o n , O n t a r i o ) and farther east exposed
graptolites, mostly dendroids (or brushy graptolites), and rare
m a j o r parts of the older C l i n t o n sediments to erosion and pro-
trilobites, such as Dalmanites, have been f o u n d in these beds.
duced a widespread beveling-surface u n c o n f o r m i t y that eroded
In
western
including
the
New York
the
large
form
Rockway
Costistricklandia,
consists
of pale
and
to
o f f the western edges of the middle and lower C l i n t o n units.
m e d i u m gray, argillaceous dolostones with interbedded greenish
H e n c e , a good deal of Middle Silurian t i m e may be missing at this
gray shales. Overall, the Rockway appears to represent an off-
important unconformity.
shore m u d d y c a r b o n a t e unit deposited well below wave-base
T h e upper part of the C l i n t o n G r o u p is a varied succession of
and under s o m e w h a t restricted c o n d i t i o n s . It grades eastward
shales a n d c a r b o n a t e s that are readily g r o u p e d into two u n c o n -
and southeastward into the Dawes S a n d s t o n e of central New
f o r m i t y - b o u n d sequences o r packages. T h e lower consists o f
York, an interbedded, h u m m o c k y c r o s s - l a m i n a t e d , fine-grained
the W e s t m o r e l a n d
sandstone and shale unit with the beautiful trilobite trace fossil,
Hematite
(locally), W i l l i a m s o n - W i l l o w v a l e
shales, and the R o c k w a y F o r m a t i o n and its lateral eastern equivalent, the Dawes S a n d s t o n e . T h e basal hematite b e d , or Westm o r e l a n d , rests u n c o n f o r m a b l y on
the beveled edges of the
Rusophycus. T h e Rockway and its equivalents are sharply overlain by a thin, widespread blanket deposit of crinoidal limestone. T h i s unit,
FIGURE 4.29. M a p s of New York. A. Early W e n l o c k times d u r i n g the d e p o s i t i o n of the Irondequoit Limestone. B. Mid Wenlock d u r i n g the deposition of the Rochester Shale. C. Cross section s h o w i n g the a p p r o a c h of Avalon with proto North A m e r i c a . C is from Isachson et al. (1991). Printed with p e r m i s s i o n of the N e w York State M u s e u m , Albany, N.Y
THE
88
FIGURE 4.30.
PALEOZOIC
GEOLOGY
OF
NEW
YORK
Irondequoit-Rochester b i o h e r m at the
upper part of the M i d d l e Silurian Clinton G r o u p . The sharp contact at the knee level of William G o o d m a n is the u n c o n f o r m a b l e c o n t a c t of R o c k w a y Dolostone (a) a n d massive Irondequoit Limestone (b). The lens-shaped b o d y at the top of the Irondequoit is a small b i o h e r m ("reefs") (arrow) of b r y o z o a n s a n d algae. It o c c u r s at the c o n t a c t of the dark gray Rochester Shale (c). N i a g a r a G o r g e , Lewiston, Niagara County.
4.29),
resent very small buildups or "reeflets" that developed on the
consists almost entirely o f the disarticulated and c o m m o n l y
seafloor during a t i m e of deepening. T h e s e m o u n d s , however,
abraded skeletal remains of crinoids and cystoids. However, there
have yielded s o m e intriguing fossil deposits that are not found
are s o m e thin, interbedded shaly units, and in the R o c h e s t e r area,
elsewhere within the Irondequoit L i m e s t o n e or the overlying
referred
to
as
the
Irondequoit
Limestone
(Figure
the Irondequoit is s o m e w h a t finer-grained and carries a lower
R o c h e s t e r Shale. For e x a m p l e , pockets of greenish shale within
diversity of fauna typified by a few species of b r a c h i o p o d s , of
the c r e a m - c o l o r e d , f i n e - g r a i n e d limestone o f the m o u n d s i n
which Whitfieldella is a c o m m o n e l e m e n t . Also in this area, as
Niagara C o u n t y yield extremely a b u n d a n t remains of the trilo-
well as in Niagara County, small (up to 5m across and 2m h i g h ) ,
bite genera Bumastus and Illaenoides. T h e s e relatively large fossils,
irregular, lenticular m o u n d s , or b i o h e r m s , o c c u r within the I r o n -
up to 3 cm across, appear to represent the remains of reef- or
dequoit (Figure 4 . 3 0 ) . In Niagara C o u n t y these o c c u r mainly at
b i o h e r m - d w e l l i n g trilobites that accumulated or collected in the
the upper c o n t a c t of the crinoidal limestone and extend upward,
small pockets and cavities within the b i o h e r m framework. Also
s o m e t i m e s as m u c h as 1 to 2 m (4 to 5 feet), into the overlying
found
R o c h e s t e r Shale. T h e s e are rather a m o r p h o u s masses of very-
rochesterense, and a new species of Diacalymene that is also rather
fine-grained ( m i c r i t i c ) l i m e s t o n e with s o m e leaflike b r y o z o a n s
large and represented by nearly c o m p l e t e articulated remains
that apparently helped s u p p o r t the m o u n d s . T h e b i o h e r m s rep-
from the Niagara G o r g e .
