The Paleozoic Era Diversification of plant and animal life (J.P. Rafferty 2011)

7/23/2019 The Paleozoic Era Diversification of plant and animal life (J.P. Rafferty 2011)

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Published in 2011 by Britannica Educational Publishing(a trademark of Encyclopdia Britannica, Inc.)in association with Rosen Educational Services, LLC29 East 21st Street, New York, NY 10010.

Copyright 2011 Encyclopdia Britannica, Inc. Britannica, Encyclopdia Britannica,and the Thistle logo are registered trademarks of Encyclopdia Britannica, Inc. Allrights reserved.

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Distributed exclusively by Rosen Educational Services.For a listing of additional Britannica Educational Publishing titles, call toll free (800) 237-9932.

First Edition

Britannica Educational PublishingMichael I. Levy: Executive EditorJ.E. Luebering: Senior ManagerMarilyn L. Barton: Senior Coordinator, Production ControlSteven Bosco: Director, Editorial TechnologiesLisa S. Braucher: Senior Producer and Data EditorYvette Charboneau: Senior Copy EditorKathy Nakamura: Manager, Media AcquisitionJohn P. Rafferty: Associate Editor, Life and Earth Sciences

Rosen Educational ServicesAlexandra Hanson-Harding: Senior EditorNelson S: Art DirectorCindy Reiman: Photography DirectorMatthew Cauli: Designer, Cover DesignIntroduction by Catherine Vanderhoof

Library of Congress Cataloging-in-Publication Data

The Paleozoic era : diversication of plant and animal life / edited by John P. Rafferty. p. cm.(The geologic history of Earth)In association with Britannica Educational Publishing, Rosen Educational Services.Includes bibliographical references and index.ISBN 978-1-61530-196 -6 ( eBook )1. Geology, StratigraphicPaleozoic. 2. PaleoecologyPaleozoic. I. Rafferty, John P.QE654.P2413 2010561'.112dc22

2009051123

On the cover: Plants such as ferns (above) and ancient creatures such as the trilobite(below) were common during the Paleozoic era. SSPL via Getty Images (above); SinclairStammers/Photolibrary/Getty Images (below and pages 5, 29, 36, 93, 138, 187, 245, 285, 325, 326, 329, 331)

On page 18: Fallen pine trees in Emerald Lake, reecting the Burgess Shale, BritishColumbia, Canada. Shutterstock.com


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C ONTENTS

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Introduction 18

Chapter 1: An Overview of Paleozoic Time 29

Paleozoic Life 30Paleozoic Geography 32

Chapter 2: The Cambrian Period 36The Cambrian Environment 38 Paleogeography 38 Paleoclimate 45Cambrian Life 46 The Fossil Record of the

Precambrian-Cambrian

Transition 47 Photosynthetic Organisms 51 Fauna 52 Deposits with Soft-Bodied

Organisms 56 Cambrian Extinction Events 59 Signicant Cambrian

Life-Forms 60 Burgess Shale 60 Conodonts 61 Denticles 64 Graptolites 65 Lamp Shells 65 Lingulids 78 Obolus 78

Stromatoporida 79 Trilobites 79Cambrian Geology 82 Types and Distribution 82

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The Boundaries andSubdivisions of theCambrian System 85

The Economic Signicance of Cambrian Deposits 87

The Correlation ofCambrian Strata 88

Stages of the Cambrian Period 90 Fortunian Stage 90 Drumian Stage 91 Guzhangian Stage 91 Paibian Stage 92

Chapter 3: The OrdovicianPeriod 93The Ordovician Environment 93 Paleogeography 96 Sea Level 97 The Circulation

of the Ocean 97 Plate Tectonics 99 Paleoclimate 101Ordovician Life 104 Marine Organisms 104 Microfossils and Plankton 104 Invertebrates 105 Early Fishes 107 Reefs 108 Terrestrial Organisms 109 Earliest Land Plants 109 Animals 110 The Ordovician Radiation 110 The Exploitation of

Habitats 112

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Causes of the Ordovician Radiation 114 Ordovician Extinction

Events 114 The Mass Extinction at the

End of the Ordovician 114 Regional Extinctions Within the Ordovician 116 Faunal Provinces 117 Signicant Ordovician

Life-Forms 118 Bellerophon 118 Bumastus 119 Byssonychia 119 Calymene 119 Ceraurus 119 Climacograptus 120 Clonograptus 120 Constellaria 120 Cryptolithus 120 Cryptostomata 121 Cystoids 121 Didymograptus 121 Diplograptus 122 Echinosphaerites 122 Favosites 122 Hallopora 122 Halysites 123 Hesperorthis 123 Horn Corals 123 Isotelus 124 Leptaena 124 Lituites 124 Lophospira 124

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Loxonema 125 Maclurites 125 Modiolopsis 125 Neoprioniodiform 125 Phyllograptus 126 Platystrophia 126 Plectoceras 126 Prasopora 126 Resserella 127 Rhynchotrema 127

Streptelasma 127 Strephomena 127 Tabulata 128 Tetragraptus 128 Trepostomata 128 Trochonema 128Ordovician Geology 129 Signicant Geologic Events 129

The Economic Signicance of Ordovician Deposits 131

The Major Subdivisions of the Ordovician System 132

The Correlation of Ordovician Strata 132

Stages of the Ordovician Period 133 Tremadocian Stage 134 Floian Stage 134 Dapingian Stage 135 Darriwilian Stage 135 Sandbian Stage 136 Katian Stage 136 Hirnantian Stage 137

Chapter 4: The Silurian Period 138The Silurian Environment 138 Paleogeography 140 Laurentia 141

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Baltica 142 Siberia, Kazakhstania, and

Other Continents 143 Gondwana 143 Paleoclimate 144Silurian Life 146 Pentamerid Communities 147 Reef Mounds and Coral

Biostromes 149 Fishes 150

Vascular Land Plants 151 Silurian Extinction Events 151 Signicant Silurian Life-Forms 152 Atrypa 153 Baragwanathia 153 Birkenia 154 Chonetes 154 Conchidium 154

Cyathocrinites 155 Cystiphyllum 155 Deiphon 155 Eospirifer 155 Giant Water Scorpions 156 Goniophora 157 Leptodesma 157 Monograptus 157 Nuculana 158 Ozarkodiniforms 158 Phacops 158 Platyceras 158 Preferns 159 Rhynchotreta 160Silurian Geology 160

The Economic Signicance of Silurian Deposits 161 The Major Subdivisions of the

Silurian System 163147


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Signicant Geologic Events 164

The Effects of LateOrdovician Glaciation 164

Silurian Sea Level 165 The Occurance and

Distribution of Silurian Deposits 166

Evaporites 167 Clastic Wedges 168

Platform Margins 170 Tillites 171 Volcanic Rocks 172 The Correlation of

Silurian Strata 173 Graptolites 173 Brachiopods 174 Conodonts 175 Isotope Stratigraphy 176 Establishing Silurian

Boundaries 177 The Work of Roderick

Murchison and OtherResearchers 177

Geochemical Analyses 179 Silurian Boundaries 180Stages of the Silurian Period 181 Rhuddanian Stage 182 Aeronian Stage 182 Telychian Stage 183 Sheinwoodian Stage 183 Homerian Stage 184

Gorstian Stage 184 Ludfordian Stage 185 Pridoli Series 185

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Chapter 5: The DevonianPeriod 187The Devonian Environment 190 Paleogeography 190 Paleoclimate 192Devonian Life 192 Invertebrates 193 Vertebrates 195 Plants 197 Faunal Realms and Migrations 199

Devonian Extinction Events 200 Signicant Devonian Life-Forms 202 Ammonoids 202 Antiarchs 203 Archaeopteris 203 Arctolepis 204 Arthrodires 204 Bactrites 205

Bothriolepis 205 Cephalaspis 206 Cheirolepis 206 Cladoselache 207 Climatius 207 Dinichthys 207 Dipterus 208 Endothyra 208 Eusthenopteron 209 Greenops 209 Heliophyllum 210 Hyenia 210 Ichthyostega 210 Monoplacophorans 211 Mucrospirifer 212

Osteolepis 212 Ostracoderms 213 Palaeospondylus 213 211

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Placoderms 213 Polygnathiforms 214 Pteraspis 215 Rensselaeria 215 Rhipidistia 215 Rhynie Plants 216 Schizodus 217 Sphenophyllum 217 Stringocephalus 218 Stropheodonta 218 Theodossia 218 Tiktaalik Roseae 218 Tornoceras 220 Trepospira 220 Tropidoleptus 220 Worthenia 221 Xenacanthus 221Devonian Geology 221 Signicant Geologic Events 222 The Economic Signicance of

Devonian Deposits 224 The Major Subdivisions of the

Devonian System 225 Establishing Devonian

Boundaries 225 The Occurance and Distribution of

Devonian Deposits 226 Sediment Types 229 Europe 230 Asia 234 Southern Hemisphere 235 North America 237 The Correlation of

