Invertebrate Palaeontology and Evolution

E.N.K. Clarkson Professor of Palaeontology Department of Geology University of Edinburgh Scotland

Fourth edition

b Blackwell Science

Invertebrate Palaeontology and Evolution

Invertebrate Palaeontology and Evolution

E.N.K. Clarkson Professor of Palaeontology Department of Geology University of Edinburgh Scotland

Fourth edition

b Blackwell Science © 1979,1986,1993,1998 by E. N. K. Clarkson Published by Blackwell Science Ltd, a Blackwell Publishing company

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Contents

Preface xv Macrofossils on CD-ROM xvi

Part One General Palaeontological Concepts 1

1 Principles ofpalaeontology 3 1.1 Introduction 3 1.2 Occurrence ofinvertebrate fossils in Phanerozoic rocks 3 Hard-part preservation 3 Soft-part preservation 6 1.3 IJivisions of invertebrate palaeontology 6 Taxonomy 8 The species concept 8 Nomenclature and identification offossil species 9 Taxonomic hierarchy 10 Use ofstatistical methods 12 Palaeobiology 13 Palaeoecolob'Y 13 Functional morphology, growth and form 19 Stratigraphy 20 Lithostratigraphy 20 Biostratigraphy 20 Chronostratigraphy 22 Bibliography 23 Books, treatises and syn'lposia 23 Individual papers and other references 25

2 Evolution and the fossil record 26 2.1 Introduction 26 2.2 Darwin, the species and natural selection 26 Inheritance and the source ofvariation 28 Where does variation come hom? 30 Significance ofalleles 32 Mutation 33 Spread of mutations through populations 34 Isolation and species formation 35 Genetic drift: gene pools 35 Molecular genetics and evolution 37 Gene regulation during development 38 2.3 Fossil record andmocles ofevolution 38 viii Contents

Microevolurion 39 Allopatric speciation 40 Heterochrony 41 Testing rnicroevoJutionary patterns 41 Analysis of case histories 42 Co-evolution 43 Macroevolution 44 Species selection 44 Origins of higher taxa 45 Rates of evolution, adaptive radiations and extinction 47 2.4 Competition and its effects 49 2.5 Summary ofpalaeontological evolution thcmy 50 Bibliography 51 Books, treatises and symposia 51 Individual papers and other references 53

3 Major events in the history oflife 55 3.1 Introduction 55 3.2 Prokaryotcs and eukaryotcs 55 3.3 Earliest metazoans 57 Ediacara E1Una: two vie"'Arpoints 58 The traditional view 58 Mcdusoids 59 Pennatulaceans 59 Annelids 60 fossils ofunknmvn afllnities 60 Vendozoan hypothesis 60 Small shelly f()ssils 61 Precambrian trace fossils 63 Causes of the Cambrian 'explosion oflife' 64 Physicochemical 6ctors 64 Biological factors 64 Biological evidence on metazoan relationships 65 3.4 Major features ofthe Phanerozoic record 68 Diversification ofinvertebrate lite 68 Changes in species diversity and habitat 69 Problematic early Palaeozoic fossils 69 Marine evolutionary tnmas 71 Climatic and sea-level changes 72 Extinctions 73 Possible causes ofmass extinctions 73 Earthbound mechanisms 74 Extraterrestrial mechanisms 74 Late Ordovician (Ashgillian) extinction event 75 Late Devonizlll (Frasnian-Famennian) extinction event 75 Late Permian extinction event 76 Late Triassic (Carnian-Norian) extinction event 76 Cretaceous-T ertialY boundary extinction 77 Bibliography 78 Books, treatises and symposia 78 Individual papers and other references 79 Contents ix

Part Two Invertebrate Phyla 83

4 Sponges 85 4.1 Phylum Porifera: sponges 85 4.2 Classification 87 4.3 Class Demospongea 88 Spicular demosponges 88 Sclerosponges 89 Chaetetids 90 Stromatoporoids 90 Sphinctozoans 92 4.4 Class Calcarea 93 4.5 Class Hexactinellida 94 4.6 Incertae sedis: Archaeocyatha 95 Soft parts, organization and ecology 96 Distribution and stratigraphic use 97 4.7 Geological importance ofsponges 98 4.8 Sponge reets 98 Spicular sponge reets 98 Calcareous sponge reefs 99 13ibliography 100 Books, treatises and symposia lOO Individual papers and other references 100

5 Cnidarians 102 5.1 Introduction 102 5.2 M;0or characteristics and classes of Phylum Cnidana 104 5.3 Class Hydrozoa 104 Order I-Iydroida 104 Order I-Iydrocorallina lOS 5.4 Class Scyphozoa 107 J.J Class Anthozoa 107 Subclass Ceriantipatharia 107 Subclass Octocorallia 108 Subclass Zoantharia: corals 108 Order R. ugosa 109 Order Tabulata 124 Order Scleractinia 128 Coral reets 132 Geological uses of corals 135 Corals as colonies: the limits of zoantharian evolution 137 Minor orders 138 Bibliography 139 Books, treatises and symposia 139 Individual papers and other references 140

6 Bryozoans 143 6.1 Introduction 143 6.2 Two examples ofliving bryozoans 143 Howerbarlkia 143 Smittina 145 x Contents

6.3 Classiflcation 147 6.4 Morphology and evolution 150 6.5 Ecology and distribution 154 Shallow-\vater bryozoans 154 Reef-dwelling bryozoans 155 Deep-water b!yozoans 156 6.6 Stratigraphical use 156 Bibliography 156 Books, treatises and symposia 156 Individu~11 papers and other references 157

7 Brachiopods 158 7.1 Introduction 15~ 7.2 Morphology 158 Subphylum Rhynchonelliformea 159 Morphology of three genera 159 Preservation, study and classification ofarticulated brachiopods 164 M(~or features of brachiopod morphology 167 Endopunctation and pseudopunctation in shells 171 Subphylum Lingulifonnea 175 Lingula 176 Other Lingulifonnea 177 Subphylum Craniitormea 178 7.3 Ontogeny 179 7.4 Classification 179 7.5 Evolutionary history 183 7.6 Ecology and distribution 184 Ecol06ry ofindividual species 1~4 Epi6unal brachiopods 1~5 Endofaunal brachiopods 185 Brachiopod assemblages and 'community' ecology 188 Ordovician palaeocommunities 188 Silurian palaeocommunities 188 Devonian brachiopod assemblages 191 Permian reefassociations 191 Mesozoic brachiopod associations 192 7.7 Faunal provinces 192 7.8 Stratigraphical use 193 Bibliography 194 Books, treatises and symposia 194 Individual papers and other references 194

8 Molluscs 197 8.1 Fundamental organization 197 8.2 Classification 199 8.3 Some aspects of shell morphology and growth 201 Coiled shell morphology 201 Septation of the shell 203 8.4 Principal fossil groups 203 Class Bivalvia 203 Contents xi

Cerastodernw 203 Range ofform and structure in bivalves 206 Classification 209 Evolutionary history 210 Functional morphology and ecology 213 Ecology and palaeoecology 219 Stratigraphical use 220 Class Rostroconchia 221 Class Gastropoda 222 Introduction and anatorny 222 Classification 224 Shell structure and morphology 224 Shell composition 226 Evolution 226 Class Cephalopoda 229 Subclass Nautiloidea 230 Subclass Ammonoidea 238 Subclass Coleoidea: dibranchiate cephalopods 251 8.5 Predation and the evolution ofmolluscs 255 Bibliography 256 Books, treatises and symposia 256 Individual papers and other references 257

9 Echinodenns 262 9.1 Introduction 262 9.2 Classification 262 9.3 Subphylum Echinozoa 263 Class Echinoidea 263 Morphology and life habits of three genera 263 Classi fication 269 Subclass Perischoechinoidea 270 Subclass Cidaroidea 273 Subclass Euechinoidea and the morphological characters of euechinoids 276 Evolution in ecbinoids 282 Class Holothuroidea 285 Class Edrioasteroidea 286 <).4 Subphylum Asterozoa 288 Subclass Asteroidea 288 Subclass Somasteroidea 289 Subclass Ophiuroidea 290 Starfish beds 290 9.5 Subphylum Crinozoa 291 Class Crinoidea 291 Main groups ofcrinOIds 293 Palaeozoic crinoids 293 Mesozoic to recent crinoids: articulates 297 Ecology of crinoids 298 Formation ofcrinoidallimestones 300 9.6 Subphylum Blastozoa 301 Classes Diploporita and Rhombifera: cystoids 301 xii Contents