in
these
associations
are
a
Cheirurus
sp.,
Scutellum
SILURIAN
89
PERIOD
T h e Irondequoit and its b i o h e r m s represent a shallow, highly
of the Rochester, like the middle p o r t i o n of the Lewiston Member,
agitated shelf e n v i r o n m e n t . Water depths were close to n o r m a l
is characterized by dark gray, nearly barren shales in m u c h of
wave-base, ranging from perhaps 10 to 20 m in western New York
western New York. Nonetheless, s o m e bedding planes contain
and no m o r e than 20 to 30 m in the slightly deeper waters a r o u n d
a b u n d a n t remains, mostly disarticulated, as well as articulated
Rochester. T h e skeletal remains of crinoids and cystoids were
s p e c i m e n s of the trilobite genera Dalmanites and Trimerus. These
reworked and accumulated to thicknesses of up 3 or 4 m. C o n -
dark, sparsely fossiliferous shale facies, representing deeper por-
sidering that it takes about 1 5 , 0 0 0 average-sized crinoids to make
tions of the R o c h e s t e r sea, were deposited during relative rises in
1 m o f limestone, the total n u m b e r o f e c h i n o d e r m s represented
the sea level. H e r e , high rates of s e d i m e n t a t i o n c o m b i n e d with
in these beds is stunningly high ( 1 0 ' ° or even 1 0
low-oxygen c o n d i t i o n s near the substrate m a d e the b o t t o m c o n -
3
12
individuals).
These organisms flourished in shallow, relatively clean waters
ditions inhospitable for m o s t invertebrate species. Nonetheless,
associated with a m a j o r lowering of relative sea level in New York
s o m e trilobites, as scavengers, appear to have thrived in these
State. To the southeast, the Irondequoit skeletal limestone grades
e n v i r o n m e n t s . Shallower p o r t i o n s of the Rochester are repre-
first into hematitic limestone (the Kirkland F o r m a t i o n ) a n d
sented by banks of b r y o z o a n s and e c h i n o d e r m s (crinoids and the
beyond into quartzose, cross-bedded sandstones of the upper
cystoid Caryocrinites). Trilobites were less d o m i n a n t here but are
Herkimer F o r m a t i o n . A few trilobites are reported from the
represented by a greater diversity of species.
Irondequoit, including the following:
H e r k i m e r S a n d s t o n e , the eastern equivalent of the Rochester Shale, represents sandy shallow shelf to n e a r s h o r e e n v i r o n m e n t s .
Bumastus sp.
Diacalymene
Calymene sp.
Cheirurus
Liocalymene clintoni
Scutellum
sp.
It
sp.
yields
Trimerus
and
Dalmanites
specimens
and
excellently
preserved trilobite trace fossils, especially the large resting trace
rochesterense
Rusophycus. T h e paleogeographic distribution
of the trilobites
in
the
Irondequoit and its lateral equivalents, the Kirkland and Keefer
Lewiston M e m b e r is of interest. T h e trilobites east of Rochester
formations, are overlain sharply, but apparently c o n f o r m a b l y , by
are different f r o m those west of this area. Differences are at the
gray m u d s t o n e s of the Rochester F o r m a t i o n . Perhaps no Silurian
species as well as genus level. C e r t a i n genera such as Arctinurus,
unit in North America is m o r e noted for its exquisite fossils than
Decoroproetus,
the Rochester. Although m u c h of the unit is relatively sparsely
have been f o u n d o n l y in the west, while Cheirurus, Maurotarion,
fossiliferous, a total fauna of over 2 0 0 species of invertebrate
and
fossils is k n o w n from the Rochester Shale, including over 20
Dalmanites and Calymene o c c u r b o t h east and west of Rochester,
species o f trilobites. T h i n lenses o f b r a c h i o p o d - and b r y o z o a n -
but
rich m u d s t o n e s and argillaceous limestone yield prolific faunas,
Trimerus are
with over 80 species of bryozoans alone k n o w n from the middle
delphinocephalus is also f o u n d in equivalent age beds in England.
beds of the Rochester. In western New York the unit is divisible into two m e m b e r s : a fossil-rich lower half, b o u n d e d on its top by
Deiphon,
Dicranopeltis,
Staurocephalus have b e e n they
are
represented
found
in
found
Illaenoides, to
the east.
by different
both
the east
and
species.
Radnoria
T h e genera Bumastus and
and the west.
Trimerus
Rochester Shale trilobites reported f r o m western New York are as follows:
a bundle of limestone bryozoa beds and referred to as the Lewiston Member, and an upper sparsely fossiliferous, d o l o m i t i c shale
Acanthopygc
unit, the Burleigh Hill M e m b e r .
Bumastus
T h e Lewiston M e m b e r has yielded most of the diverse trilo-
Calymene
Arctinurus
sp.
boltoni
Calymene
ioxus
Cheirurus
sp.
niagarensis sp.
bite and e c h i n o d e r m faunas for which the Rochester is justifiably
Dalmanites
famous. Fossils are exceptionally well preserved, especially in beds
Decoroproetus
near the transition from a lower, highly shell-rich interval into a
Diacalymene
middle barren portion of the Lewiston and in the upper transi-
Illaenoides
tion from this sparsely fossiliferous interval to the overlying b r y -
Odontopleurid
Proetid
ozoan-rich beds. T h e s e beds c o n t a i n a n u m b e r of trilobites, s o m e
Radnoria
Staurocephalus
of them in an extraordinary state of preservation. T h e well-
Trimerus
Dalmanites
limulurus corycoeus /.
sp. pisum
Dicranopeltis
sp. cf.