Devonian Strata 238

Stages of the Devonian Period 239 Lochkovian Stage 239 Pragian Stage 240 Emsian Stage 241

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Eifelian Stage 241 Givetian Stage 242 Frasnian Stage 243 Famennian Stage 244

Chapter 6: The CarboniferousPeriod 245The Carboniferous Environment 245 Paleogeography 246 Paleoclimate 247

Carboniferous Life 248 Invertebrates 249 Plants 251 Fishes 252 Amphibians and Early Reptiles 253 Signicant Carboniferous

Life-Forms 254 Calamites 254

Composita 255 Cordaitales 255 Cryptoblastus 255 Dictyoclostus 256 Dielasma 256 Diplovertebron 256 Edaphosaurus 257 Euphemites 257 Fenestella 258 Fusulina 258 Fusulinella 259 Fusulinids 259 Gastrioceras 260 Labyrinthodonts 260 Lebachia 261

Lepidodendron 261 Linoproductus 262 Lophophyllum 263 Myalina 263

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Neospirifer 263 Nuculopsis 264 Pentremites 264 Phillipsia 264 Platycrinites 264 Seed Ferns 265 Sigillaria 266 Wedekindellina 266Carboniferous Geology 267 Signicant Geologic Events 268

The Major Subdivisions of the Carboniferous System 269 The Economic Signicance of

Carboniferous Deposits 270 The Occurance and Distribution of

Carboniferous Deposits 270 Mississippian Limestones 271 Pennsylvanian Cyclothems,

Tillites, and Turbidites 273The Correlation of Carboniferous Strata 276

Missippian Subsystem 276 Pennsylvanian Subsystem 277Stages of the Carboniferous Period 278 Mississippian Subperiod 278 Tournaisian Stage 279 Visean Stage 280 Serpukhovian Stage 281 Pennsylvanian Subperiod 282 Bashkirian Stage 282 Moscovian Stage 283 Kasimovian Stage 283 Gzhelian Stage 284

Chapter 7: The Permian Period 285The Permian Environment 286 Paleogeography 286

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Paleoclimate 288Permian Life 289 The Emergence of Important

Reptiles 291 Permian Mass Extinction 293 Temperature Crises and the

Permian Extinction 294 The Alteration of the

Carbon Cycle 295 Other Potential Causes of the

Permian Extinction 296 Signicant Permian Life-Forms 296 Bradysaurus 396 Cacops 297 Captorhinus 297 Cynodonts 298 Diadectes 299 Dimetrodon 300 Eryops 300 Glossopteris 301 Leptodus 302 Limnoscelis 302 Mesosaurus 303 Moschops 303 Parafusulina 304

Paraschwagerina 304 Pseudoschwagerina 304 Schwaserina 305 Seymouria 305 Tapinocephalus 306 Venyukovia 306 Permian Geology 306

The Major Subdivisions of the Permian System 307 The Economic Signicance of

Permian Deposits 309

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INTRODUCTIONINTRODUCTION


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T he Paleozoic Era is probably less familiar and perhapsless dramatic than the age of the dinosaurs that woulddominate the Mesozoic Era that followed. However, thePaleozoic Era contained one of the most intense increasesin biodiversity in Earths history, the Cambrian explosionand the subsequent Ordovician radiation. It also con-tained the largest extinction event the world has everknown, the Permian extinction, which wiped out morethan 90 percent of marine species and roughly 70 percentof species on land. It was also a time of great geologicalchanges as landmasses migrated and collided, eventuallycreating the supercontinent called Pangea. Sea levels rose,drowning whole continents, fell, and rose again. Some ofEarths oldest mountain ranges, such as the Appalachiansand the Urals, were formed during the Paleozoic. Lifemoved from the oceans to dry land and insects took wingfor the rst time. Many evolutionary advances took place,

which set the stage for life as we know it today. Some ofthese advances include the development of plants withseeds, shelled eggs, and organisms capable of breathingair. So in geologic history the Paleozoic Era was prettydramatic after all. In the pages that follow, all of thesedevelopments, as well as the clues that scientists have usedto decipher the history of Earths changes, will be explored.

Spanning nearly 300 million years of history, from 542to 251 million years ago, the Paleozoic Era covers morethan half of the Phanerozoic Eon, also known as the Ageof Lifethe geologic time in which humans still live.Scientists divide the Paleozoic into a number of smallerperiods, beginning with the Cambrian, approximately 542million to 488 million years ago, followed by the

Ordovician, Silurian, Devonian, Carboniferous, andPermian periods. Most of these names are derived fromthe locations in which rocks and fossils from that time

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were rst discovered and studied. The exception is theCarboniferous period, so named because it was the periodin which great coal beds were formed all around the world.These coal beds still provide a signicant portion of the worlds energy needs today.

The Cambrian period, the rst period of the PaleozoicEra, was a time of great transition in the Earths environ-ment. Global temperatures, which had been cooler thantoday, suddenly warmed to average temperatures of 22 C(72 F), much warmer than todays global temperatures ofapproximately 14 C (57 F). There were no polar ice caps,and the oceans ooded over large portions of the worldscontinents. These changes led to increased levels of oxygen in the ocean waters. This increase in turn set the stagefor the Cambrian explosion a sudden burst of evolution-ary development leading to an amazing diversication oflife in the sea. Although some primitive life forms existed

prior to this time, they have left little information in thefossil record. In the Cambrian period, however, life formssuch as trilobites and other species with hard skeletonsleft fossils that can be found in present-day rocks through-out the world.

The most famous fossil eld for studying the Cambrianperiod is the Burgess Shale, a formation in western Canada.

The rock in this area is thought to have beenformed froma giant mudslide that rolled into the sea and buried a hugearea of the ocean oor. Instantly, the mudslide killed all liv-ing organisms unlucky enough to live beneath it andpreserved their remains as the mud hardened into rock.The fossils in the Burgess Shale are therefore extremelydetailed and include soft body parts and animals without

an exoskeleton that would normally have decomposedbefore becoming fossilized. This remarkable nd has givenscientists a much more complete picture of life in theCambrian period than would be afforded by more typical


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fossil formations. More than 60,000 specimens have beenidentied from fossils in the Burgess Shale. Many of theseorganisms have no living counterparts today.

The Ordovician period, approximately 488 to 443 mil-lion years ago, is when almost every modern phylum ofmarine invertebrate began to appear, as well as the rstshes. During this period, the rst land plants occurred.Scientists contend that the earliest terrestrial animals,early arthropods similar to millipedes, also appeared.Scientists have not found any fossils of these animals, buthave found fossilized burrows, which, evidence suggests,could have been created by such creatures.

Besides the diversication of life forms, the Ordovicianperiod was also characterized by signicant geologicalactivity. Tectonic shifts caused extensive and created vastridges deep in the ocean oor. The continental landmasses were all in motion, shifting their positions on the globe.

Signicant parts of the continents were submerged duringa good portion of this period due to rising sea levels andhigh global temperatures

Sedimentary rock from this period continues to con-tain a multitude of fossils. These rocks reveal much aboutlife on earth and in the seas, but much of the Ordovicianrock layer attests to the volcanic eruptions that character-

ized this period. Comparison of the makeup of the volcanic rocks from different regions allows scientists toestimate the incredible magnitude of these eruptions.Studying the rock layers from this period also providesevidence of huge hurricanes and typhoons across theoceans. The Ordovician period ended with a mass extinc-tion second only to the Permian extinction some 200

million years later.By contrast, the Silurian period (443.7-416 million years ago) was relatively quiet. Shallow seas extended overmost of the world. Marine life rebounded after the

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Geologic Time Scale

Encyclopdia Britannica, Inc. Source: International Commission onStratigraphy (ICS)


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Ordovician extinction, and similar types of life formsthrived worldwide. Coral reefs were common, and sh were widely distributed. Also present in this period werenow-extinct giant arthropods known as sea scorpions.These creatures grew up to 2.5 metres (8 feet) in length with large grasping pincers to trap their prey. On land,plants continued to evolve, although most were still quitesmalljust a few inches in heightand conned to areasnear the coastline. These early plants did not yet haveleaves or seeds. They consisted of smooth branched stems with spore sacs for reproduction.

Rocks from the Silurian period include a crystallizedrock called dolomite. Dolomite is harder than the shalethat characterized much of the sedimentary rock fromearlier periods. It is rare for dolomite to be formed today.Dolomite can be observed in such natural features asNiagara Falls, formed by erosion washing away the under-

lying shale below a layer of harder Silurian stone.The Devonian period, spanning 416 million to 359 mil-

lion years ago, is sometimes called the Age of Fishes butalso sees the emergence of the rst land-based vertebrates.Fossil evidence also indicates the presence of forests bythe end of this period, signifying a rapid surge in the evolu-tion of plant life.