Structural char3cteristics 301 Pore structures 30] Classification 303 Ecology 304 Class Blastoidea 304 Diversity and tlll1ction ofhydrospires 305 Classification and evolution ofblastoids 306 Ecology and distribution ofblastoids 307 9.7 Subphylum Homalozoa, otherWIse calcichordates 307 9.8 Evolution 311 Earliest echinoderms and their radiations 311 Evolution of the tube feet 3J3 Why pentamery? 313 Convergent evolution and intermediate forms 314 Bibliography 314 Books, treatises and symposia 314 Individual papers and other references 315

10 Graptolites 318 10.1 Structure 318 Order Graptoloidea 318 Sacto,~raptus chirnaera 318 Diplograptus !eplotheca 320 Order Dendroide;j 320 Dcndrograptus 320 Preservation and study ofgraptolites 322 Ultrastructure and chcmistlY ofgraptolite periderm 324 10.2 ClassifIcation 326 10.3 Diological atlinities 329 10.4 Evolution 329 Shape ofgraptolite rhabdosomes 329 Proxirnal end in graptoloids 333 Thecal structure 335 Cladia 337 Structure of rctiolitids 338 10.5 Hmv did graptolites live? 338 Passive drifting 339 Automobility 340 Use of models in interpreting the mode of life of graptoloids 340 10.6 Faunal provinces 343 10.7 Stratigraphical usc 344 Bibliography 345 Books, treatises and symposia 345 Individual papers and other references 346

11 Arthropods 348 11.1 Introduction 348 11.2 Classification and general rnorphology 348 Diversity ofarthropod types 348 Features ofarthropod organization 349 Contents xiii

11.3 Trilobita 351 General morphology 351 Ilcastc dOll/l1irlgiac 352 Detailed morphology of trilobites 354 Cuticle 355 Cephalon 357 Glabella 357 Cephalic sutures 357 Hypostome 359 Eyes 360 Cephalic fringes 363 Enrollment and coaptative stnlctures 365 Thorax 366 Pygidiurn 367 Appendages 367 Trilobite tracks and trails 370 Walking movements in arthropods 372 Different kinds of trilobite trails 372 Life attitudes, habits and ecolo[,,'y 374 Ecdysis and ontogeny 377 Classification 380 Evolution 382 General pattern of evolution 382 Microevolution 383 Faunal provinces 386 Stratigraphical use 387 11.4 Phylum Chelicerata 388 Class Merostomata 388 Subclass Xiphosura 388 Subclass Eurypterida 392 11.5 Phylum Crustacea 397 Bibliography 400 Books, treatises and symposia 400 Individual papers and other rderences 400

12 Exceptional faunas; ichnology 406 12.1 Introduction 406 12.2 Burgess Shale tllma 409 Arthropods 410 Lobopods 412 Other invertebrates 413 Significance of the Burgess Shale faunas 415 Ecology 415 Geographical distribution 416 Diversity 416 Persistence 416 12.3 Upper Cambrian ofsouthern Sweden 418 12.4 I IUllSrLickschiefer [nma 420 12.5 Mazon Creek buna 422 12.6 Solnhofen lithographic limestone, Bavaria 424 xiv Contents

l2.7 Ichnology 426 Classification of trace fossils 426 l'Vlorphological and preservational classification 426 Behavioural classification 426 Phylogenetic classification 429 Uses of lchnolobY)T 429 Sedimentary environment 429 Stratigraphy 430 Fossil behaviour 430 Bibliography 431 Books, treatises and symposia (exceptional faunas) 431 Individual papers and other references (exceptional bunas) 431 Books, treatises and symposia (ichnol06,),) 434 Individual papers (ichnology) 434

Systematic index 435

General index 443 Preface

The first three editions of this textbook were pub­ given me at every stage. I Iad Cecilia not undertaken lished in 1979, 1986 and 1992, and I trust that this the vast job of rebuilding our teaching collections new one, necessitated by so many advances both in and preparing new course materials, I would never fact and theory, ,vill retain its function as a course ha ve had the time to complete this book before the text for students of palaeontology from their second deadline. year onwards. I have made substantial changes to To lZoisin Moran of University College, Galway Chapters 1-3,7,8 and 12, and the Trace Fossils sec­ I extend my grateful thanks for her two beautiful tion has been transferred from Chapter 1 to the last paintings for the Part Title pages. chapter. All the other chapters have been revised to I wish to thank also my editorial colleagues, a greater or lesser extent. I have redrawn about half Dr Ian Francis and Jane Plowman, who have given ofthe illustrations: a singularly congenial occupation all possible assistance. From Ian came the suggestion, fl,r long winter evenings, and I hope that these will which we have followed up, that the new edition prove ofvalue in helping students to understand the should be coupled with a CD-ROM of Fossil anatomy and terminology off()ssil invertebrates. Images. 'rlllS has been undertaken in cor~unction Key words appear in bold at their first mention in with the Natural IIistory Museum, London, and to the text. There is no specific index for them, but Norman McLeod and Paul Taylor of that institution they appear in the General Index. lowe a great deal. I am likewise gratefil1 to David As before, I wish to thank all my fi-iends, col­ Hicks and Eve Daintith for their meticulous copy­ leagues and family who have given me such assist­ editing and proofreading, respectively. ance in preparing this new edition. Derek Briggs, On a wet November day, over SO years ago, I was Colin Scrutton and Rachel Wood gave excellent taken into a museum in my native Newcastle-upon­ advice at the beginning of the project as to how I Tyne, to escape fimn the rain. There, in a dusty glass should proceed with the fourth edition, and I have case were two giant ammonites (probably Titanites taken on board most oftheir suggestions. As always, ,!Zigantcus) , and I was given to understand that they these colleagues, together with David tlarper, Peter had lived millions ofyears ago. They excited a £.sci­ Sheldon, Susan Rigby, Liz Harper, Dick Jefferies, nation which has continued until now, and I am still John Cope, Alan Owen and various others have glad to escape to the hills on a fj-esh summer morn­ helped me at all stages, and so have many others, too ing, to search for fossils, and then to bring them back numerous to Inention, in my own country and to the laboratory for study. I hope that this book abroad, and my thanks are due to all. will be of value to any student wishing to explore I would like, above all, to record my heartfelt something of the richness and diversity of ancient thanks to my true and stalwart fi-iend Cecilia Taylor, life, and of the methods available for its scientific f()l- all the support and helpfill suggestions she has study. Ifso, I will have achieved what I set out to do_

Euan Clarkson Edinburgh PaleoBase-Macrofossils on CD-ROM

The Natural History Museum London and Dr E.N.K. Clarkson have collaborated on the development of this important new initiative in paleontology teaching. PaleoBase is a combined image library and database, containing records for 1000 key fossil genera. Each record contains a set of images, and information on stratigraphic range, hard-part mineralogy, palaeoecology, palaeobiogeography, etc. The images have been captured using the Natural History Museum's high resolution PALAEOVISION digital imaging system, and the data and images reside in the CompuStrat database manager. This system allows the user great flexibility in displaying data. For example, users can simply browse from record to record; pull up fossils from a taxonomic index; or select and sort records by geographic range, life habit, stratigraphic range, or by a variety of other criteria. Range charts and palaeobiographic maps are given where appropriate, and the user can print records, or export them into other applications. Within each of the major groups there are a number of labelled images and diagrams to allow the user to become familiiar with key morphological terms. PaleoBase may be used as an adjunct to Invertebrate Palaeontology, or as a standalone product. Taxa have been selected for their relevance to Earth science teaching worldwide.

PC or MAC, minimum 8 Mb RAM, CD-ROM drive, 640 x 480 colour monitor.