Deiphon
trilobita
sp. delphinocephalus
nereus
Maurotarion
Trochurus
sp. sp. halli
known examples of the large lichid Arctinurus boltoni, as well as abundant
and
T h e R o c h e s t e r Shale seems to reflect siliciclastic muds that were
exquisite
derived from erosion of newly developed highlands to the east or
preservation of s o m e of the fossils suggests that they were buried
southeast. Initiation of this new input is represented through
Trimerus
Dalmanites delphinocephalus,
limulurus, occur
in
Calymene these
levels.
niagarensis, The
instantly by sediment plumes or turbidity currents. T h e Burleigh Hill M e m b e r consists of m e d i u m to dark gray
m u c h o f the Appalachian Basin b y the o c c u r r e n c e o f rather thick sands k n o w n as Keefer S a n d s t o n e in Pennsylvania and
shales with thin calcisiltites, especially toward the top, that grade
the H e r k i m e r S a n d s t o n e in central New York State (Figure 4 . 2 9 ) .
upward into the overlying D e C e w F o r m a t i o n . T h i s upper p o r t i o n
O t h e r circumstantial
evidence suggests
that a new pulse of
THE
90
PALEOZOIC
GEOLOGY
OF
NEW
YORK
tectonic activity o c c u r r e d a b o u t the middle part of the Silurian.
the L o c k p o r t G r o u p c o m e s to rest on the underlying crinoidal
T h i s c o m e s in the f o r m of the shifting of the depositional basin
grainstone o f the I r o n d e q u o i t .
itself. In the t i m e period represented by the W i l l i a m s o n Shale
D u r i n g the Late Silurian a vast c a r b o n a t e bank, somewhat like
to the upper part of the Rochester Shale, the area of thickest
that which existed in the C a m b r o - O r d o v i c i a n , developed o n c e
sediment a c c u m u l a t i o n , d o m i n a n t l y of shale, shifted westward
again in Laurentia. Very widespread shoals were developed from
approximately 100 km from the region of O n e i d a Lake to the area
sand- and gravel-sized skeletal pieces ( m a i n l y columnals) of
of Wayne C o u n t y . T h i s m i g r a t o r y pattern together with the
crinoids and cystoids. Under similar, high-energy environments,
increased input of coarser sediments suggests that renewed thrust
tabulate corals and s t r o m a t o p o r o i d s established reefs in the Great
faulting may have taken place in the f o r m e r T a c o n i c landmass.
Lakes area. A barrier reef c o m p l e x developed around a circular,
Possibly this is the result of new o c e a n - f l o o r s u b d u c t i o n u n d e r -
subsiding a r e a — t h e M i c h i g a n B a s i n — a n d patch reefs developed
neath the eastern margin o f N o r t h A m e r i c a (Figure 4 . 2 9 C ) .
on the tops of crinoidal skeletal shoals from Indiana, to O h i o , and
T h e Rochester Shale represents a relatively shallow but m u d d y -
into New York. M a n y such reefs or b i o h e r m s show a good suc-
b o t t o m sea that ranged upward to over a h u n d r e d meters of water
cession of pioneer thicket formers to climax c o m m u n i t i e s d o m -
depth. T h e Rochester Shale facies grades eastward to the sandy
inated by head corals and s t r o m a t o p o r o i d s . Trilobites were a
shoreline facies of the H e r k i m e r S a n d s t o n e in central New York
m i n o r b u t significant c o m p o n e n t of the reefal faunas.
and to the northwest into c a r b o n a t e banks.
T h e basal L o c k p o r t
unit, G a s p o r t
L i m e s t o n e , consists o f
T h r o u g h o u t m u c h of western New York, the R o c h e s t e r Shale
d o l o m i t i c limestones and s o m e argillaceous dolostone. M u c h o f
is overlain by a rather unusual unit in the D e C e w D o l o s t o n e . T h i s
it can be described as an e c h i n o d e r m packstone or grainstone,
unit is a thin (3 to 5 m) and u n i f o r m interval of b u f f - c o l o r e d
like the underlying I r o n d e q u o i t , that consists of disarticulated
c a r b o n a t e silt with h u m m o c k y cross-stratification. T h e result of
and c o m m o n l y abraded fragments of c r i n o i d and cystoid plates
storm deposition, the D e C e w is also highly convoluted and c o n -
and
torted and in s o m e places displays o v e r t u r n e d folds. We suggest
deposits were m o v e d by s u b m a r i n e currents. Consequently the
columnals.
Typically
these
crinoidal
sands
and
gravel
that this represents a period of seismic s h o c k ( e a r t h q u a k e ) activ-
unit shows m e d i u m - to small-scale cross-stratification (Figure
ity on the floor of the foreland basin or interior continental sea.