Devonian sh could be found in both saltwater andfreshwater environments. Many Devonian sh had exter-nal armor or scales. These exoskeletons allowed forexcellent fossil preservation of those species. By the endof the period, most of these armored sh had becomeextinct, however. Of more importance for evolutionaryhistory are early sharklike shes, lungshes, and a group of

sh known as rhipidistians. Although the Rhipidstiabecame extinct by the early Permian period, many scien-tists maintain that their body features suggest that these were most likely the branch from which early amphibians


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and all later terrestrial vertebrates developed. This issue,however, is not settled.

An even more dramatic evolutionary development was taking place in the plant world, however. By the end ofthe Devonian period, plants had established themselvesthroughout the land masses, not only at the margins of thesea, and had evolved from primitive leaess forms to a wide variety of ferns, leafy stemmed plants, and even therst true woody trees, some with trunks up to 1.8 metres(6 feet) in diameter. Fossils of plant forms from this periodare frequently found in a layer of rock known as Old RedSandstone, characteristic of the Devonian period. Thepreserved remains are both beautiful in their detail andextremely important for scientists to understand the anat-omy and function of these early plants.

The presence of this sandstone rock layer in diverseareas of the globe also helps scientists to understand the

movement of the continents during this period, whichfeatured the creation of a single Northern Hemispherecontinent named Laurussia, comprising most of NorthAmerica and Europe. Laurussia was formed by the unionof two smaller landmassesLaurentia and Baltica. SouthAmerica, Africa, India, Australia, and Antarctica were allpart of a single supercontinent in the Southern Hemisphere

called Gondwana.The vast coal deposits across Europe, Asia, Australia,and North America that gave the Carboniferous period itsname formed as giant swamps full of vegetation graduallyand repeatedly decayed. The movements of the conti-nents during this period (from 359-299 million years ago)eventually joined the two major landmasses of Laurussia

and Gondwana, forming the Appalachian mountain rangeat the point of their collision. A later collision of Siberia with Eastern Europe formed the Urals. The movement ofthe continents also changed the global climate, causing

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the formation of glacial ice in Gondwana. These ice sheetsemerged and retreated, causing times of continental ood-ing interspersed with times of drier terrestrial terrain,particularly during the latter half of this period.

With the increase of terrestrial habitats, severalimportant evolutionary advances occurred during theCarboniferous period. Perhaps the most important ofthese was the development of amniotic eggs (that is, eggsthat were encased in shells to prevent drying). TheCarboniferous period was the time of peak amphibiandevelopment and the emergence of the reptiles (which layamniotic eggs). One of the earliest reptiles, Hylonomus, was recovered from Lower Pennsylvanian tree stumps inNova Scotia.

The Carboniferous period also saw the rst wingedinsects. By the Pennsylvanian subperiod (the second halfof the Carboniferous period), dragonies and mayies

were abundant and had reached large sizes. Fossils of moreadvanced insects capable of folding their wings, particu-larly cockroaches, have been dated to the Pennsylvaniansubperiod. Other Pennsylvanian insects include the ances-tral forms of grasshoppers and crickets and the rstterrestrial scorpions.

The Permian period (from 299 to 251 million years ago)

ended the Paleozoic Era, and ended it in dramatic fashion with the largest extermination event in Earths history.The cause of this massive disruption was most likely cli-mate change, perhaps caused in part by the continuingunication of continental landmasses. During the EarlyPermian period this process was largely complete, withthe merging of Gondwana and Laurussia.

Worldwide climate warmed throughout the Permianperiod, and much of the world became drier. There werealso signicant drops in sea level and the shallow surface


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waters of Permian seas warmed, which would havedestroyed the habitat of many marine species. The warmerand drier climate during the early to middle Permianperiod also allowed for increasing diversity and develop-ment of many types of ora and fauna, however, particularlythose suited to the terrestrial environment. Early conifer-ous plants and the precursors to owering plants allappeared in the Middle and Late Permian periods. Asplants diversied and spread, so did the insect populationsthat adapted to these new species.

The Permian period also saw the emergence of severalimportant reptile types, including early ancestors of dino-saurs, crocodiles, and birds, as well as precursors to snakes,lizards, turtles, and other reptile species. The fossil recordfrom this period also reveals a group of transitional reptilescalled synapsids that had begun to show mammal-like toothand bone features. Evidence of these animals at relatively

high and colder latitudes would suggest that they also hadthe ability to maintain an elevated body temperature,another feature that separates mammals from reptiles.

By the end of the Permian period, the overwhelmingmajority of marine and terrestrial species had becomeextinct. Marine invertebrates were especially hard hit, andseveral life forms that had been common since the early

Paleozoic period disappeared from the fossil record. Alllife on Earth today is descended from the survivors of thePermian extinction.

Our understanding of the Paleozoic Era comes fromabout 175 years of work with fossils and the various rocklayers that contained. There is still much to learn aboutthe Paleozoic world, and several debates still need to be

resolved. This book will reveal what scientists do knowabout the Paleozoic Era, how they know it, and what stillremains to be explained.

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CHAPTER 1 A N O VERVIEW OF P ALEOZOIC T IME

Spanning nearly 300 million years, the Paleozoic Era was one of the dening intervals shaping life on Earth.

It was characterized by the sudden emergence of multi-tudes of new life-forms as well as the migration of severalof Earths continents from the Southern Hemisphere tothe Northern Hemisphere. By the end of the era, all ofEarths major landmasses were merged into one. ThePaleozoic was a time in which many bizarre creaturesthrived. Long, tubular Hallucigenia oated through ancient

seas along with trilobites and armoured shes, while earlyamphibians rst crawled onto solid ground. These were just a few of the notable members of Paleozoic faunas.However, life on Earth was subjected to three of the mostsevere extinctions of all time.

The Paleozoic Era was a major interval of geologictime that began 542 million years ago with the Cambrian

explosion, an extraordinary diversication of marine ani-mals, and ended 251 million years ago with the end-Permianextinction, the greatest extinction event in Earth history.The major divisions of the Paleozoic Era, from oldest to youngest, are the Cambrian (542 to 488.3 million yearsago), Ordovician (488.3 to 443.7 million years ago), Silurian(443.7 to 416 million years ago), Devonian (416 to 359.2

million years ago), Carboniferous (359.2 to 299 million years ago), and Permian (299 to 251 million years ago)periods. The Paleozoic takes its name from the Greek word for ancient life.


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PALEOZOIC LIFE

The story of the earliest Paleozoic animals is one of life inthe sea. Presumably simple fungi and related forms existedin freshwater environments, but the fossil record providesno evidence of these modes of life. The terrestrial envi-ronment of the early Paleozoic was barren of the simplestof life-forms.

The Cambrian explosion was a sharp and suddenincrease in the rate of evolution. About 542 million yearsago, at the onset of the Cambrian Period, intense diversi-cation resulted in more than 35 new animal phyla.However, new discoveries show that the explosionstarted roughly 575 million years ago, near the end of theProterozoic Eon, with the Ediacara fauna. The biota rap-idly diversied throughout the Cambrian and Ordovician

An early Silurian coral-stromatoporoid community.


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periods as life-forms adapted to virtually all marine envi-ronments. In numbers of described marine species, fossilsof trilobites dominate Cambrian rocks, whereas brachio-pods (lamp shells) predominate in strata from theOrdovician through the Permian Period.

Several different kinds of organisms adapted indepen-dently to life on land, primarily during the middlePaleozoic. Leaess vascular plants (psilophytes) andinvertebrate animals (centipede-like arthropods) wereboth established on land at least by Silurian time.

Vertebrate animals made the transition to land via theevolution of amphibians from air-breathing crossopter-

ygian sh during Devonian times. Further conquest ofthe land became possible during the CarboniferousPeriod, when plants and animals evolved solutions toovercome their dependence on moist environments for

The lone tree with horizontal grooves in the right foreground is a jointed sphe- nopsid (Calamites). The large trees with scar patterns are lycopsids.Courtesyof the Department Library Services, American Museum of NaturalHistory, neg. #333983

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reproduction: waterborne spores were replaced by seedsin plants of seed-fern origin, and shell-less eggs werereplaced by amniote eggs with protective shells in animalsof reptilian origin. Flight was rst achieved also duringthe Carboniferous Period as insects evolved wings.

The Permian extinction, at the end of the PaleozoicEra, eliminated such major invertebrate groups as theblastoids (an extinct group of echinoderms related tothe modern starsh and sea lilies), fusulinids, and trilo-bites. Other major groups, which included theammonoids, brachiopods, bryozoans (small colonial ani-mals that produce a skeletal framework of calciumcarbonate), corals, and crinoids (cuplike echinoderms with ve or more feathery arms), were severely decimatedbut managed to survive. It has been estimated that asmany as 95 percent of the marine invertebrate speciesperished during the late Permian Period. Extinction rates

were much lower among vertebrates, both aquatic andterrestrial, and among plants. Causes of this extinctionevent remain unclear, but they may be related to thechanging climate and exceptionally low sea levels ofthe time. Although of lesser magnitude, other importantPaleozoic mass extinctions occurred at the end of theOrdovician Period and during the late Devonian Period.