For fllrther int(mnation aud ordering details, please contact: ianJi':mcisCd!blacksci.co.uk PART ONE General Palaeontological Concepts Amioceras cf. hartmanni (OppelI, an assemblage of immature ammonites from the Lower Jurassic of Black Ven, Charmouth, England (Lower Sinemurian). These specimens were probably catastrophically buried, since the soft parts must have been in place when they died preventing sediment penetrating the chambers. Painting by Raisin Moran; original specimen in the James Mitchell Museum, University College, Galway, Ireland. Pri ncipies of po loeontology

no record of terrestrial animals until much later. In terms of our understanding of the history of life, perhaps the most significant of all events took place Once upon a time ... abont 543 Ma ago at about the beginning of the Some 4600 miIlion years ago the Earth came into Cambrian Period, for at this stage there was a sud­ being, probably forming from a condensing disc of den proliferation of ditferent kinds of marine inver­ particles, dust and gas, which slowly rotated round tebrates. During this critical period the principal the Sun. Larger particles, or planetismals, formed invertebrate groups were established, and they then from this nebular disc, and as these collided they diversified and expanded. Sonle of these organisms accreted, eventually tanning the planets. acquired hard shells and were capable of being fos­ Of all the nine planets in the Solar System only silized, and only because of this can there be any Earth, as f,1r as is known, supports advanced life, chance of understanding the history of invertebrate thongh at the time of writing nlUch interest has lite. been generated by the discovery of organic material The stratified sedimentary rocks laid down since on Jupiter's satellite Europa. It is, however, a strik­ the early Cambrian, and built up throughout the ing tact that life on Earth began very early indeed, whole of Phanerozoic time, are distinguished by a within the first 30% of the planet's history. There rich heritage of the fossil remains of the inverte­ are remains ofsimple organisms (bacteria and 'blue­ brates that evolved throuuhb successive historical green algae', or cyanobacteria) in rocks 3400 1\11a periods; their study is the domain of invertebrate old, so Ii fe presumably originated before then. palaeontology and the subject of this book. These simple t<:)fJm of life seem to have dominated the scene for the next 2000 Ma, and evolution at that time was very slow. Nevertheless, the cyanobacteria and photosynthetic bacteria were instrumental in changing the environment, for they gave off oxygen into an atmosphere that was previ­ ously devoid of it, so that animal life eventually became possible. Hard-port preservation Only when some of the early living beings of this Earth had reached a high level of physiological and rossil invertebrates occur in many kinds ofsedimen­ reproductive organization (and most particularly tary rock deposited in the seas during the when sexual reproduction originated) was the rate Phanerozoic. They may be very abundant in of evolutionary change accelerated, and with it all limestones, shales, siltstones and mudstones but on manner of new possibilities were opened up to the whole are not common In sandstones. evolving life. This was not until comparatively late Sedimentary ironstones may have rich fossil in geological history, and there are no fossil animals remains. Occasionally they are found in some coarse known from sediments older than about 700 Ma. rocks such as greywackes and even conglomerates. Needless to say, these are all invertebrate animals The state of preservation of fossils varies greatly, lacking backbones. All of them are marine; there is depending on the structure and composition of the 4 Principles of palaeontology

original shell, the nature and grain size ofthe enclos­ floor during these erosion periods would probably ing sediment, the chemical conditions at the time of be transported or destroyed - another limitation on sedimentation, and the subsequent processes of dia­ the adequacy ofthe fossil record. genesis (chemical and physical changes) taking On the other hand, some 1nari ne invertebrates place in the rock after deposition. found in certain rock types have been preserved The study of the processes leading to fossilization abundantly and in exquisite detail, so that it is possi­ is known as taphonomy. In most caseS only the ble to infer much about their biology hom their hard parts of fossil animals are preserved, and for remains. Many of the best-preserved fossils come these to be fossilized, rapid burial is normally a pre­ fl-om limestones or from silty sediments vvith a high requisite. The soft-bodied elements in the E1lll1a, calcareous content. In these (Fig. 1.1) the original and those forms with thin organic shells, did not calcareous shells may be retained in the t(:)ssil state normally survive diagenesis and hence have left little \'vith relatively little alteration, depending upon the or no evidence oftheir existence other than records chemical conditions within the sediment at the time oftheir activity in the form of trace fossils. What we of deposition and after. can see in the rocks is therefore only a narrow band A sediI11ent often consists of components derived in a whole spectrum of the organisms that were from vanous environments, and when all of these, once living; only very rarely have there been found including decaying organisms, dead shells and sedi­ beds containing some or all of the soft-bodied ele­ mentary particles, are thrown together the chemical ments in the fauna as well. These arc immensely sig­ balance is unstable. The sediment will be in chemi­ nificant for palaeontolobry. cal equilibriunl only after diagenetic physicochemi­ The oldest such fauna is of late Precambrian age, cal alterations have taken place. These may involve some 615 Ma old, and is our only record of animal rec1)7stallization and the growth of new minerals life before the Cambrian. Another such 'window' is (authigenesis) as well as cementation and com­ known in Middle Cambrian rocks from British paction of the rock (lithification), and during any Columbia, \'vhere in addition to the normally one of these processes the fossils may be altered or expected trilobites and brachiopods there is a great destroyed. Shells that are originally of calcite pre­ range of soft-bodied and thin-shelled animals ­ serve best; aragonite is a less stable form of calcium sponges, wonns, jellyfish, small shrimp-like crea­ carbonate secreted by certain living organisms (e.g. tures and animals of quite unknown affinities ­ corals) and is often recrystallized to calcite during which are the only trace of a diverse f2una which diagenesis or dissolved away completely. would otherwise be quite unknovvn (Chapter 12). Calcareous skeletons preserved in more sandy or There arc similar 'windows' at other levels in the silty sediment.s may dissolve after the sediment has geological column, likewise illuminating. hardened or during weathering of the rock long The :f()ssil record is, as a guide to the evolution of after its induration. Moulds (often miscalled casts) ancient life, unquestionably limited, patchy and of the external and internal suliaces ofthe fossil may incomplete. Usually only the hard-shelled elements be left, and ifthe sediment is fine enough the details in the biota (apart from trace f()ssils) arc preserved, these show may be very good. Some methods for and the fossil assemblages present in the rock may the study of such moulds are described in section have been transported some distance, abraded, dam­ 7.2, with reference to brachiopods. If a fossil aged and mixed with elements ofother Elllnas. Even encloses an originally hollow space, as for instance if thick-shelled animals were oribrinally present in a between the pair of shells of a bivalve or brachio­ fauna, they may not be preserved; in sandy sedi­ pod, this space may either be lett empty or become ments in which the circulating waters are acidic, for filled with sediment. In the latter case a sedirnent instance, calcareous shells may dissolve withill a few core is preserved, which comes out intact when the years bef()re the sediment is compacted into rock. rock is cracked open. This bears upon its surfclCe an Since the sea floor is not ahvays a region ofcontinu­ internal mould of the fossil shell, whereas exter­ ous sediment deposition, 111any apparently continu­ nal moulds arc left in the clVity tram which it ous sedinlentary sequences contain nUlTlerous came. In rare circumstances the core or the shell, or small-scale breaks (diastems) representing periods both, may be replaced by an entirely different min­ of winnowing and erosion. Any shells on the sea eraI, as happens in fossils preserved in ironstones. If Occurrence of invertebrate fossils in Phanerozoic rocks 5

(j) recrystallized calcite ~~

Ironstone -':-~~~8B1 / '. / (f) core -'-:'-+j;""""

Figure 1.1 possible processes of fossilization of a bivalve shell: (a) original shell, buried in mud (left) or carbonate (right); (b) the shell was calcite, was buried in a carbonate sediment and was preserved intact other than as a small crystallized patch; (c) shell orig­ inally of aragonite, now recrystallized to calcite which destroys the fine structure; (d) original calcite shell retained surrounding a dia­ genetic core of silica; (e) a silica rim grawing on the outside of the shell; (f) tectonic distortion of a shell preserved in mudstone; (gl shell preserved in mud with original shell material leached away, leaving an external and an internal mould, surrounding a mudstone core; (h) a calcareous concretion growing round the shell and inside (if the original cavity was empty), with patches of pyrite in places; (jl ironstone replacement of core and part of shell.

the original spaces in the shell are impregnated with the silica increases markedly, and the silica so extra minerals, it is said to be permineralized, released will travel through the rock and precipitate while the growth of secondary minerals at the wherever the pH is lower. The inside ofa sea urchin expense of the shell is replacement. Cores may decaying under different conditions would trap just sometimes be of pyrite. Graptolites are often pre­ such all internal microenvironment, within which served like this, anaeorbic decay having released the silica could precipitate as a gel. Such siliceous hydrogen sulphide, which reacted with ferrous cores retain excellent features, preserving the intel11a1 (Fe2+) ions in the water to allow an internal pyrite morphology of the shell. On the other hand, silica COl"e to form. Sometimes a core of silica is !()und may replace calcite as a very thin shell over the sur­ within an unaltered calcite shell. This has happened f~\ce of a fossil as a result of SOlTle complex surface with some ofthe Cretaceous sea urchins ofsouthern reactions. These siliceous crusts may retain a very England. They lived in or on a sediment ofcalcare­ detailed expression of the surElce of the fossil and, (ms mud along with many sponges, which secreted since they can be treed from the rock by dissolving spicules of biogenic silica as a skeleton. In alkaline the limestone with hydrochloric acid, individual conditions (above pH 9), which may sometimes be small fossils preserved in this way can be studied in generated during bacterial decay, the solubility of three dimensiollS. Some of the most exquisite of all 6 Principles of palaeontology