4 . 3 2 A ) . S o m e p o r t i o n s o f the Gasport also show bimodal cross-
T h i s effect was widespread; we have f o u n d evidence for a similar
stratification, two opposite o r i e n t a t i o n s of cross-bedding that
d e f o r m a t i o n in D e C e w age strata in central Pennsylvania and
might reflect the influence of oscillating tidal currents. T h e
southern O h i o . Because o f the rapid deposition o f the c a r b o n a t e
Gasport sediments evidently accumulated during a time of initial
silts, this is not a highly fossiliferous unit, although occasional
transgression following a m a j o r regression of seas from western
remains of s o m e fossil crinoids and the trilobite Trimerus have
New York that p r o d u c e d the u n c o n f o r m i t y beneath the unit.
been found within the unit. T h e source o f the D e C e w c a r b o n a t e
D u r i n g a t i m e of rising sea level, b u t still in e n v i r o n m e n t s of
silts and fine sands is s o m e w h a t unclear, but it probably was rep-
shallow, agitated waters, perhaps no m o r e than a few meters deep,
resented by c a r b o n a t e shoals in the f o r m of c r i n o i d gardens that
c r i n o i d banks developed extensively over m u c h of western and
existed to the northwest of the m a i n New York depositional area.
west-central New York. In the Rochester area, these banks appar-
T h e s e shoal facies e n c r o a c h e d u p o n New York d u r i n g the d e p o -
ently interfingered with quartz sand bars represented by the
sition of the succeeding L o c k p o r t G r o u p .
Penfield F o r m a t i o n . T h e s e cross-stratified dolomitic sandstone
An eastern shaly facies equivalent to the DeCew, the G l e n m a r k
units seem to reflect the input of quartzose sediment from a
Shale, yields a diverse fauna of small rugose corals, b r a c h i o p o d s ,
n o r t h e r n or northeasterly source terrain. T h e y form an elongate
and trilobite genera, including Daltnanites sp.,
shoal or barlike feature that extended southward from Rochester
Trimerus, Encrin-
urus sp., calymenid, Maurotarion sp., Cheirurus sp., and proetids.
to the subsurface of New York State. To the east of this area, in Wayne C o u n t y , the sands give way to m u d d y dolostones and d o l o m i t i c limestones, m a n y o f t h e m containing remains o f ostra-
Lockport Group T h e L o c k p o r t G r o u p is an interval over 65 m thick of pre-
cods, s o m e stromatolites, and the trilobite genera Trimerus, Caly-
d o m i n a t e l y d o l o m i t i c c a r b o n a t e rocks. However, to the east of the
mene, Encrinurus, and Maurotarion.
Rochester
Still farther to the east, these
limestones
interfinger with dark gray shale of the Ilion F o r m a t i o n . W i t h i n
interfinger with dark gray shales of the Ilion F o r m a t i o n . In the
the upper part of the Gasport F o r m a t i o n , deepening produced
central Appalachians the L o c k p o r t G r o u p is represented by a
a c h a n g e in s e d i m e n t a t i o n patterns, in western New York, to
d o m i n a n t l y dark, shaly interval with thin r i b b o n - l i k e limestones
t h i n n e r - b e d d e d , m o r e argillaceous dolostones. At the same t i m e ,
termed
small patch reefs, which had begun developing on the now-
area,
the
these
Mackenzie
dolostones
Formation.
and
The
dolomitic
Lockport
represents
a
depositional s e q u e n c e . It is b o u n d e d at its base by a m a j o r erosion
stabilized surface of the crinoidal shoals, built upward, f o r m i n g
surface that locally cuts into the D e C e w D o l o s t o n e and removes
the famed G a s p o r t reefs or b i o h e r m s (Figure 4 . 3 2 B ) . T h e s e reefs
it in areas near H a m i l t o n , O n t a r i o , where the u n c o n f o r m i t y c o n -
were built primarily of favositid corals and stromatoporoids. T h e
tinues to cut d o w n w a r d into the Rochester Shale until the base of
G a s p o r t is not particularly n o t e d for trilobites, although s o m e
FIGURE 4 . 3 1 . M i d Silurian s u c c e s s i o n s . A. Lowest light limestone l e d g e , Irondequoit L i m e s t o n e (a) is overlain by about 20 m of dark gray Rochester Shale (b). Thick layers of the L o c k p o r t G r o u p (c) protrude at the t o p of the cliff. Niagara River G o r g e , south of L e w i s t o n - Q u e e n s t o n B r i d g e , N i a g a r a County. B. Lowest rhythmic thin b e d s are Rockway Formation (a). These are sharply overlain by Irondequoit Limestone (b) b e a r i n g small b i o h e r m s ("reefs") (arrow) that protrude slightly into the overlying Rochester Shale (c). G e n e s e e River G o r g e , Rochester, Monroe County.
FIGURE 4.32. Silurian c a r b o n a t e s . A. C r o s s - b e d d e d crinoidal limestone, G a s p o r t Formation, cut along Robert M o s e s Parkway, Lewiston, N i a g a r a County. B. B i o h e r m or reef m o u n d of s t r o m a t o p o r o i d s a n d tabulate corals, Goat Island Formation, overlies t h i n - b e d d e d u p p e r G a s p o r t Formation on the right e n d of the cut. Road cut on Rte. 429, N i a g a r a e s c a r p m e n t , Pekin, N i a g a r a County.