PALEOZOIC GEOGRAPHY

On a global scale, the Paleozoic was a time of continentalassembly. The majority of Cambrian landmasses were gathered together to form Gondwana, a supercontinentmade up of the present-day continents of Africa, South

America, Australia, and Antarctica and the Indian subcon-tinent. It extended from the northern tropics to thesouthern polar regions. With the exception of three majorcratons (landmasses forming the stable interiors of


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continents) not part of the initial conguration ofGondwana, the remainder of Earth was covered by the global Panthalassic Ocean.

Laurentia, a craton primarily made up of present-dayNorth America and Greenland, was rotated 90 clockwisefrom its present orientation and sat astride the paleoequa-tor during Cambrian times. Laurentia (a name derivedfrom Quebecs portion of the Canadian Shield) was sepa-rated from Gondwana by the Iapetus Ocean. The smallerBaltica craton was positioned within the Iapetus Ocean,lying to the south of Laurentia and just off the northernmargin of Gondwana. Baltica was made up of much ofScandinavia and western Europe. To the east of Laurentia,the Siberian craton was positioned just south of the paleo-equator between Laurentia and the western coast ofGondwana. Until the late Carboniferous Period, Siberia was rotated 180 from its present orientation.

While a portion of Gondwana was positioned at ornear the South Pole, there is no evidence of glaciation dur-ing Cambrian time. While little is known about the nerdetails of the Cambrian climate, geologic evidence showsthat the margins of all continents were ooded by shallowseas. It is in the rock formed within these shallow seasthat the greatest explosion of life ever recorded occurred.

By Ordovician time, part of Gondwana had begun to moveover the South Pole. The distribution of extensive glacialdeposits, which formed later in the Paleozoic, has beenused to track the movement of parts of Gondwana overand around the South Pole.

Siberia, Baltica, and Laurentia also moved to new loca-tions during the course of the Paleozoic. Siberia, essentially

the large Asian portion of present-day Russia, was a sepa-rate continent during the early and middle Paleozoic, whenit moved from equatorial to northern temperate latitudes.Baltica moved across the paleo-equator from southern



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cool temperate latitudes into northern warm latitudes dur-ing the Paleozoic Era. It collided with and joined Laurentiaduring the early Devonian Period. The beginnings of suchmountainous regions as the Appalachians, Caledonides,and Urals resulted from the Paleozoic collision of the litho-

spheric plates. By the end of the Paleozoic, continuedtectonic plate movements had forced these cratonstogether to form the supercontinent of Pangea. Largeareas of all continents were episodically inundated by shal-low seas, with the greatest inundations occurring duringthe Ordovician and early Carboniferous (Mississippian)periods.

Paleozoic rocks are widely distributed on all conti-nents. Most are of sedimentary origin, and many showevidence of deposition in or near shallow oceans. Among

Devonian volcanics formed Ancient Wall. Once the oor of a shallow ocean, it now juts up to a peak in Canadas Jasper National Park. Jonathan S. Blair/ National Geographic/Getty Images


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the more useful guide fossils for correlation are trilobites(an extinct group of aquatic arthropods), for Cambrianthrough Ordovician strata; graptolites (small, colonial,planktonic animals), for rocks dated from Ordovicianthrough Silurian times; conodonts (primitive chordates with tooth-shaped fossil remains), for Ordovician toPermian rocks; ammonoids (widely distributed extinctmollusks resembling the modern pearly nautilus), forDevonian through Cretaceous strata; and fusulinids (single-celled amoeba-like organisms with complex shells),for rocks dating from the Carboniferous through thePermian Period.



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CHAPTER 2T HE C AMBRIAN P ERIOD

The earliest time division of the Paleozoic Era wasthe Cambrian Period, an interval extending from

about 542 to 488.3 million years ago. The CambrianPeriod is divided into four stratigraphic series: Series 1(542 to 521 million years ago), Series 2 (521 to 510 million years ago), Series 3 (510 to 501 million years ago), and theFurongian Series (499 to 488.3 million years ago).

Rocks formed or deposited during this time areassigned to the Cambrian System, which was named in

1835 by English geologist Adam Sedgwick for successionsof slaty rocks in southern Wales and southwesternEngland. These rocks contain the earliest record of abun-dant and varied life-forms. The corresponding period andsystem names are derived from Cambria, the Roman namefor Wales. As originally described, the Cambrian System was overlain by the Silurian System, which was named,

also in 1835, by Scottish geologist Roderick I. Murchison.Subsequent disagreement between Sedgwick andMurchison over the denition and placement of theCambrian-Silurian boundary led to a bitter controversythat involved many British geologists. The problem per-sisted until after the deaths of both Sedgwick andMurchison in the 1870s and the eventual adoption of an

intervening system, the Ordovician, which was proposedin 1879 by English geologist Charles Lapworth.The Cambrian world differed greatly from that of the

present, but it was also quite different from the preceding


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Proterozoic Eon (2.5 billion to 542 million years ago) interms of climate, geography, and life. Average global tem-peratures during much of the Neoproterozoic Era (1billion to 542 million years ago) were cooler (around 12C[54F]) than the average global temperatures (around14C [57 F]) of the present day, whereas the global tem-perature of Cambrian times averaged 22C (72F). Lowtemperatures during the Neoproterozoic helped to sus-tain a series of worldwide events known as the Sturtian(748 to 713 million years ago), Marinoan (650 to 600 mil-lion years ago), and Gaskiers (595 to 565 million years ago) glaciations. Climate studies suggest that Cambrian tem-peratures were the norm for most of the Phanerozoic Eon(the last 542 million years), and these were exceeded onlyby a brief increase during the Permian Period near the endof the Paleozoic Era. Cooler periods, similar to the aver-age global temperature of the present day, occurred during

the end of the Ordovician, Late Carboniferous, EarlyPermian, Late Jurassic, and Early Cretaceous periods, as well as near the end of the Oligocene Epoch.

Just prior to the beginning of the Neoproterozoic Era,Earth experienced a period of continental suturing thatorganized all of the major landmasses into the huge super-continent of Rodinia.

Rodinia was fully assembled by one billion years agoand rivaled Pangaea (a supercontinent that formed laterduring the Phanerozoic Eon) in size. Before the beginningof the Cambrian, Rodinia split in half, resulting in the cre-ation of the Pacic Ocean west of what would becomeNorth America. By the middle and later parts of theCambrian, continued rifting had sent the paleocontinents

of Laurentia (made up of present-day North America andGreenland), Baltica (made up of present-day westernEurope and Scandinavia), and Siberia on their separate ways. In addition, new collisional events led to the


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formation of Gondwana, a supercontinent composed of what would become Australia, Antarctica, India, Africa,and South America.

The tectonic events involved in the breakup of Rodiniaalso modied the ocean basins, forcing their expansionand ooding portions of many continents. The melting ofthe Varanger glaciers during the Neoproterozoic Era alsoplayed a role in the ooding of continents. This episoderepresented one of the largest and most persistent rises insea level of the Phanerozoic Eon. Though the extent ofcontinental ooding varied, for most continents sea levelreached its maximum by the middle and later parts of theCambrian time. This ooding, combined with the elevated Cambrian temperatures and changes in Earths geography, led to increased rates of erosion that alteredocean chemistry. The most notable result was an increasein the oxygen content of seawater, which helped set the

stage for the rise and later diversication of lifean eventthat has come to be known as the Cambrian explosion.

THE CAMBRIAN ENVIRONMENT

Known from rocks and fossils surrounding three of theperiods major landmasses, the Cambrian was a time of

volcanism across Gondwana and several other continents,ice-free polar regions, and warmer conditions than today.Scientists contend that global sea levels dropped and roserelatively suddenly during this period that created as wellas destroyed many biological communities.

Paleogeography

The geography of the Cambrian world differed greatly fromthat of the present day. The geographic reconstruction is


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based on integrated geologic and biological evidence.Fossils in continental-shelf deposits indicate the presenceof at least three major faunal provinces (or biogeographicalregions) during much of the Cambrian Period.

The most distinct faunal province surrounded thecontinent of Laurentia. Paleomagnetic evidence indicatesthat Laurentia was located over the paleoequator duringmost or all of Cambrian time. This geographic interpreta-tion is supported by the presence of thick, warm-watercarbonate-platform deposits that accumulated in a broadbelt encircling the continent. These carbonates are com-monly anked on the inner shelf by lagoonal shale andnearshore sandstone deposits. On the outer shelf, the car-bonates commonly grade into laminated mudstone andshale that accumulated in deeper water. At times, twoalmost mutually exclusive ecosystems are separated bytemperature and salinity barriers in the shallow water on

the carbonate platforms. Inner restricted-shelf deposits were characterized by sparse low-diversity communitiesthat tended to be highly endemic (conned to a particularregion). Outer open-shelf deposits are characterized byhigh-diversity ecosystems that were widely distributedaround the continent. Fossils are usually most abundantand most diverse near the outer margins of the carbonate

platform. Because Laurentia remained nearly intact struc-turally, it is ideal for studying the relationships betweenCambrian environments and communities of organismsaround a low-latitude Cambrian continent.