trilobite,; and brachiopods are kno\vl1 from material remains ofsome part, usually a shell ofskeleton, ofa such as this. once-living organism; (2) trace fossils, which arc A relatively uncommon but exquisite mode of tracks, trails, burrows or other evidcnce ofthe activ­ preservation is phosphatization. Sometimes the ity of an animal of former times··- sometimes these external skeleton, especially of thin organic-shelled are the only guide to the former presence of soft­ animals, may be replaced or overgrown by a thin bodied animals in a particular environment; (3) sheet of phosphate, or the latter may reinforce an chemical fossils, relics of biogenic organic com­ originally phosphatic shelL In the former situation pounds which may be detected geochemically in the external form ofthe body is precisely replicated. the rocks. Alternatively a phosphatic filling of the interior of At the heart of invertebrate palaeontology stands the shell may form a core, picking out internal taxonoluy; the classification of fossil and modern structures in remarkable detail. Such preservation is animals into ordered and natural groupings. These probably associated with bacterial activity directly groupings, known as taxa, must be narned and after the death ofthe animal. Many small Cambrian arranged in a hierarchial system in which their rela­ fossils have been preserved by phosphatization tionships are made clear, and as £1r as possible must (Chapters 3 and 12), but much larger fossils may be be seen in evolutionary perspective. preserved also, for example crustaceans with a Evolution theory is compounded ofvarious dis­ fluorapatite infilling and with all their delicate ciplines - pure biology, comparative anatomy, appendages intact. embryolobry, genetics and population biolob'Y - but Fossils arc often found in concretions: calcare­ it is only the palaeontological aspect that allows the ous or siliceous masses formed around the fossil predictions of evolutionary science to be tested shortly aiter its death and burial. Concretions form against the background of geological time, permits under certain conditions only, where a delicate the tracing of evolving lineages and illustrates some chemical balance exists bet\veen the water and sedi­ of the patterns of evolution that actually have ment, by processes as yet not fully understood. occurred. The rates at which animals have evolved have varied through time, but most animal types Soft-part preservation (species) have had a geological life of only a tt'vv million years. Some ofthese evolved rapidly, such as In very rare circumstances soft-bodied organisms the ammonites, others very slowly (Chapter 2). A can be preserved as fossils, and these provide other­ rock succession of marine sediments built up over wise unobtainable evidence ofthe diversity ofmeta­ many millions of years may therefore have several zoans living at particular periods; this is discussed in fossil species occurring in a particular sequence, each Chapter 12. species confined to one part of the succession only and representing the time when that species was living. Herein lies the oldest and Inost general appli­ cation of invertebrate palaeontolob'Y: biostratig­ raphy. Using the sequence of f()ssil faunas, the geological column has been divided up into a series Invertebrate palaeontology is normally studied as a of m~~or geological time units (periods), each of subdivision of geology, as it is within Earth sciencc which is further divided into a hierarchy of small that its greatest applications lie. It can also be seen as units. The whole basis for this historical chronology a biological subject, but one that has the unique per­ is the documented sequence of fossils in the rocks. spective of geological time. Within the domain of But different kinds of fossils have different strati­ invertebrate palaeontolob'Y there are a nmnber of graphical values, and certain parts of the geological interrelated topics (Fig. 1.2), all of which have a record are more closely subdivided than others. bearing on the others and \vhich also link up with Some 'absolute' ages based on radiometric dating other sciences. have been fixed at particular points to this relative Three main categories of fossils may be distin­ scale, and these provide a framework for under­ guished: (1) body fossils, in other words the actual standing the geological record in terms of real time Divisions of invertebrate palaeontology 7

Figure 1.2 The various subdisciplines of palaeontology.

(i.e. known periods of millions of years) rather than 1950s, is concerned with the relationships of fossil as just a purely relative scale. This is only possible at animals to their environment, both as individuals certain horizons, however, and for practical pur­ (autecology) and in the faunal communities in poses the geological timescale based on biostratigra­ which they occur naturally; the latter is sometimes phy is unsurpassed for Phanerozoic sediments. known as synecology. Although stratigraphy is the basis of the primary Since the soft parts of fossil animals are not nor­ discipline of geochronology, a small [lcet of mally preserved, but only their hard shells, there are palaeontological study has a bearing on what may be relatively few ways in which their biology and life termed 'geochronometry'. By counting daily habits can be understood. Studies in functional growth rings in extinct corals and bivalves, informa­ morphology, however, which deal with the inter­ tion has been obtained bearing upon the number of pretation of the biology of fossilized skeletons or days in the lunar month and year in ancient times. structures in terms of their original function, have This has helped to confirm geophysical estimates on been successfully attempted with many kinds offos­ the slowing ofthe Earth's rotation (Chapter 5). sils, restricted in scope though these endeavours may Since stratigraphical applications of palaeontology necessarily be. Ichnology is the study of trace fos­ have always been so important, the nlOre biological sils: the tracks, surfice trails, burrows and borings aspects of palaeontology were relatively neglected made by once-living animals and preserved in sedi­ until comparatively recently. Palaeoecology, ments. This topic has proved valuable both in which has developed particularly since the early understanding the behaviour of the animals that 8 Principles of palaeontology

lived when the sediment \vas being deposited and in These words should make entirely clear the fun­ interpreting the contemporaneous environment. damental nature oftaxonomy. For, as has often been Finally, it is only by the integration of taxonomic said, to identify a fossil correctly is the first step, and data on local tltll1as that the global distribution of indeed the key, to fmding out further infcH"l1utlon marine invertebrates through time can be eluci­ abollt it. Sound classification and nomenclature lie dated. Such studies of palaeobiogeography (or at the root of all biological and palaeontological palaeozoogeography in the case of aniInals) can work; without them no coherent and ordered be used in cOl~junction with geophysical data in systerll of data storage and retrieval is possible. understanding thc former relativc positions and Taxonomy, or systematics as it is sometimes nlovernents of continental rnasses. known, is the science of classitlcation or organisms. All of these aspects of pabeontoloh'Y arc interre­ It is the oldest of all the biological disciplines, and lated, and an advance in one may have a bearing the principles outlined by Carl Custav Linnaeus upon any other. Thus a particular study in func­ (17CJ7.--177H) in his pioneering Systema fwtllrae are tional morphology may give int<->rIl1ation on still in use today, though greatly modifIed and palaeoecology and possibly some feedback to taxon­ extended. omy as well. Likewise, recent refinemcnts ill taxo­ nomic practice have enabled the development of a The species concept much more precise stratigraphy. The fi.mdarncntal unit of taxonomy is the species. Chemical cOlnpounds of biological origin can Animal species (e.g. Sylvester-Bradley, 1951l) are now be recovered from ancient rocks and fc)rm the groups of individuals that generally look like each basis of bionlolecular palaeontology. Such fossil other and can interbreed to produce off'lpring of the nlOlecules may help to diagnose which organisms same kind. They cannot interbreed \vith other they come from and their breakdown patlnvays l11ay species. Since it is reproductive isolation alone that say something about the environment. l\101ecular ddines species it is only really possible to distinguish phylogcnetics based upon protein sequencing may closely related species if their breeding habits arc show how far two or more related organisms have kno\vn. Of all the described 'species' of living ani­ diverged f1-o111 a cornmon ancestor, and to sor11C mals, ho\vever, only about a sixth arc 'gooe!', or extent the available techniques can be applied to the properly defined, species. Inf(wI11dtioll upon the recent fossil record. Immunological-determinant reproductive preferences of the other £i ve-sixths of techniques can be used to detect proteins and poly­ all naturally occurring animal populations is just not saccharides in fossil shells but, tor the moment, only documented. shells younger than 2 Ma have proved anlcnable to The differentiation of most living and all fossil analysis. There arc also promising developments also species therefore has to be based upon other and in palaeobiochemistry and organic geochemistry technically less valid criteria. \vhich are applicable to the fossil record, though Of these by tIl' the most important, especially in these arc beyond the scope ofthis book. palaeontology, is morphology, the SCIence of form, since most natural species tend to be com­ posed of individuals of similar enough external Taxonomy appearance to be identifiable as of the same kind. Distinguishing species of living animals by morpho­ Taxonomy is often undervalued as a glorified logical criteria alone is not \vithout hazards, espe­ form offrling - \\/ith each species in its folder, like cially where the species in question are sinlilar and a stamp in its prescribed place in an album; but closely related. Supplementary information, such as taxonomy is a fundanlental and dynamic science, the analysis of species-specific proteins, may be of dedicated to exploring the causes of relationships help in some cases where there is good rcason [or and similarities among organisms. Classifications it to be sought (e.g. for disease-carrying insects). For are theories about the basis of natural order, not the rest some degree ofsubjectivity in taxonomy has dull catalogues compiled only to avoid chaos. to be accepted, though this can be minimized if The best fnOflOl:raphs are works c:Igcniu3 ... enough morphological criteria arc used. (S.j. Gould, 19(0) Divisions of invertebrate palaeontology 9