SILURIAN
93
PERIOD
specimens of calymenids are f o u n d . An area of interfingering
dolostones are characterized by small thickets or biostromes of
(shaly)
grainstones
corals, especially c l a d o p o r i d s . T h e y represent thickets of small
occurs in the Sweden-Walker Q u a r r y at B r o c k p o r t , M o n r o e
tabulates and o t h e r corals that grew in shallow waters. S t r o m a t o -
County. This transitional facies has yielded the r e m a i n s of
poroids are also c o m m o n as isolated heads within these beds, typ-
dolostones with
Dalmanites,
Trimerus,
and
typical
another
Gasport
rare
crinoid
homalonotid
trilobite.
ically up to a foot across b u t highly altered by diagenesis. T h e s e
Also, s o m e of the b i o h e r m a l faunas have yielded f r a g m e n t a r y
beds, like the underlying G o a t Island, are typically vuggy and
remains of the calymenid, Diacalymene, and o t h e r trilobites.
c o n t a i n a b u n d a n t mineralized pockets that are well known to
T h e overlying G o a t Island F o r m a t i o n is separated in most
local mineralogists. In O n t a r i o , C a n a d a , however, the basal beds
places from the Gasport by a m i n o r u n c o n f o r m i t y . It represents
of the E r a m o s a that overlie E r a m o s a Shales and appear almost
the second cycle of shallowing, followed by deepening and the
gradational into the latter, are platy argillaceous dolostones that
development of crinoidal shoals and in places such as at B r o c k -
seem to represent s o m e w h a t m o r e offshore c a r b o n a t e a c c u m u l a -
port, the development of small patch reefs of s t r o m a t o p o r o i d s
t i o n . T h e s e basal E r a m o s a c a r b o n a t e s are n o t e d for abundant,
and corals (Figure 4 . 3 1 B ) . T h e s e reefs have m a n y features in
although m a i n l y disarticulated, trilobites. Q u a r r i e s in the vicin-
c o m m o n with the underlying G a s p o r t . However, in m a n y places,
ity o f D u n d a s , O n t a r i o , have yielded t h o u s a n d s o f specimens o f
the G o a t Island has been affected severely by late diagenetic
the otherwise rare trilobite Encrinurus raybesti, along with caly-
dolomitization, which has altered its texture and m a d e it difficult
m e n i d s , and a new d a l m a n i t i d species. T h e E r a m o s a represents
to decipher the original fossil content and o t h e r features.
a return to c o n d i t i o n s s o m e w h a t m o r e like those of the Rochester
An argillaceous d o l o s t o n e bed near the base of the G o a t
Shale, although pulses of rapid burial were rare in the E r a m o s a ;
Island, however, has yielded an e x t r a o r d i n a r y biota of s o f t - b o d i e d
hence, most
trilobites
are disarticulated.
In
New York and
organisms, particularly algae, s o m e w o r m s , and graptolites. T h i s
t h r o u g h o u t at least the eastern Niagara Peninsula of O n t a r i o , the
deposit, which L o D u c a ( 1 9 9 5 ) studied extensively, appears to rep-
upper beds of the E r a m o s a reflect an abrupt shallowing. Layers
resent an a c c u m u l a t i o n of muds in shallow waters between patch
o f stromatolites o r bacterial mat b o u n d s t o n e s were c o m m o n and
reefs in the G o a t Island F o r m a t i o n . Unfortunately, to date, it has
widespread at this t i m e . T h e s e stromatolites f o r m e d d o m a l heads
yielded few, if any, trilobite remains.
up to 2 or 3 m across and a m e t e r tall (Figure 4 . 3 3 ) . T h e s t r o m a -
T h e upper Goat Island in western New York, and particularly
tolites are generally associated with a sparse low diversity of
been
favositid corals, o s t r a c o d s , and a few species of b r a c h i o p o d s . T h e
misidentified as the E r a m o s a F o r m a t i o n . It is presently t e r m e d
i n c o m i n g of these beds appears to reflect the stress of increased
in
O n t a r i o , is
highly argillaceous and
frequently has
the Vinemount Shale Member. V i n e m o u n t Shale beds have yielded
hypersalinity in the Appalachian Basin. To the east, beds of the
several fragmentary fossil r e m a i n s , including trilobites, small
S c o n o n d o a and upper Ilion reflect these s a m e sorts of changes.
rugose corals, and b r a c h i o p o d s , although the extensive d o l o m i t i -
S t r o m a t o l i t i c limestones give way abruptly eastward to dark
zation of these argillaceous c a r b o n a t e s has o b s c u r e d m o s t of the
shales c o n t a i n i n g few n o r m a l m a r i n e o r g a n i s m s but remains o f lingulid b r a c h i o p o d s and eurypterids. T h e s e in turn pass upward
details. Near Ancaster, O n t a r i o , and locally in Niagara C o u n t y , New York, an upper portion of the G o a t Island yields white chert nodules that contain the remains of n u m e r o u s species of fossils
to the red beds of the V e r n o n F o r m a t i o n . Although not generally considered a trilobite-rich interval, the L o c k p o r t G r o u p in total has yielded the following trilobites:
in a rather g o o d state of preservation. Particularly notable are small siliceous sponges that may hint at the source for the silica of the light-colored cherts. Also, a variety of b r a c h i o p o d s and several trilobite species, including calymenids and dalmanitids, have been obtained as fragments f r o m these chert nodules. Evidently, early replacement by silica protected fossils f r o m d o l o m i tization, thereby leaving a reasonable record of these fossils.
Calymene
singularis
Calymene
Dalmanites
sp.