Another Cambrian faunal province surrounded thesmall continent of Baltica, which was located in middle tohigh southern latitudes. Cambrian shelf deposits of Baltica

are relatively thin, rarely exceeding 250 metres (820 feet)in thickness, and are composed primarily of sandstoneand shale. Seemingly as a consequence of cool-water

7 The Cambrian Period 7


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environments, carbonate deposits are relatively minor and very thin. The wide distribution of many species from thenearshore to deep-shelf environments of Baltica suggeststhat no signicant restrictions in shelf dispersal occurred,unlike the shallow carbonate platforms of Laurentia.

The largest Cambrian faunal province is locatedaround Gondwana, which extended from the low north-ern latitudes to the high southern latitudes, just shortof the South Pole. The rocks and fossil assemblages ofGondwana show major changes that correspond to its great size and wide range of climates and environments.The Antarctic and Australian sectors of Gondwana restedin low latitudes during the Cambrian and have extensivecarbonate deposits, although those of Antarctica arepoorly exposed through the present-day polar ice cap.Differences in their fossil assemblages in addition topaleomagnetic evidence suggest that present-day North

and South China were on separate tectonic plates.However, extensive carbonate deposits in both regionsindicate that both plates were found in low latitudes dur-ing the Cambrian. The fossil assemblages of South Chinahave strong similarities with those of both Australia andKazakhstan, but details of the Cambrian geographic rela-tionships remain unclear.

Several terranes (fault-bounded fragments of theEarths crust) seem to have been located near or attachedto the margin of the northern Africa sector of Gondwanain the high southern latitudes, but many details of theirCambrian geographic relations are unknown. These ter-ranes now make up much of southern Europe and parts ofeastern North America. Cambrian deposits in all the

terranes are chiey sandstone and shale and include fewor no carbonates. Their faunas closely resemble thoseof Baltica at generic and higher taxonomic levels, but


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differences at the species level suggest some geographicseparation.

Siberia was a separate continent located in the low lati-tudes between Laurentia and Gondwana. Fossil similaritiesbetween this continent and Gondwana suggest that it waspositioned relatively close to the equatorial region ofGondwana. Present-day Kazakhstan seems to be com-posed of several microcontinental blocks that were in alllikelihood separated during the Cambrian. These blocksbecame amalgamated after the Cambrian, and the subse-quent composite continent known as Kazakhstaniacollided with Siberia during the late Paleozoic Era.

Few Cambrian faunas from continental-slope anddeep-ocean environments are known. Limited informa-tion from these is important, however, for demonstratingafnities between deep, cool-water ecosystems at all lati-tudes and shallow, cool-water communities of higher

latitudes. There exists a close similarity between theobserved distribution patterns of Cambrian and modernmarine arthropods. This has been used as persuasive evi-dence for the existence of thermally stratied Cambrianoceans in lower latitudes with a thermocline separating warm-water from cool-water layers. This inferred thermo-cline and the wide oceanic separation of biogeographic

regions are seen as the likely causes for the high endemismof the biological communities surrounding Laurentia.This interpretation is supported by deposits that dateback to the middle of the Cambrian in northern Greenland where, within a few tens of kilometres (miles), normalLaurentian shelf-margin trilobite (distinctive three-lobedmarine arthropods) communities grade into deepwater

faunas like those in the shallow-shelf deposits of Baltica.Similarly, trilobite species in later Cambrian deepwaterfaunas found in the western United States also appear in



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southeastern China, whereas shallow-water communitiesof the two regions have few genera in common.

As large tectonic plates continued to move during thePhanerozoic Eon, terranes of various sizes were displaced.Endemic Cambrian fossils, in conjunction with other geo-logic evidence such as physical stratigraphy, have been usefulfor identifying the geographic origins of some terranes, par-ticularly those that have undergone substantial displacement.Examples of displaced terranes include northern Scotlandand west-central Argentina (Precordillera) with Laurentianfossils, eastern Newfoundland with fossils from Baltica,and southern Mexico (Oaxaca) with Gondwanan (SouthAmerican) fossils.

Another important consequence of continued platemovement has been the formation of large mountainranges at the sites of plate collision. Pressure and heat gen-erated during collisions since the Cambrian have folded,

faulted, and metamorphosed signicant volumes ofCambrian rock, especially from the outer margins andslopes of many continental shelves. No crustal rocks foundin todays oceans are older than the Mesozoic Era (251 to65.5 million years ago). Most pre-Mesozoic deposits thataccumulated in the deep ocean basins are thought to havebeen destroyed after they subducted into Earths interior.

Relatively abrupt changes in sea level may have sig-nicantly inuenced Cambrian environments and life. A global drop in sea level is suggested by extensive uncon-formities (interruptions in the deposition of sedimentaryrock). The time represented by such unconformities insectors of Laurentia and Baltica bounding the IapetusOcean has been called the Hawke Bay event. An apparent

absence of a coeval unconformity in western NorthAmerica seems to be an anomaly. Thick uninterruptedshelf deposits in this sector of Laurentia, however, may


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have resulted from abnormal shelf subsidence caused bythe cooling of crustal rocks following a late Precambrianplate-rifting event. The timing of unconformities foundin widely separated continents cannot be correlated withprecision. However, it is perhaps signicant that a num-ber of characteristic animal groups from the earlyportions of the Cambrian were either exterminated orseverely restricted in their geographic distribution atabout the same time in the worlds shallow-shelf environ-ments. Among biostratigraphically important trilobites,the olenellids were exterminated near Laurentia, the hol-miids went extinct at the margins of Baltica, and theredlichiids vanished from the shallow-shelf ecosystemsnear Gondwana. Also, diverse and abundant reef-dwell-ing archaeocyathans (extinct group of sponges thought tohave helped construct the rst reefs) disappeared frommost low-latitude warm-water continental shelves.

The signicant rise in sea level is suggested by ratherabrupt and extensive displacements in sedimentary envi-ronments and biotas during the middle of the Cambrian(the Ptychagnostus gibbus zone). Lowland areas were ooded,as in parts of Baltica. In warm-water shelf sections of the world, coarse-grained, shallow-water, carbonate rocks were abruptly overlain by ne-grained, deeper-water, lami-

nated limestone or shale. Adaptive radiation of the pelagicagnostoid trilobites was greatly accelerated in open-oce-anic environments following this event, perhaps inresponse to newly expanded habitats.

In Laurentia, a signicant drop in sea level near theend of the Cambrian is suggested by both the disappear-ance of some faunas and a single unconformity that denes

the boundary between the Dresbachian and Franconianstages, two regional stages in peripheral areas of NorthAmerica. However, there is no evidence for a comparably



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large change in sea level at or near the Cambrian-Ordovician boundary. Associated minor unconformitieshave made it difcult to select a dening Global StandardSection and Point (GSSP) for the boundary, which ideallyshould be located in an uninterrupted stratigraphic section.

Several regions of Cambrian volcanism have beenidentied. Australia was especially active. Large areas inthe northern and central regions were covered by oodbasalts during the early parts of the Cambrian with resid-ual activity extending into the middle of the period.Basalts and mac intrusives (molten rock derived frombasic magmas) in southeastern Australia formed in a vol-canic island arc setting during the early and middle partsof the Cambrian. Volcanic suites of similar age are alsopresent in New Zealand and in parts of Antarctica (north-ern Victoria Land, Ellsworth Mountains, and PensacolaMountains). Other signicant Cambrian volcanic depos-

its occurring during the early and middle parts of theperiod are found in southern Siberia and western Mongolia(Altai and Sayan mountains), eastern Kazakhstan andnorthwestern China (Tian Shan), and northeastern China.Cambrian volcanics are scattered along the easternmostmargin of the United States, but most are probably islandarc deposits that were accreted to Laurentia after the

Cambrian. In the southern United States (Oklahoma), granitic intrusives and basaltic and rhyolitic extrusives areassociated with a large tectonic trough that was formed byrifting, or crustal extension, throughout the rst half ofthe period.

Minor volcanic deposits, mainly ash beds and thinows, are widely known. In general, these have received

little study, but some are suitable for the determination ofisotopic ages. Zircons from a lower Lower Cambrian (pre-trilobite) volcanic ash bed in New Brunswick, Can., have a


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uranium-lead age of 531 million years. Volcanic tuffs nearinferred Tommotian-Atdabanian boundaries (Russiandesignations for the pretrilobite and trilobite portions ofthe Cambrian explosion) in both Morocco and southwest-ern China have yielded similar dates of 521 million years.