Nomenclature and identification of fossil species comparing the species point by point. Some of the In the formal nomenclature of any species, living or species may prove to be identical with already fossil, taxonomists follow the biological system of described species, or show only minor variation ofa Linnaeus, whereby each species is defined by two kind that would be expected in a local variant names: the generic and specific (or trivial) names. within the same species. Other species in the fauna For example, all cats, large and small, are related, may be new, and if so a full technical description and one particular group has been placed in the with illustrations must be prepared for each new genus Felis. Of the various species of Felis the spe­ species, which should be published in a palaeonto­ cific names F. catttS, F. leo and F. pardus formally logical journal or monograph. This description is refer to the domestic cat, lion and leopard, respec­ based upon type specilllens, which are always tively. In full taxonomic nomenclature the author's thereafter kept in a museum or research institute. name and the date of publication are given after the Usually one of these, the holotype, is selected as species, e.g. Felis catus (Linnaeus 1778) (see below the reference specimen and fully illustrated; COIn­ for further discussion). parative detail may be added from other specimens In palaeontol06'Y it can never be known for called the paratypes. There are various other kinds certain whether a population with a particular mor­ of type specimens; for example, a neotype may be phology was reproductively isolated or not. I-Ience erected when a holotype has been lost, or when a the definition ofspecies in palaeontology, as in most species is being redescribed in fi.lller and more up­ living specimens, must be based almost entirely OIl to-date terms when no type specimen has previously morphological criteria. Moreover, only the hard been designated. parts of the fossil animal are preserved, and much A new genus will be designated as gen. nov. by its useful data has vanished. A careful exarnination and author, this following the generic name, a new documentation of all the anatomical features of the species as sp. nov. and a new subspecies as ssp. nov. fossil has to be the main guide in establishing that The new species must be named and allocated to one specIes is dittcrent from a related species. In rare an existing genus, or if there is no described genus cases this can be supplemented by a comparison of to which it pertains then a new genus must also be the chemistry of the shell, as has proved especially erected. 'To appreciate the method let us consider useful in the erection of higher taxonomic cate­ the following historical tale. In the early nineteenth gories. Within any interbreeding population there is century brachiopods were poorly known and few usually quite a spread of morphological variation. distinct genera had been erected. One of these was On a broader scale there may be both geographical the living Terebratula, narned by O.P. Mliller in and stratigraphic variations, and all these must be 1776. When E.F. von Schlotheim first studied carefully documented if the species is to be ideally Devonian fossils from North Germany in 1H20 he established. Such studies may be very signitlcant in recognized that some of the shells were bra­ evolutionary palaeontology. chiopods, and he described one of the most abun­ When a palaeontologist is attempting to distin­ dant fonus as the new species Terebratula sarcinulatus. guish the species in a newly discovered f:mna, say of By 1830, however, much more was known about fossil brachiopods, she or he has to separate the indi­ brachiopods, and G. Fischer de Waldheim proposed vidual fossils out into groups of morphologically a new genus fiJI' this species, so that it became cor­ similar individuals. There may, to take an example, rectly designated Chonetes sarcinulattts (Schlotheim). perhaps be eight such groups, each distinguished by This is the 'type species' of Chonetes, a well-known a particular set of characters. Some of these groups Siluro-Devonian genus ofthe Class Strophomcnata. may be clearly distinct from one another; in others Note that where a species was originally described the distinction may be considerably less, increasing under a different generic name, the original author's the risk of greater subjectivity. These groups are name is quoted in parentheses. In 1917 F.R. prOVIsionally considered as species, which must then Cowper Reed, then working on Ordovician and be identified. This is done by consulting palaeonto­ Silurian brachiopods ofthe Girvan district, Scotland, logical monographs or papers containing detailed recognized many new species. One of these had technical descriptions and illustrations of previously similarities in rnorphology to Chonetes, but it was described brachiopod f:lLInas of similar age, and suHiciently diHcrent to be regarded as a species of a 10 Principles of palaeontology

new subgenus; this is \vritten Chonefes (Eochonetes) There may be various subdivisions of these cate­ advena Reed 1917. When in 1928 the taxonomic gories, e.g. super£lrnilies, suborders, etc., and in cer­ problerns of Chonefes and similar forms were tain groups there is even a case for erecting addressed by O.T. Jones, the nevv Superfamily 'superphyla'. There are only about 30 phyla In the Plectambonitacea was erected to acconllllodate animal kingdom, and only about a dozen of these, adlJena and many other related brachiopods. At a e.g. Mollusca and Brachiopoda, leave any fossil later stage }:'ochonetes was elevated to the rank of a re111<1Il1S. full genus. In the most recent treatment, D.A.T. In taxonomy higher taxa are usually distinguished Harper described a large fauna from the Girvan by their suffix (i.e. -ea, -a, etc.). As an example, the district of Scotland, and within this he recognized following documents the classification of the two subspecies of E. advena, of somewhat different Ordovician brachiopod Eochorlctes adl'cna gracilis, ages and distinguished by minor differences in referred to earlier, according to a taxonomic scheIne morphology. These, in Harper's (1989) monograph, in which the author of the taxon and the year of are written Eochonete.' advena ad/Jena Reed 1917 (des­ publication are quoted. ignating Reed's original material), and Eochom:,tes adlJeruIR.. eed 1917 J!.ra cilis ssp. nov. Subsequent Phylum Brachiopoda ULlmeril 1806 authors \,vill refer to the latter, in full, as For/tonetes Subphylum Rhynchonelliformea Williams ct al. adlJcrw gracilis Harper 1989, or in abbreviated form as 1996 E. advena l,racilis. Class StrophomenataWilhams ct al. 1996 Where, due to indiHerent preservation, or lack of Order Strophomenida ()pik 1934 an up-to-date monographic base, a species cannot Suborder Strophomenidina Opik1934 be identified with certainty, it may be designated as Superfunily Plectambonitacea Jones 1928 afT. (related to) or cf (may be compared \vith), an Family Sowerbyellidae Opik 1930 existing species (e.g. iVIono<.t,!,raptu5 cf. lJomerinus). Subfamily Sowerbyellinae Opik 1930 Where the fossil can be identified as belonging to a Genus Eochonetes Reed 1917 known genus, but cannot be ascribed to a species (as Species advena Reed 1917 may be the case where preservation is poor or if Subspecies gracilis Harper 1989 only a fi'agment is preserved, the s'_lffix sp. (plural spp.) is used (e.g. Caloceras sp.). Ifthe specimen, in a In the above section we have seen the divisions of more extreme case, can only tentatively be referred the taxonomic hierarchy, but in defining these to a genus, one would write e.g. ?Kutorgin!1 sp. If groupings how do taxonomists actually go about it? only the species is dubious such an ascription as The basic principle is that morphological and other Loplectodonta ?pcnkillensis might be used. siluilarities reflect phylogenetic affinity (honlol­ All taxonomic work such as this must t()llovv a ogy). This is always so unless, for other reasons, particular set of rules, which have been worked out similarity results from convergent evolution. But in by a series of International Commissions and are assessing 'similarities' how does one decide upon documented in full in the opening pages of each which characters are Important? How should they volume of the Treatise on invertebrate Paleontology (a be chosen to minimize subjectivity and produce continuing series of volumes published by the natural order groups? There is no universally Geological Society ofAmerica). accepted method of facilitating these ends and taxonomists have used different rnethods. In recent Taxonomic hierarchy years three sharply contrasting schools have Although all taxonomic categories above the species em.erged: these are the schools of (1) evolutionary level are to some extent artificial and subjective, taxonomy, (2) numerical taxonomy and (3) c1adism. ideally they should as far as possible reflect evolu­ Until the 19705 most palaeontologists, especially tionary relationships. those working with fossil material which they have Similar species arc grouped in genera (singular collected in the field, were evolutionary taxono­ genus), genera in fanlilies, families in orders, mists. In erecting a hierarchical classifIcation, such orders in classes and classes in the largest division of classical taxonomists used a traditional and very flex­ the animal kingdom: phyla (singular phylunl). ible combination of criteria. First there is nlorpho- Divisions of invertebrate palaeontology 11 logical (or phenetic) resemblance, the extent to £11' the most effective method for reconstructing which the animals resemble one another. Second, phylogeny. Smith's (1994) explanation of cladistic phylogenetic relationships are along with phe­ methodology is so comprehensive that only brief netic resernblancc considered irnportant. By this is COlTunents arc given here. meant the way (as Elf as can be determined) in HemTig was of the opinion that recency of com­ which animals are actually related to each other, i.e. mon origin could best be shown by the shared pos­ in terms of recency of common origin, which of seSSIOn of evolutionary novelties or 'derived course grades into evolutionary taxon0111Y. The characters'. Thus in closely related groups we would order of succession in the rock record and geo­ see 'shared derived characters' (synapomorphies) graphical distribution may play an important part in which would distinguish this group from others. deciding relationships. This prJctical approach to Henmg's central concept was that in any group taxonomy, which took all factors into considerJtion, characters arc either 'primitive' (symplesiomor­ has for a long time been the backbone ofpalaeonto­ phic) or 'derived'. Thus all vertebrates have back­ logical classitlcation, and is still considered to be the bones; the possession of a backbone is a primitive best method by stratophenetic palaeontologists, who character for all vertebrates and is not, of course, place much emphasis on time in seeking ancestor­ indicative of any close relationship between any descendant relationships (Henry, 1984; Gingerich, group of vertebrates. What is a primitive character 1990). tor all vertebrates is of course a derived character as For some taxonomists, however, the uncertainties compared to invertebrates - synapomorphy and and subjectivity which arc almost inescapable in any symplesiomorphy thus delineate the relative status kind of c1assitlcation seemed to be particularly acute of particular characters with respect to a specific in classical taxonomy, as did the limitations of the problem. fossil record in terms ofpreservation. The numerical Hennig endeavoured to provide an objective taxonomists tried to escape £rorn this problem by methodology for determining recency of common opting for quantified phenetic resemblance as the origin in related taxa, based upon primitive and only realistic guide to natural groupings. It was their derived characters. Such relationships arc expressed view that if enough characters were measured, in a cladogram (Fig. 1.3), in which dichotomous quantitled and computed and represented by the use branching points are arranged in nested hierarchies. of cluster statistics, the distances between clusters Here taxa A and J:) share a unique common could be used as a measure of their differences. ancestor. They are said to be sister groups. They Numerical taxonomy has been found very useful both share an evolutionary novelty or synapomor­ in some instances, but subjectivity cannot be elimi­ phy, not possessed by taxon C. Now C is the sister nated since the operator has to choose (subjectively) group of the combined taxa A and B, and D is the how best to analyse the measurements made, and sister group of combined taxa A, Band C. in per­ may need to 'weight' them, giving certain characters forming a cladistic analysis, therefore, a taxonomist more importance (again subjectively) in order to assumes that dichotomous splitting had occurred obtain meaningful results. Hence the objectivity of Il1 each lineage and compiles an (unweighted) numerical taxonomy is less than it might appear. The third school relics upon phylogenetic critena A B c D alone, emphasizing that features shared by organisms manifest, in nature, a hierarchial pattern, evident in the distribution of characters shared amongst organ­ isms. It is known as cladism or phylogenetic systelnatics - a school founded by the German entomologist Willi Hennig (1966) and was soon applied vigorously to palaeontology (e.g. Eldredge and CrJcraft, 1980). In the eJrly days of cladistics there were many doubters (including, as I have to admit, the present author). But, as the method itself evolved, cladistics has come to be recognized as by Figure 1.3 A c1adogram (for explanation see text). 12 Principles of palaeontology