Diacalymene
sp.
Maurotarion
sp.
Encrinurus Proetus Scutellum
raybesti artiaxis wardi
Proetus Trimerus
sp.
tenuisulcatus delphinocephalus
Howell and Sanford ( 1 9 4 7 ) described trilobites from the G o a t Island
Formation
(formerly
misidentified
as
Oak
Orchard
M e m b e r ) in the Rochester area:
Late Silurian Salina G r o u p
Calymene Proetus
singularis artiaxis
Encrinurus
sp.
Scutellum
wardi
D u r i n g the Late Silurian, a m o r e arid climate prevailed over eastern Laurentia, and a landlocking of seas o c c u r r e d in two areas. T h e later part of the Silurian in eastern N o r t h America is
T h e Eramosa F o r m a t i o n is represented over m o s t of central and
represented by the Salina G r o u p . As the n a m e implies, these
western New York by vuggy, massive dolostones that have c o m -
deposits represent a m i x t u r e of s o m e shales and dolostones but
m o n l y been termed Oak Orchard Formation
are primarily n o t e d for evaporites. In the M i c h i g a n Basin, the
in the past. T h e s e
94
THE
PALEOZOIC
GEOLOGY
OF
NEW
YORK
FIGURE 4 . 3 3 . Domal stromatolites (arrow). Guelph Dolostone, Robert M o s e s Power Plant a c c e s s road, Lewiston, Niagara County.
barrier reef c o m p l e x restricted circulation, while in the eastern
ite beds within the Vernon reflects restriction of the Appalachian
Appalachian region, a new pulse of s e d i m e n t s eroded o f f coastal
Basin. T h e Vernon clastic sediments appear to represent a final
m o u n t a i n s . T h e V e r n o n - B l o o m s b u r g d e l t a — w a s derived f r o m
pulse of elastics derived f r o m erosion of the newly uplifted m o u n -
Salinic O r o g e n i c uplifts southeast (present directions) of New
tainous source terrain to the southeast of New York State. In
York State. T h e outward building ( p r o g r a d a t i o n ) o f the V e r n o n -
Pennsylvania, extremely thick red mudstones of the B l o o m s b u r g
B l o o m s b u r g delta
a
Shale take the place of the Vernon and seem to point to a nearby
partial barrier to o c e a n i c circulation in the n o r t h e r n Appalachian
source of m u d s and silts. North America at this time appears to
Basin of New York. T h e s e restrictive barriers prevented o p e n cir-
have lain within the subtropical desert belt, probably in paleolat-
culation to the sea and enabled salinity to build up to the p o i n t
itudes between 20 and 30° south of the equator (Figure 4 . 5 ) .
into
south-central
Pennsylvania
formed
of precipitating evaporites. A cyclic process of seawater influx
U n d e r such c i r c u m s t a n c e s , the development of a large wedge of
then evaporation enabled evaporites, such as anhydrite, g y p s u m ,
deltaic m u d s t o n e s and sandstones in the B l o o m s b u r g / V e r n o n
and halite, to a c c u m u l a t e to considerable thickness. T h e salt layer
F o r m a t i o n produced a restriction in the circulation in the n o r t h -
under Detroit, M i c h i g a n , is over 1 km thick.
ern Appalachian Basin region. T h i s restricted circulation in c o m -
T h e Upper Silurian V e r n o n F o r m a t i o n consists o f probably
b i n a t i o n with the arid climate led to the development of a
n o n m a r i n e or very shallow tidal-flat deposits of a red, rather
hypersaline seaway, probably similar in s o m e ways to the m o d e r n
massive m u d s t o n e . T h e s e beds c o n t a i n few fossils, although an
D e a d Sea. As seawater evaporated away, salt and anhydrite or
occasional interbedded d o l o s t o n e yields remains of fossil jawless,
gypsum deposits began to a c c u m u l a t e in the lower sags of the
a r m o r e d fish. Small clams and b r a c h i o p o d s also m a y be present
basin, while on the margins s o m e , at least seasonal, input of
in these beds. To the west the Vernon gives way from red m u d -
siliciclastic m u d s c o n t i n u e d .
stones to greenish gray d o l o m i t i c shales a n d d o l o s t o n e s , m a n y of
Although salt deposition o c c u r r e d within the western New
which contain evaporite crystal m o l d s . T h e V e r n o n in western
York/Genesee Valley region during deposition of the Vernon
New York is probably m o s t n o t e d as the source of key salt beds
Shales at the base of the Salina G r o u p , the locus of salt deposi-
that have been m i n e d for m a n y decades at the R e t s o f M i n e s in
tion
the area of the Genesee Valley. T h e a p p e a r a n c e of these evapor-
the Salina G r o u p sediments. T h i s eastward shifting also may
shifted progressively eastward during the deposition
of
DEVONIAN
PERIOD
have been associated with the m i n o r uplift of the old Vernon/
95 position (Figure 4 . 3 6 A ) . O n l y a few species of normal marine
B l o o m s b u r g deltaic land above sea level, which p r o d u c e d an e r o -
fossils are f o u n d within parts of the Bertie. T h e y include lingulid
sional beveling of Late to Middle Silurian sediments in areas east
b r a c h i o p o d s , a few o t h e r species of articulate b r a c h i o p o d s , and
of Utica, O n e i d a County. A large salt depositional basin existed
a
in the central region of the Finger Lakes and eventually in the
cephalopods).