The tectonic history of the Precambrian is not as wellknown as that of the Paleozoic. In general, however, latePrecambrian history seems to have been characterized bycontinental fragmentation, whereas Paleozoic history wascharacterized by the continental accretion of terranes.The Cambrian was a period of transition between thosetectonic modes, and continents were scattered, appar-ently by the fragmentation of Rodinia. Major Cambrianand early Ordovician tectonism affected large areas ofGondwana in what are now Australia, Antarctica, andArgentina. Multiphase tectonism in Antarctica is calledthe Ross Orogeny, and in Australia it is known as the

Delamerian Orogeny. At least some of the volcanic activ-ity noted previously, particularly that of volcanic islandarcs, is evidence that seaoor spreading and crustal sub-duction were active geologic processes.

Paleoclimate

Global climate during the Cambrian time was probably warmer and more equable than today. An absence of eitherland or landlocked seas at the Cambrian poles may haveprevented the accumulation of polar ice caps. The generalabsence of glacial till deposits of the Cambrian age is morenotable, because these deposits are common and wide-spread in upper Precambrian strata. They accumulated

again during the Ordovician Period in northern Africa asGondwana began to move over the South Pole. Otherwise,the presence of persistent and widespread limestone



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deposits found on the margins of a centralized transconti-nental arc in North America, for example, indicates that asubtropical climate existed in latitudes between 30N and30S. In addition, arid to semiarid conditions at latitudesaround the Tropics of Cancer and Capricorn (approxi-mately 2327' N and S latitude, respectively) are suggestedby deposits that include sandstone with quartz grainsfrosted by abrasion through wind transport, ventifacts(wind-polished stones), and evaporites.

More sophisticated research on paleoclimates relieson the detection of changing patterns in the amounts ofisotopic oxygen, carbon, and strontium retained in lime-stone samples to correlate the timing of different geological events. Much remains to be accomplishedregarding this promising line of research on Cambrianstrata, but certain trends and events are becoming betterdened. Strong reductions in isotopic carbon ( 13C), for

example, are correlated from Lower Cambrian strata atlocalities as distant from one another as the Lena Riverarea of Siberia and the Atlas Mountains of Morocco.Another substantial drop in 13C is believed to mark thetransition between the early and middle parts of theCambrian in the Great Basin of North America. Suchdecreases may represent a global rise in temperature

occurring at the same time as a rise in global sea level. Inaddition, the delivery of anoxic (oxygen-depleted) ocean waters with reduced amounts of 13C also may have beenaided by the rising sea level.

CAMBRIAN LIFE

The long history of life on Earth has been punctuated byrelatively abrupt changes. Some have argued that the greatest change of all occurred in marine environmentsnear the Precambrian-Cambrian boundary. Fossils from


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Cambrian rocks include the oldest representatives ofmost animal phyla having mineralized shells or skele-tons. A lack of observed connecting links suggests thatprocesses of biomineralization (specically, the forma-tion of bones, shells, and teeth) evolved independentlyin several phyla. Whether or not soft-bodied representa-tives of some of these phyla originated during thePrecambrian Era but have no preserved record is adebated question. Nevertheless, the hard parts ofCambrian animals had a much greater potential for pres-ervation than the soft parts, and they mark the beginningof a diverse fossil record.

The Fossil Record of the Precambrian-Cambrian Transition

The preservation of the record of the Precambrian-

Cambrian transition was signicantly affected by globalchanges in sea level. During the latest Precambrian time,the sea level was relatively low, resulting in spatiallyrestricted oceans and expanded continents. Throughoutmuch of the Cambrian, rising seas gradually ooded vastland areas. Sediment was eroded from the continentsand deposited in adjacent seas. Because of low sea level,

the sedimentary and fossil records of the Precambrian-Cambrian transition are generally most complete towardthe outer margins of continental shelves. As a corollary, thetime gaps, represented by the boundary surface, generallyincrease in landward directions. This has led to anabsence or serious incompleteness of the transitionalrecord in most areas, particularly in those of classical

Cambrian studies. As a result, it is thought that thisincompleteness, combined with a general deciency inknowledgeprior to the mid-1900sof Precambriancommunities, contributed signicantly to the long-held



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notion of an abrupt or sudden appearance of Cambrianfossils.

Considering the biological importance of thePrecambrian-Cambrian transition, it is somewhat surpris-ing that the primary impetus for its detailed study camefrom a project designed to establish a suitable interna-tional boundary stratotype (a rock layer that serves as abenchmark of geologic time). Before the project was initi-ated in 1972, reasonably complete stratigraphic sectionsacross the transition were either largely unrecognized orignored. Since 1972, information about the transition hasaccumulated at an accelerating rate. Although manydetails remain to be learned, the general history of thismomentous interval is becoming clear.

The Precambrian-Cambrian biotic transition, oncethought to be sudden or abrupt, has been found to includea succession of events spread over many millions of years.

It commenced with the appearance of the animal king-dom (i.e., multicelled organisms that ingest food), but thedate and details of that event remain obscure. At leastthree informal phases in the transition can be identiedby progressively more diverse and complex biologicalcommunities.

The earliest phase of the late Precambrian age is char-

acterized by fossils of soft-bodied animals known frommany localities around the world. Based on fossils of ani-mal embryos, it is thought that elements of the Ediacaranfauna appeared as early as 590 to 600 million years ago.The fossils are predominantly the imprints of soft-bodiedanimals. Their extraordinary preservation, usually in sand-stone or shale, was probably the result of rapid burial and

protection by smothering sediment. Most of the fossilsare relatively simple, and many resemble worms, sea pens,and jellysh. Dwelling traces (fossilized burrows and otherexcavations) like those of modern sea anemones are also


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common. Placing Ediacaran fauna in higher (more gener-alized) taxonomic levels is controversial, however, becausecritical diagnostic features are not evident. Some paleon-tologists have assigned Ediacaran body fossils to theextant phyla Annelida, Coelenterata, and Arthropoda, whereas others have regarded them as members of extincttaxonomic groups of high rank. Some adherents of thislatter viewpoint have suggested that the Ediacaran fauna was terminated by a major extinction event, but direct evi-dence of an abrupt replacement of species has not beenfound in the geologic record.

Other kinds of fossils also provide valuable cluesabout life during Ediacaran time. Photosynthetic organ-isms include unicellular blue-green algae (cyanobacteria)and acritarchs (probable algae), both of low diversity.Individuals of some species were probably abundant,however, and may have been an important source of food

for Ediacaran animals. Hard parts of animals, primarilyknown from Africa and China, are mainly dwelling tubescomposed of calcium carbonate and other compounds.Most were probably secreted by sessile, lter-feeding, wormlike animals. Although rare and of low diversity,these forms are signicant because they signal the adventof biomineralization. The oldest unequivocal trace fos-

sils, mainly crawling trails, are also of Ediacaran age. Thetrails suggest that locomotion of the trace makers wasaccomplished by waves of muscular contraction, like thatin annelids and sea slugs, and not by legs. All but the latestEdiacaran trace fossils are relatively simple, suggestinglimited and primitive behaviour patterns. Their low diver-sity further suggests that few kinds of mobile animals

lived on the Ediacaran seaoor.The second phase of the Precambrian-Cambrianbiotic transition is characterized by a marked increase inthe diversity of its shelly fauna and a lack of trilobites. It is



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near the lowest stratigraphic occurrence of this fauna thatthe Precambrian-Cambrian boundary stratotype has beenplaced. The fauna includes that of the Tommotian Stage,as applied in Russia, and it has often been referred to asthe Tommotian fauna. It is known from many localitiesaround the world, but time correlations lack precision. A general acceleration in biotic diversity during this secondphase is the beginning of the Cambrian explosion.

Fossils of the second phase, which may be locally abun-dant, represent several new animal groups of Paleozoicaspect. Calcied archaeocyathans (extinct group ofsponges thought to have helped construct the rst reefs)diversied rapidly and were the rst skeletal metazoans(multicellular animals with differentiated tissues) todevelop a modular growth habit. They also evolved a com-plex symbiotic relationship with reef-building blue-green

algae. Mollusks, preserved

in both shale and lime-stone, include at least fourclasses (Monoplacophora,Gastropoda, Hyolitha, andRostroconchia). Brachio-pods (lamp shells) madetheir appearance but are

low in diversity. Several groups are represented byan astonishing array ofsmall mineralized tubes,scales, and spicules. Thepresence of arthropods,the rst animals to develop

legs, is indicated by charac-teristic trace fossils. Theskeletal remains of arthro-pods are not preserved in

Now extinct, the small, conical-shelled hyoliths are usually only found preserved in Cambrian rocks.O. LouisMazzatenta/National Geographic/ Getty Images


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the fauna, however, presumably because they were notmineralized. Other trace fossils show a marked increase inabundance and diversity as well as an expansion of behav-iour patterns that reect improvements in locomotion, greater ability to penetrate sediment, and new foragingstrategies.