character data matrix. The more characters and opposed by Ridley, 1985), the successive appearance character states there are available the larger the of taxa in stratigraphy cannot be denied as an essen­ database, and large databases are often rOlltinely tial source of data, however imperfect the fossil processed and the construction of a cladogram record actually is. Thus as Gingerich (1990) com­ speeded up by using one of several computer pro­ ments, 'Time is a fundamental dimension in evolu­ grams. The PAUP (Phylogenetic Analysis Using tionary studies, and a major goal of palaeontology Parsimony) program. tor example, is a technique should continue to be the study ofthe diversification which makes the fewest assumptions in ordering a of major groups in relationship to geological time'. set ofobservations. So, having constructed an appropriate cladogram, How can "\ve distinguish symplcsiolT1orphic the next stage in exploring relationships is to com­ (shared primitive) fi-om synapomorphic (shared bine this information with biostratigraphic data. For derived) character states? The most useful way is this, Smith (1994) discusses methodology in detail. 'outgroup cornparison'. Here an 'ingronp' (ofwhich The result is a phylogenetic tree, which shows the the relationships arc under investigation) is specifi­ splitting of lineages through time and is efFectively cally designated, and compared with a closely 'a"best estimate" of the tree oflife'. Very commonly related ·outgroup'. Any character present in a vari­ there is an excellent correspondence between the able state in the 'ingronp' must be plesiomorphic if cladogram and the rock record; on the other hand it is also found in the outgroup. Like"\vise, apomor­ the combined cladistic and biostratigraphic approach phil' characters are only present in the ingroup. may throvv up unexpected patterns. Hennig distinguished three kinds of cladistic The ultimate problem, not only tor cladism, but groupings. Monophyletic groups contain the com­ for all taxonomic methodology. results ti-om con­ mon ancestor and all of its descendants (D, C, B, A vergent evolution. Resemblances in characters or in Fig. 1.3); paraphyletic groups are descended character states m.ay have nothing to do with from a common ancestor (usually now extinct and recency of comInon orif.,rin, but from convergence, known as the stem group) but do not include all and it may not always be possible to disentangle the descendants (B and C, for example, in Fig. 1.3); results of the two. Thus Willmer (1990) and Moore polyphyletic groups on the other hand, arc the and Willmer (1997) in considering the relationships result of convergent evolution. Their representa­ between major invertebrate groups argue that tives arc descended from different ancestors and 'cladistic analysis based on parsimony will tend to hence, although these may look superfIcially similar, minimize and thus conceal convergence', and they any polyphyletic group comprising them is artificial. contend that convergence at all levels is t~lr 1110re A dadogram is not an evolutionary tree; it is an important than has generally been believed. There is analysis of relationships. As such it is a valuable and certainly a problem here. Even so, cladistic method­ rigorous way of working out and showing graphi­ ology, coupled with biostratigraphy seems to be the cally how organisms are related, and it forces taxon­ best way f()l"ward, and an essential prerequisite tor omists to be explicit about patterns and groups. The drawing up meaningful phylogenetic trees. methodology of cladistics is especially good when dealing with discrete groups with large morphologi­ Use of statistical methods cal and stratigraphic gaps and to these it brings the Inevitably palaeontological taxonomy carries a cer­ potential for real objectivity. A cladograrn shows tain element of subjectivity since the information how sister groups dre hierarchically related on the coded in fossilized shells does not give a complete basis of shared-derived homologies, but although it record of the structure and life of the animals that portrays taxonomic relationships in ten11S ofrecency bore them. There arc particular complications that of common origin, the order of succession in the cause trouble. For instance, palaeontological taxon­ rock record is not taken into account (though omy can do little to distinguish sibling species, implicitly cladograms have a tirne axis). which look alike and live in the same area but can­ So where does that leave the potential contribu­ not interbreed. Polymorphic species, in which tion of stratigraphy in reconstructing phylogeny? many forms arc present within one biological Whilst a few 'transformed cladists' negate the value species, may likewise be hard to speciate cOlrectly. of the fossil record altogether (3 vievv vigorously In particular, where sexual dimorphism is strong Divisions of invertebrate palaeontology 13 the males and females of the one species may be so ancient organisms in their environmental context. dissirnilar in appearance that they have sornetimes All animals are adapted to their environment in all been described as different species, and the true situ­ ofits physical, chemical and biological aspects. Each ation may be hard to disentangle (as with species is precisely adapted to a particular ecological ammonites; Chapter 8). niche in which it feeds and breeds. It is the task of When it comes to the distinction of closely palaeoecology to fmd out about the nature of these related species, however, there are a number ofsta­ adaptations in fossil organisms and about the rela­ tistical tests that may help to give a higher degree of tionships of the animals with each other and their objectivity. One simple bivariate test in common environments; it involves the exploration of both use, for example, can be used when a series of present and past ecosystems (Schafer, 1972) growth stages are found together. If a collection of Although palaeoecology is obviously related to brachiopods is made frarn locality A, the length/ ecosystenr ecology, it is not and cannot be the same. width ratios or SOIne other appropriate parameters In modern community ecology much emphasis is may be plotted on a graph as a scatter diagranl. A placed on energy flow through the system, but this line of best fit (e.g. a reduced major axis) may then kind ofdetermination is just not possible when deal­ be drawn through the scatter. This gives a simple ing with dead communities. Instead palaeontologists y = ax + b graph, where a is the gradient and b the perforce must concentrate on establishing the com­ intercept on the y axis. A similar scatter from a pop­ position, structure and organization of palaeocom­ ulation collected from locality 13 may be plotted on munities, in attempting from here to work out the same graph, and the reduced major axis drawn linkage patterns in food webs and in investigating fi-om this too. The relative slopes and separations of the autecology ofindividual species. the two axes may then be compared statistically. If Many attempts have been made to summarize these lie within a certain threshold the populations categories of fossil residue, to provide the back­ can then be regarded as being of the same species; if ground for interpreting original community struc­ outside it then the species are different. ture. The scheme proposed by Pickerill and This is only one ofa whole series ofpossible tests, 13renchley (1975) and amended by Lockley (1983) is and more elaborate techniques of multivariate used here: analysis are becoming increasingly important in tax­ onomic evolutionary studies. 1. An assemblage refers to a single sample trom a With the advent of microcomputers and the pro­ particular horizon. vision of specialist software packages designed 2. An association refers to a group of assemblages, specifically to meet the needs of palaeontologists all showing similar recurrent patterns of species (PALSTAT; 13ruton and Harper, 1990; Ryan et composition. al., 1995), the usc of numerical analysis is becon'ling 3. A palaeocommunity (or fossil community) standard. Statistical methods are likewise essential refers to an assemblage, association or group of in defining palaeocommunities, in 'undef()rming' associations mferred to represent a once-distinc­ populations ofdet{)fmed fossils and comparing them tive biological entity. Normally this only repre­ to unaltered rnaterial. sents the preservable parts ofan original biological community, since the soft-bodied animals are not preserved. This definition corresponds more or Palaeobiology less exactly to that of Kauffman and Scott (1976).