classic Syracuse area. T h e s e deposits, the Syracuse F o r m a t i o n ,
e n o u g h that n o r m a l m a r i n e c o m m u n i t i e s still were not estab-
take their n a m e from that city. T h e f o r m a t i o n consists of thin-
lished in the offshore areas.
few
species
of
mollusks
(snails,
clams,
and
orthoconic
Evidently, salinity remained high and variable
bedded dolostones and shales and in places thick a c c u m u l a t i o n s
A new, unusual trilobite, apparently a lichid, was recently
of anhydrite or salt. Although the remains of n o r m a l m a r i n e
collected f r o m the e u r y p t e r i d - b e a r i n g waterlimes in Fort Erie,
organisms are u n c o m m o n within these beds, there are stray
O n t a r i o , near Buffalo. On the whole t h o u g h , these beds reflect
reports o f s o m e b r a c h i o p o d s and even o f o n e o c c u r r e n c e o f a
unusual hypersaline c o n d i t i o n s , and trilobites are very rare.
trilobite, a calymenid, within the Syracuse F o r m a t i o n in the Syra-
However, finally within the latest part of the Silurian, normal
cuse area. O t h e r w i s e , c o n d i t i o n s generally were far t o o harsh,
m a r i n e c o n d i t i o n s returned over New York, Pennsylvania, and
owing to the elevated salinity, to s u p p o r t n o r m a l m a r i n e c o m -
m u c h o f the Appalachian Basin. T h e Keyser F o r m a t i o n o f Penn-
munities, and most o f the beds o f the Salina G r o u p are barren o f
sylvania and the laterally equivalent R o n d o u t G r o u p and Decker
fossils. M o r e diverse m a r i n e fossils, including c a l y m e n i d trilo-
F o r m a t i o n in New York State c o n t a i n a far m o r e diverse assem-
bites, are k n o w n from the laterally equivalent Tonoloway F o r m a -
blage of m a r i n e fossils than do the underlying Bertie and Salina
tion in Pennsylvania, M a r y l a n d , and West Virginia.
beds. At least d u r i n g times of the deposition of the R o n d o u t ,
Salina deposition was terminated by the a c c u m u l a t i o n of the
shallow marine c o n d i t i o n s rather akin to those that developed
Camillus F o r m a t i o n , a relatively thick interval of m o t t l e d gray to
during the L o c k p o r t deposition existed over p o r t i o n s of eastern
slightly reddish barren shales with salt crystal molds and s o m e
and central New York State. At times, units such as the Cobleskill
dolostones, c o m m o n l y with molds of small gypsum or anhydrite
L i m e s t o n e a c c u m u l a t e d with a b u n d a n t c r i n o i d , coral, s t r o m a t o -
lathlike crystals. Not surprisingly, the Camillus is nearly barren of
poroid, and o t h e r remains. Trilobites are scarce but are repre-
fossils. It too has served as a source for e c o n o m i c a l l y i m p o r t a n t
sented
gypsum deposits in western New York.
camerata,
Hedstroemia
Dalmanites
aspinosus.
Bertie D o l o s t o n e and R o n d o u t G r o u p During the very late phases of Silurian deposition in New York
by
a
fair
diversity
of species
pachydermata, In
such
Richtcrarges
Pennsylvania
the
as
the
Calymene
ptyonurus,
laterally
and
equivalent
Keyser F o r m a t i o n c o n t a i n s small reef buildups of corals and strom a t o p o r o i d s and o t h e r shaly beds, rather reminiscent of the
State, the Bertie G r o u p sediments a c c u m u l a t e d . T h e Bertie is an
m u c h older Rochester Shale, that c o n t a i n diverse bryozoan and
unusual rock unit that consists of argillaceous, b u f f - c o l o r e d d o l o -
b r a c h i o p o d faunas and a b u n d a n t , well-preserved cystoids. Again,
stone, referred to in the past as waterlimes because of their p r o p -
s o m e trilobites are reported f r o m these Keyser beds. Preservation
erty as natural cement r o c k s , yielding c e m e n t s , which hardened
within the Keyser ranges f r o m disarticulated to exquisite arti-
underwater. T h e Bertie, as with the underlying Salina G r o u p , c o n -
culated remains. T h u s , the last chapter of Silurian m a r i n e de-
tains m u c h evidence for deposition at or near sea level. W i t h i n
position records a b r e a k d o w n of barriers to n o r m a l marine
the f o r m a t i o n are beds of low d o m a l stromatolites, extensive
circulation, as well as possible climatic changes from the m o r e
layers of fine, m u d - c r a c k e d d o l o s t o n e , gypsum crystal m o l d s ,
arid c o n d i t i o n s that characterized deposition of the Salina and
very shallow water ripples, and o t h e r evidence for deposition in
Bertie
a restricted tidal flat to shallow lagoonal setting. T h e Bertie is
Appalachian Basin, so t o o did n o r m a l m a r i n e representatives,
most noted for its extraordinary eurypterids at certain horizons.
which must have i m m i g r a t e d in f r o m outside the basin.
Groups.