The third phase of the Precambrian-Cambrian biotictransition commenced with the appearance of mineralizedtrilobite skeletons. The subsequent adaptive radiation ofthe trilobites was exceptional, and their remains dominatemost later Cambrian deposits. For this reason, the CambrianPeriod has sometimes been called the Age of Trilobites.

The known Cambrian biota was restricted to marineenvironments. At least 11 extant animal phyla (Annelida,Arthropoda, Brachiopoda, Chordata, Ctenophora,Echinodermata, Hemichordata, Mollusca, Onychophora[velvet worms], Porifera, and Priapulida), including most of

those with a fossil record, rst appear in Cambrian rocks.Most of these rapidly diversied as they seemingly adaptedto numerous unlled ecological niches. Another ve phyla(Nemertea, Phoronida, Platyhelminthes, Pogonophora,and Sipuncula) may also trace their origin back to theCambrian fossils, though questions still remain aboutthem. The only extant animal phylum with a good fossil

record that is not known from Cambrian rocks are theBryozoa (moss animals), which rst appear in rocks ofEarly Ordovician age. A summary of the principal biotic groups of the Cambrian is given below.

Photosynthetic Organisms

Cambrian photosynthetic organisms, the primary food ofanimals, are entirely unicellular. These organisms includea variety of prokaryotic (nonnucleated) and eukaryotic(nucleated) bacteria and algae. Their evolution, like that



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in associated animals, shows a marked acceleration inadaptive radiation and biomineralization near the base ofthe Cambrian. A new calcareous bottom-dwelling oradominated by blue-green algae appeared. Some of theseorganisms formed mounds on the seaoor. Others formedsmall, concentrically laminated, marble- or biscuit-shapedstructures called oncoids, which were locally abundant.Although it was rarely preserved, there existed a non-calcareous benthic ora that also was dominated byblue-green algae. By at least the middle of the Cambrian,some noncalcareous green algae (Chlorophyta) hadbecome common. In North America and Siberia, the axesof one species, Margaretia dorus, exceeded 2 centimetres(0.8 inch) in diameter and were probably more than 1metre (3.3 feet) in height. Such large size is attained bymodern green algae only in warm, equatorial oceans. Thephytoplankton, consisting of acritarchs and blue-green

algae, also diversied near the base of the Cambrian.Acritarchs are widespread in many kinds of marine rocksand seem to have the potential for an improved zonationof Lower Cambrian rocks. They are difcult to study, how-ever, because of their microscopic size.

Fauna

Cambrian faunas, like those of the present day, are com-monly dominated in numbers and kind by members ofthe phylum Arthropoda. Calcication of skeletons by thebeginning of Atdabanian time contributed to an abundantfossil record of the class Trilobita, of which some detailshave been discussed above. Many hundreds of genera and

thousands of species of Cambrian trilobites have beendescribed worldwide. Rates of evolution in Cambrian tri-lobites were relatively rapid, resulting in short stratigraphicranges and giving them much value for biostratigraphic


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correlation. Representatives of the class Ostracoda, char-acteristically enclosed by a bivalved carapace, alsoappeared near the base of the Atdabanian. Compared withtrilobites, however, ostracods are generally rare and of lowdiversity throughout the Cambrian, except in some rocksof Australia and China. Extraordinary preservation at rarelocalities indicates that many other kinds of arthropods were at least locally more abundant and more diverse thanthe trilobites. These other arthropods had unmineralizedskeletons, and some may represent extinct classes.

Sponges (phylum Porifera) are commonly representedin Cambrian faunas. Archaeocyathan sponges, character-ized by cup-shaped skeletons with double calcareous wallsand numerous pores, are abundant and diverse in someearly Cambrian deposits. They have been used for provin-cial biostratigraphic zonation, especially in Australia andSiberia. Archaeocyathans are common only in regions that

were positioned in low latitudes during the CambrianAntarctica, Australia, China, Kazakhstan, Siberia, andNorth America. Their latitudinal distribution is similar tothat of modern colonial corals, suggesting adaptations tosimilar ecological controls in warm shallow seas.Archaeocyathans nearly disappeared about the middle ofthe Cambrian, but rare species survived until much later

in the period, after which the group became extinct. Othercommon Cambrian sponges had skeletons of siliceous (sil-ica-derived) spicules, which readily disaggregated afterdeath, making their identication at lower taxonomic lev-els difcult, if not impossible. At rare locations ofexceptional preservation, where articulated skeletons andassociated soft-bodied taxa and others were found, spicu-

lar sponges are second only to arthropods in speciesdiversity. This suggests that Cambrian sponges were muchmore common and more diverse than is indicated by theknown fossil record. Limited information indicates that



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species of spicular sponges evolved slowly during theCambrian, resulting in relatively long ranges within thestratigraphic record.

Brachiopod shells are present in many Cambriancontinental-shelf deposits. In terms of the total numberof species that have been described from Cambrian rocks,brachiopods are second only to trilobites. Species diver-sity, however, is generally low to moderate at mostlocalities. Phosphatic shells of the class Inarticulata arenormally much more common and more diverse thanare calcareous shells of the class Articulata. These abun-dance and diversity relationships are usually reversed inpost-Cambrian rocks.

The phylum Echinodermata (some present-day repre-sentatives of which are sea urchins and starsh) had amajor adaptive radiation during the Cambrian Period.The number of classes increased from three early in the

Cambrian to eight toward the middle of the period. Onlyone of these, the Eocrinoidea, is known from many spe-cies, but the described record seems to be grosslyincomplete. Skeletal plates in early echinoderms were notrigidly connected, and they readily disaggregated after thedeath of an animal. Consequently, it is rare to nd articu-lated skeletons that can be classied to lower taxonomic

levels. In some Cambrian limestones, however, skeletalplates of echinoderms are a dominant sedimentary con-stituent, indicating the existence of innumerable animalsand suggesting far greater diversity, especially at low taxo-nomic levels, than has been recorded. As in some modernechinoderm species, it is common for those in theCambrian to show evidence of a gregarious habit and

patchy distribution. Most of the Cambrian echinoderms were suspension and detritus feeders, and it was only afterthe Cambrian that herbivores and carnivores became


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common. All classes of echinoderms that were presentduring the Cambrian, except for the Crinoidea, subse-quently became extinct.

The phylum Mollusca also underwent signicant adap-tive radiation during the Cambrian, with the appearanceof the classes Monoplacophora, Gastropoda, Pelecypoda(synonymous with Bivalvia), Cephalopoda, Polyplacophora,Rostroconchia, Hyolitha, and Stenothecoida. (The latterthree are now extinct.) The only molluscan class thatappeared after the Cambrian is the Scaphopoda (tusk ortooth shells), which originated during the Ordovician. Asmall variety of mollusks is present in the shelly fauna ofthe earliest Cambrian. Mollusk shells usually are absent orrare in later Cambrian rocks, but at a few localities they arecommon to abundant. The small conical shells of hyolithsare the kind most commonly preserved in Cambrian rocks.

Other new Cambrian phyla largely lack biomineraliza-

tion and have a poor fossil record. The Hemichordata isrepresented by rare sessile graptolites (order Dendroidea)of the class Graptolithina, which appeared during the mid-dle of the Cambrian. Appearances of the more commonplanktonic graptolites (order Graptoloidea) have beenused as informal indicators of the Cambrian-Ordovicianboundary. The formal boundary stratotype coincides with

the rst appearance of Iapetognathus uctivagus, a conodont(primitive chordate with tooth-shaped fossil remains).Cambrian worm phyla (Annelida, Priapulida, and probablePogonophora) are mainly known from localities wherepreservation was extraordinary. Other rarely representedphyla include the Onychophora, with leglike lobopodia (acatchall category of Cambrian life), and the Ctenophora

(comb jellies).The origin of the phylum Chordata is unclear. If primi-tive conodont-like fossils (paraconodonts) are included, as



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argued by some paleontologists, the phylum appearedduring the late Precambrian. Rare soft-bodied possiblechordates have been described from Lower Cambrianrocks. The oldest unequivocal chordate remains comefrom the Lower Cambrian of south China, where small jawless sh similar to present-day lamprey and hagshoccur as part of the Chengjiang Biota. The fossils fromthis Lagersttte (a fossil-rich deposit of sedimentary rock)preserve the remains of different soft body tissuestracesof gill pouches, dorsal ns, and likely the notochord itself.

Trace fossils, as discussed above, provide indepen-dent evidence of accelerated animal diversication and adistinct increase in the complexity of animal behaviournear the beginning of the Cambrian Period. Other evi-dence from trace fossils indicates changes in Cambrianbioturbation, the churning and stirring of seaoor sedi-ment by animal forms. Late Precambrian (Ediacaran)

trace fossils from around the world are essentially surfacetrails that show little evidence of sediment burrowing.Quantitative study in the western United States hasshown that a signicant increase in bioturbation occursbetween pretrilobite (Tommotian) and trilobite-bearing(Atdabanian) Lower Cambrian rocks. Throughout theCambrian, bioturbation was more intensive in nearshore

and inner-shelf environments than in more offshoresettings. The depth of bioturbation in carbonate envi-ronments of the inner shelf was consistently less than afew centimetres throughout Cambrian time.