Various categories may be included in palaeobiol­ Palaeoecology must always remain a partial and 0S'Y: palaeoecology (here discussed with palaeobio­ incomplete science, for so much of the information geography), functional morphology and ichnology, available for the study ofmodern ecosystems is sim­ each of which requires some discussion. ply not preserved in ancient ones. The animals themselves are all dead and their soft parts have Palaeoecology gone; the original physics and chemistry ofthe envi­ Since ecolos'Y is the study of animals in relation to ronment is not directly observable and can only their environment, palaeoecology is the study of be inferred from such secondary evidence as is 14 Principles of palaeontology

available; the shells may have been transported a\vay Modern marine environments are graded accord­ frOlll their original enviromnents by currents, and ingly to depth. The littoral environments of the the fossil assemblages that are found nlaY well be shore grade into the subtidal shelf, and at the edge mixed or incomplete; post-depositional diagenetic of the shelf the continental slope goes down to processes may have altered the evidence still fllrther. depth; this is the bathyal zone. Deloyv this lie the Despite this palaeoecology remains a valid, if partial flat abyssal plains and the hadal zones of the deep­ science. Much is now known about the post­ ocean trenches. There is often a pronounced zona­ mortem history of organic remains (taphonomy; tion oflife forms in depth zones more or less parallel Chapter 12), of which an additional dimension with the shore. In addition there is a general involves burial processes (biostratinomy). This decrease in abundance (number ofindividuals) but helps to disentangle the various factors responsible not necessarily diversity (number of species) on for deposition of a particular fossil assemblage, so descent into deeper water from the edge ofthe shelf that assemblages preserved in situ, which can yield The faunas of the abyssal and hacial regions were valuable palaeoecological information, can be dis­ originally derived ti'om those of shallow waters but tinguished from assemblages that have been trans­ arc highly adapted for catching the limited food ported. available at great depths. These regions ,1re, how­ Biostratinorny or preservation history has both ever, impoverished relative to the shallo\v-vvater pre-burial and post-burial clements. The tanner regIOns. include transportation, physical, chemical or biolog­ Animals and plants that live on the sea floor arc ical damage to the shell, and the attaclullent of epi­ benthic; those that drift passively or s\vim feebly in fauna. Post-burial processes may involve the water column are planktic (= planktonic) disturbance by burrowers and sediment eaters (bio­ since they arc the plankton. Nekton (nektic or turbation), current reworking, solution and other nektonic [lUna) on the other hand comprises active diagenetic preservation changes. swimmers. Neritic animals belong to the shallow vvaters ncar land and include demersal clements Modern environments and vertical distribution of \vhich live above the continental shelves and feed animals on benthic animals thereon. Pelagic or oceanic Figure 1.4 shows the main environments within the faunas inhabit the sur£1Cc waters or middle depths of Earth's oceans at the present day and the nomen­ the open oceans; bathypelagic and the usually clature for the distribution of marine animals within benthic, abyssal and hadal organisms inhabit the the oceans. great depths.

medn sea level NERITIC --:-7:":"""~-- __-~-----~~---

OCEANIC

if) 4000 :!: ----- high.tide a:; mean sea ievel E -- low tide 5000 ~ABC

PLAIN--.L...... --==-=-'~~'---'10 CONTINENTAL SHELF SLOPE ABYSSAL 000

Figure 7.4 Modern marine environments. A, Band C in the inset refer to supralittoral, littoral and sublittoral environments. (Based on a drawing in laporte, 1968.) Divisions of invertebrate palaeontology 15

Only the shelf and slope environments are nor­ Water turbulence may exercise a substantJal con­ mally preserved in the geological record, the trench trol over distribution, and the characters offaunas in sediments rardy so. The abyssal plains are underlain high- and low-energy environments are otten very by basaltic rock, formed at the mid-oceanic ridges disparate. Robust, thick-shelled and rounded species and slowly moving away from them to become arc normally adapted for high-ener!:,'Y conditions, fmally consumed at the subduction zones lying whereas thin-shelled and fragile f(wms point to a below the oceanic trenches; it moves as rigid plates. much quieter water environment, and it may be The ocean basins arc very young geologically, the possible to infer much about relative turbulence in a oldest sediments known therein being of Triassic fossil environment merely from the type of shells age. These are now approaching a subduction zone that occur. and arc soon to be consllIned without trace. Hence there are very few indications of abyssal sediment MODERN AND ANCIENT COMMUNITIES now uplifted and on the continents. In shallow cold-temperature seas marine mverte­ What is preserved in the geological record is brates are normally found in recurrent ecological therefore only a fraction, albeit the most populous comrnunities or associations, which are usually sub­ part, of the biotic realm of ancient times. The sedi­ strate related. In these a particular set of species ments of the continental shelf include those of the are usually found together since they have the same littoral, lagoonal, shallow subtidal, rnedian and outer habitat preferences. Within these communities the shelf realms. Generally sediments become finer animals either do not compete directly, being towards the edges of the shelf, the muddier regions adapted to microniches within the same habitat, or lying of1shore. There may be reefs close to the shore have a stable predator-prey relationship. or where there is a pronounced break in slope. Community structure is normally well defined in cold-temperature areas, but in warmer seas where Horizontal distribution of marine animals diversity is higher it is generally less clear. The main controls affecting the horizontal distribu­ Petersen (1918), working on the faunas of the tion ofrecent and f()ssil animals are temperature, the Kattegat, first studied and defined some ofthese nat­ nature of the substrate, salinity and water turbu­ urally occurring communities. I Ie also recognized lence.fhe large-scale distribution of animals in the two categories of bottom-dwelling animals: infau­ oceans is largely a function of temperature, whereas nal (buried and living within the sedim.ent) and the other factors generally operate on a more local epifaunal (living on the sea floor or on rocks or scale. Tropical shelf regions carry the most diverse seaweed). It was soon found that parallel communi­ faunas, and in these the species are very nUlnerous ties occur, with the same genera but not the same but the number of individuals of anyone species is species, on the opposite sides of the Atlantic. Since relatively low. In temperate through boreal regions this pioneer work a whole science of community the species diversity is less, though the number of ecology has grown up, having its counterpart in individuals per species can be very large. palaeoecology. Much effort has been expended in Salinity in the sea is of the order of 35 parts per trying to understand the composition of fossil thousand. Most marine animals are stenohaline, communities, the habits of the animals composing i.e. confined to waters of near-normal salinity. A them, community evolution through time and, as few are euryhaline, i.e. very tolerant of reduced £lr as possible, the controls acting upon them (e.g. salinity. The brackish water environment is physio­ Thorson, 1957, 1971). This is perhaps the most logically 'difficult', and £nmas living in brackish active field of palaeoecology at present, as a host of waters arc normally composed of very few species, recent original works testifIes (e.g. Craig, 1954; especially bivalves and gastropods belonging to spe­ Ziegler ct al., 1968; Doucot, 1975, 1981; Scott and cialized and often long-ranged genera. 'These same West, 1976; McKerrow, 1978; Skinner et al., 1981; genera can be found in sedimentary rocks as old as Dodd and Stanton, 1990; Bosence and Allison, the Jurassic, and their occurrence in particular sedi­ 1995; Brenchley and Harper, 1997). ments which lack normal marine fossils is a valuable As FUrsich (1977) makes it clear, however, most pointer to reduced salinity in the environment in fossil assemblages 'lack soft-bodied animals. Where which they lived. possible trace fossils can be used to compensate for 16 Principles of palaeontology