As
normal
marine
seas
returned
to
the
T h e e n v i r o n m e n t of these interesting c h e l i c e r a t e a r t h r o p o d s is still debatable. Although they are often preserved in rocks that contain evidence for hypersalinity, such as salt crystal casts, the
Devonian Period
eurypterids probably did not live under these highly saline c o n -
T h e Devonian Period ( 4 1 5 to 3 6 0 million years ago) was
ditions. Rather, the local heavy a c c u m u l a t i o n s of carcasses of
relatively long, with m a n y i m p o r t a n t events in Earth and life
these animals probably represent dead remains that were washed
history (Figure 4 . 1 ) . M a j o r orogenies took place in Europe (end
out from estuaries that fed into the m o r e saline Bertie sea; c a r -
of C a l e d o n i a n ) , eastern N o r t h A m e r i c a ( A c a d i a n ) , and for the
casses were rapidly buried in briney sediments. T h e a p p e a r a n c e
first t i m e in the P h a n e r o z o i c , western North America (Antler
of small land plants, s o m e of the oldest k n o w n in the world, along
O r o g e n y ) (Figure 4 . 3 4 ) . M u c h o f the Devonian was characterized
with the eurypterids suggests that these sediments a c c u m u l a t e d
by w a r m , " g r e e n h o u s e " - t y p e climates and a strong tendency for
in very close proximity to low-lying lands of the upper tidal flats
stagnation in deeper sea e n v i r o n m e n t s , leading to the f o r m a -
and probably in small estuaries that e m a n a t e d o f f the exposed
tion of very widespread a n o x i c (oxygen-deprived) black shale
land that were, at least periodically, brackish water in their c o m -
deposits (Figure 4 . 3 5 ) . However, toward the end of the period
96
THE
PALEOZOIC
GEOLOGY
OF
NEW
YORK
FIGURE 4.34. C o m p o s i t e stratigraphic chart for the northern a n d central parts of the A p p a l a c h i a n Basin, s h o w i n g the depositional s e q u e n c e s a n d their relationship to t e c t o p h a s e s . From Ettensohn (1987), © 1987 by the University of C h i c a g o . All rights r e s e r v e d . R e p r o d u c e d with p e r m i s s i o n . there is evidence for climatic stress associated with renewed
lying Tristates groups, including the m o r e offshore facies, are c o n -
glaciation in G o n d w a n a (especially S o u t h A m e r i c a ) .
fined to the Hudson River Valley and probably o n c e extended northeastward into New England and Q u e b e c .
Early Devonian T h e Lower D e v o n i a n rocks of New York State are represented primarily by the Helderberg a n d Tristates G r o u p s . T h e final,
Late Early Devonian H e l d e r b e r g Facies and Trilobites
regressive phases of Sloss's T i p p e c a n o e (Tutelo phase) Superse-
T h e lowest unit of the Helderberg G r o u p is the T h a c h e r
quence are recorded in the Helderberg c a r b o n a t e s of eastern New
M e m b e r o f the M a n l i u s F o r m a t i o n (Figure 4 . 3 5 C ) . However, the
York and Pennsylvania (Figure 4 . 3 5 ) . T h e Helderberg G r o u p is
M a n l i u s facies are distinctly d i a c h r o n o u s , being younger in the
a series of limestones and m i n o r shales that c r o p out in central
area near Syracuse, New York, than in the Hudson Valley and
to eastern New York State. O u t s t a n d i n g exposures of these rocks
equivalent in age to the C o e y m a n s or even Kalkberg F o r m a t i o n s
are cuts along Rte. 1-88 in the S c h o h a r i e Valley, and a n u m b e r of
in the H u d s o n Valley. T h e M a n l i u s is considered to be close to the
quarries and road cuts from the area of Albany, at the Helderberg
S i l u r i a n - D e v o n i a n b o u n d a r y (Rickard 1 9 7 5 , 1 9 8 1 ; K l a p p e r 1 9 8 1 ) .
E s c a r p m e n t , southward to the state line at Port Jervis, O r a n g e County.
T h e M a n l i u s F o r m a t i o n bears m a n y resemblances to the O r d o v i c i a n Black River G r o u p . B o t h intervals contain a series of
It should be noted that d u r i n g Helderberg deposition, the axis
peritidal to very shallow subtidal facies. T h e Manlius represents
(deepest part) of the Appalachian Basin was at a position s u b -
the belt of low-energy but very shallow lagoonal to tidal-fiat envi-
stantially farther east than d u r i n g Silurian or later Devonian
r o n m e n t s that were sheltered to shoreward by offshore shoals and
t i m e . T h e basin axis appears to have shifted eastward d u r i n g a
bars. T h i s suite o f e n v i r o n m e n t s has been called the " Z " zone
t i m e of tectonic quiescence b e g i n n i n g in the late part of the
(Irwin 1 9 6 5 ) . T h e s e facies typically are arranged in 1- to 3 - m
Silurian, and to have been in a position east of the H u d s o n Valley
scale, shallowing-upward cycles referred to as punctuated aggra-
extending northeastward into New England at this p o i n t in Early
dational cycles (PACs)
D e v o n i a n t i m e . For this reason, m o s t of the Helderberg and over-
cyclic facies are typical of the older T h a c h e r M e m b e r of the
( G o o d w i n and Anderson
1 9 8 5 ) . Such
I Acadian O r o g e n y - Collision of Avalon and P r o t o North A m e r i c a