Deposits with Soft-Bodied Organisms

Modern biota are largely dominated by soft-bodied organ-isms, whereas the fossil record is overwhelminglydominated by the hard parts of organisms. Rare deposits offossils with soft parts are therefore of great importance in


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helping to establish the original diversity and ecology ofancient communities. Among the most famous soft-bodied biota are those found in the Burgess Shale of western Canada (British Columbia), which was formedduring the middle of the period, and the Chengjian Biotafrom southern China (Yunnan), which was formed earlierin the period. In the case of the Burgess Shale, tens of thou-sands of complete specimens, many with soft partspreserved in remarkable detail, were apparently buried bysubmarine slumping of sediment on the continental shelfof Laurentia. Fossils from the Burgess Shale have been usedto demonstrate the presence of a complex community asdiverse in habit, structure, and adaptation as many moderncommunities. If isolated, fossils with hard parts would con-stitute a typical Cambrian fauna, but they represent onlyabout 40 percent of the genera in the Burgess Shale,

The largest predator to swim the Cambrian seas, theAnomalocaris exes its mighty appendages to feed on the haplessOpabinia . De Agostini PictureLibrary/Getty Images



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a proportion similar to that in modern faunas on continen-tal shelves. Approximately 15 percent of the genera knownfrom the Burgess Shale also occur in the older Chengjiandeposit. Mollusks and echinoderms appear to be the onlymajor groups absent in the Chengjian Biota that occur inthe Burgess Shale.

The most celebrated invertebrate from the BurgessShale is attributed to the genus Anomalocaris. This creature was the largest predator that swam in Cambrian seas. It wasouttted with a pair of giant exible appendages thatcould grasp and move prey toward a peculiar mouth struc-ture consisting of armoured plates arranged in a circularpattern. The articulated mouth of Anomalocaris has beendescribed as resembling a pineapple ring with the centercut out. Trilobites with telltale scars on their carapace arebelieved to represent the cookie-cutter bite of Anomalocaris.

In the Canadian Rockies, a photographer zooms in on the famous BurgessShale fossil formation, formed by Middle Cambrian mudslides that buried and preserved in startling detail more than 60,000 soft-bodied specimens.Michael Melford/National Geographic/Getty Images


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A smaller relative of Anomalocaris is the bizarreOpabinia, with its ve eyes mounted on an arthropod-likebody and a long nozzlelike structure tted with a singlepair of claws that protrudes forward from the head. It isthought that Opabinia moved across the seaoor, using itsexible nozzle to stir up bottom sediment and grasp hid-den prey with extended claws.

Other less-diverse Cambrian deposits with soft-bod-ied organisms have been discovered in places such asSouth Australia, northern Greenland, Sweden, and theUnited States (Utah and Pennsylvania). Some of these areimportant in demonstrating that the biota of the BurgessShale is unusual only in preservation and not in composi-tion. They also demonstrate that some of the soft-bodiedtaxa have substantial geologic ranges and wide geographic distributions. Extraordinary preservation ofarthropods dating to the later parts of the Cambrian

Period in Sweden is especially notable, as the bodies andappendages remain largely uncrushed. The integumentretains many ne structures, including setae (bristly hairsor organs) and pores.

Cambrian Extinction Events

Minor extinction events occurred sporadically throughoutthe Cambrian Period. One near the middle of the Cambrian was apparently related to global marine regression. At leastthree later Cambrian events primarily affected low-latitudeshelf communities and have been used in North Americato dene biostratigraphic units called biomeres. (Suchunits are bounded by sudden nonevolutionary changes in

the dominant elements of a phylum.) Each of the Cambrianbiomere events eliminated several trilobite families, whichcollectively contained most of the genera and species that were living on the continental shelves. Less attention has



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been paid to extinction patterns among other inverte-brates, but some evidence of corresponding extinctionsamong brachiopods and conodonts is available.

Geochemical evidence suggests that the biomereextinctions were probably caused by abrupt drops in watertemperature. Oxygen isotopes from the skeletons of bot-tom-dwelling trilobites associated with one biomereboundary in Texas indicate a drop in water temperature ofabout 5C (9F) at the boundary. A comparable decrease intemperature would kill the larvae of many modern marineinvertebrates that live in warm oceans. Following eachCambrian extinction, shelf environments were repopu-lated by low-diversity trilobite faunas of relatively simpleform, which apparently emigrated from deeper and cooleroff-shelf environments. In effect, every one of the bio-mere events was followed by an adaptive radiation of newtaxa, especially among the trilobites.

Signicant Cambrian Life-Forms

Several different kinds of life emerged during the period,and much of what is known about Cambrian life comesfrom the Burgess Shale formation of western Canada.Some organisms, such as the conodonts, trilobites, and

graptolites, have been useful in the global correlation ofCambrian rocks and those of subsequent geologic peri-ods. The brachiopods, or lamp shells, which is a group thatcan also be used in geologic correlation, have modern liv-ing representatives.

Burgess Shale

The celebrated Burgess Shale, as mentioned previously,is a fossil formation containing remarkably detailedtraces of soft-bodied biota of the Middle CambrianEpoch (520 to 512 million years ago). Collected from a


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fossil bed in the Burgess Pass of the Canadian Rockies,the Burgess Shale is one of the best preserved and mostimportant fossil formations in the world. Since it was dis-covered in 1909, over 60,000 specimens have beenretrieved from the bed.

The Burgess Shale captures a complex marine envi-ronment containing a rich diversity of arthropods,miscellaneous worms, sponges, lophophorates, echino-derms, mollusks, priapulids, chordates, hemichordates,annelids, and coelenterates. The fossil bed is likely theresult of mud slides from the Laurentian shelf that rap-idly buried the fauna, preserving great morphologicaldetail. While many of the fossils clearly belong to estab-lished phyla and reveal important information aboutphylogenetic development, there are many other generathat do not t so easily into modern phyla. Such unusualfossils as Hallucigenia, a creature with a long tubular body

and two rows of tall dorsal spines; Wiwaxia, an oval crea-ture with two rows of spines down its plated back; andOpabinia, which had ve eyes and a long nozzle, have ledmany scientists to conclude that the Cambrian Periodmay have produced many unique phyla. However, depos-its discovered in China, Greenland, and elsewhere havedemonstrated that at least some of the shales oddities

(including Hallucigenia and Wiwaxia ) belong to known groups of animalsthough they were members of lin-eages that diverged early from the others and soonbecame extinctand that the Burgess Shale is unique inpreservation, but probably not in composition.

Conodonts

Conodonts are among the most frequently occurring fos-sils in marine sedimentary rocks of Paleozoic age. Theseminute toothlike fossils are composed of the mineral apa-tite (calcium phosphate). Between 0.2 millimetres (0.008



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inch) and 6 millimetres (.236 inch) in length, they areknown as microfossils and come from rocks ranging in agefrom the Cambrian Period to the end of the TriassicPeriod. They are thus the remains of animals that livedduring the interval of time from 542 million to 200 million years ago and that are believed to have been small marineinvertebrates living in the open oceans and coastal watersthroughout the tropical and temperate realms. Onlyrecently has the conodont-bearing animal been found,preserved in ne-grained rock from North America.Conodont shapes are commonly described as either sim-ple cones (like sharp teeth), bar types (a thin bent shaft with needlelike cusps or fangs along one edge), blade types(attened rows of cones of ranging size), or platform types(like blades, with broad anges on each side making asmall ledge or platform around the blade). Well over 1,000different species or shapes of conodonts are now known.

Some conodonts exist in two forms, right and left.They are known to have occurred in bilaterally symmetri-cal pair assemblages in the animal, like teeth but moredelicate and fragile. The few assemblages discovered so farappear to contain as many as nine different species, orforms, of conodonts. Bars, blades, and platforms may allbe present in a single assemblage or apparatus. How single

cones tted into assemblages is uncertain. The conodontapparatus seems to have been placed at the entrance to the gut and to have assisted in food-particle movement. Therelationship of this little animal (3040 mm long) tothe known wormlike animal groups is still debatable, andno exactly compatible creature is known to exist today.

Conodonts are very useful fossils in the identication

and correlation of strata, as they evolved rapidly, chang-ing many details of their shapes as geologic time passed.Each successive group of strata thus may be character-ized by distinctive conodont assemblages or faunas.


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Moreover, conodonts are very widespread, and identicalor similar species occur in many parts of the world. Blackshales and limestones are especially rich in conodonts,but other sedimentary rock types may also be productive.In some parts of the world assemblages of conodonts,regarded as those of animals living out in the open ocean,can be distinguished from others thought to belong toi

The Paleozoic Era Diversification of plant and animal life (J.P. Rafferty 2011)

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