this but they arc no real substitute'. Hence assem­ environment. Living commtmities are therefore gen­ blages, associations and even palaeocommunitics erally well balanced, the number of species and indi­ must not be considered as directly equivalent to the viduals of particular species being controlled by the sea-floor communities of the present day. Using nature and availability offood resources. Fossil assem­ biostratinornic and scdimcntological data the blages may be tested according to this concept. Ifthey 'degree of distortion' trom the original community are 'unbalanced' then either (1) there may have been can in some cases be estimated. soft-bodied unprescrvable organisms which originally completed the balance or (2) the assemblage has been FEEDiNG RELATIONSHIPS AND COMMUNITIES mixed through transportation and thus does not All Illodern animals feed on plants, other animals, reflect the true original community. organic detritus or degradation products. The tiny plants of the plankton arc thc prhnary producers FAUNAL PROVINCES (autotrophs), as are seaweeds. Small planktic ani­ Marine zoogeography (Ekman, 1953; Briggs, 1974; mals arc the primary consumers (herbivores and Hallam, 1996) is primarily concerned with the detritus eaters); there are secondary (carnivorcs) global distribution of marine faunas and vvith the and tertiary consumers (top carnivores) in turn. definition of faunal provinces. These are large Each animal species is therefore part of a food web geographical regions of the sea (and most particu­ oftrophic (i.c. feeding) relationships wherein there larly the continental shelves) vllithin which the fau­ are a number oftrophic levels. In palaeoecology it nas at the specific, generic and sometimes familial is rarely possible to draw up a realistic f()od web level have a distinct identity. In faunal provinces (though this is one of the more important aspects of many of the animals are endemic, i.e. not ±cmnd modern ecology), but most fossil animals can usually outside a particular province. Such provinces are be assigned to their correct feeding type and so the often scparated from neighbouring provinces by trophic level may be estimated reasonably. ±~lir1y sharp boundaries, though in other cases the Of primary consumers the following types are boundaries may be more gradational. important: Figure 1.5 shows the main zoogeographical regions of the present continental shelves as defined fllterers or suspension feeders, vvhich are in"tmnal by Briggs (1974). or epi±:clUnal animals sucking in suspended organic Tropical shelf, warm temperate, cold temperate material fro111 the water; and polar regions can be distinguished, whose lirnits epifnmal 'collectors' or detritus fceders, which are controlled by latitude but also by the spread of sweep up organic m~lterial £i-0111 the sea floor; y·varrncr or colder water through major marine cur­ some infaunal bivalves and wornlS are also collec­ rents. T wpical shelf faunas occur in four separate tors: provinces. Of these the large 1nelo-West Pacific svvallowers or deposit feeders, vvhich are int;umal province, extending from southern Afi-ica to eastern animals unselectively scooping up mud rich in Australia, is the richest and most diverse and has organic material. been a m~or centre of dispersal throughout the Tertiary. Smaller and generally poorer provinces of Secondary and tertiary consumers, the carnivores, may tropical faunas arc found in the East Pacific, West prey on any of these, but it is the comrnunities ofthe Atlantic and (least diverse ofall) East Atlantic. These primary trophic level that are most commonly pre­ are separated both by land barriers (e.g. the Panama served because oftheir sheer number ofindividuals. Isthmus) and by regions of cooler water (e.g. \A/here In many living communities most of the 'bio­ the cold Hmnboldt Current sweeping up the \vest­ mass' is actually contributed by very few taxa, usu­ ern side ofSouth America restricts the tropical fauna ally not more than five (the trophic nucleus), but to vvithin a fe\\' degrees south ofthe equator). there may be representatives of a number of other The shelf faunas of cooler regions of the vvorld species in small numbers. In this system competition are likewise restricted by temperature, and again between the species concerned seems to be mini­ vvann or cold currents exercise a strong control of mized. It is thus mutually beneficial since the ditter­ their distribution. Some zoogeographic 'islands' can cnt taxa are exploiting different resources vvithin the be isolated by regions of warmer or cooler water. Divisions of invertebrate palaeontology 17

Key - (()/.?)) tropical shelf ~ warm temperate

P-~.=@ cold temperate

• polar

0;:-----; ~ warm currents

-+- cold currents - -

Figure 1.5 Distribution of modern marine shelf-living animals in faunal provinces, using Winkel's 'Tripe!' projection. (Based on a drawing in Briggs, 1974.)

For example, the tip ofFlorida ca rries a tropical shelf Modern and ancient reefs [luna, isolated to the northeast and northwest by Throughout geological time animals have not only cooler water areas, but though this is part of the become adapted to particular environments but also West Atlantic tropical shelt- province it has been iso­ themselves created new habitats and enVirOl1lTlents. lated for some time and therefore its [luna has Within these there has been scope for almost unlim­ diverged somewhat trorn that of the Caribbean ited ecological differentiation. shelf Likewise some oceanic islands (e.g. the Perhaps the most striking examples of such bio­ Galapagos) may be considered as part of a general genic environments are the reefs of the past and zoogeographic region or province, but as their present. Reefs (Fig. 1.6) are massive accumulations shelves have been initially colonized by chance oflimestone built up by lime-secreting algae and by migrants they may have very many endemic ele­ various kinds ofinvertebrates. ments \vhich have evolved in isolation. How many Through the activities ofthese frame-builders great of these endemics there arc may depend largely on mounds may be built up to sea level, with caves how long the islands have been isolated. and channels within them providing a residence for The development of today's faunal provinces has been charted throughout most of the Tertiary. At BASIN SLOPE RIM lAGOON OR SHFLF present Mesozoic and Palaeozoic provinces are like­ wise being documented (Middlerniss et al., /971; Hallam, 1973; Hughes, 1975; Gray and Boucot, 1979; McKerrow and Scotese, 19(0). When used in conjunction with palaeocontinental maps it may be possible to sec how the distribution pattern of ancient faunas related to the position ofancient con­ tinental masses and their shelves. Sometimes palaeo­ zoogeographical and geological evidence may have a bearing upon palaeotemperatures and even allow some inference to be made regarding ancient ocean current systerns. Figure 1.6 Generalized section through a reef (algal or coral). 18 Principles of palaeontology

innumerable kinds of animals, all ecologically differ­ ing in clumps and thickets on the reef surface, entiated for their particular niches. When these earliest of reef invertebrates became In the barrier and patch reef" of the tropical seas extinct in late Middle Cambrian time there \vere no of today, which grmv up to the surface, the warm, 1110re reef anin131s until some 6() Ma later; the only oA)Tgen-rich, turbulent waters allow rapid calcium reefs \VelT stromatolitic. In 1'vliddk Ordovician time metabolism and hence cominuollS gn)\vth. The these algae were joined by corals and stromato­ principal frame-builders are algae and corals, but poroids (lime-secreting sponges; Chapter 4) as well there are many other kinds of invertebrates in the as by red algae, vlhich together formed a reef envi­ reef community: sponges, bryozoans and molluscs ronment attracting a host of other invertebrates ;nnongst others. SOllIe ofthese add in minor yvays to ll1c1uding brachiopods and trilobites. ror unknown the reef framework; others break it down by boring reasons this type of reef complex did not continue and gr3zing. The growth of the reef to sea level beyond late I)evonian time. '1'he reefs that arose in continually keeps pace vvith subsidence, but it is also the Carboniferous were THainly algal, stromato­ being continually eroded. In a typical coral reef poroids and corals no longer playing such an impor­ complex the reef itself is a hard core of celnented tant part in their construction. algal and coral skeletons £Icing seawards, and as the The same kind of reef continued into the reefsubsides it grows outwards over a forerccf slope Permian, and many of these fringed the shrinking of tumbled boulders broken from the reef fi-ont. in1Jnd seas of thJt time. They rose at the edge of Behind it is a lagoon with a coral sand sediment and deeper basins in which the water periodically tidal flats along the shore colonized by cyanobacteria became more saline as it evaporated; there was like­ ('blue-greens'). Green algae are commonest in the wise evaporation in giant salt pans behind the reef as back-reef£.tcies; red algae are the main lirne sencters water drawn through the reef dried out in the of the reef itself lagoons behind. In the Permian reds of 'Texas and Large reefs such as the above are knovvn as bio­ northern England, which are very large, the reef herms; they form discrete mounds rising (i'om the front rose as a vertical \:vall oflaminated algal sheets, sea floor. Biostromes, on the other hand, are flat turning over at the reef crest where it reached sea laminar communities ofreef-type an imals and barely level (Fig. 1.7). rise above the sea floor. The upper sUli~lce \vas intensely colonized by Throughout geological history there have been stromatolites, which died out towards the lagoonal various kinds of reef cormnunities, which have back-reef f.1.cies. This kind of reef in general mor­ arisen, flourished and become extinct. In all of these phology therefore does not closely approximate the the frame-builders have included algae, but the standard pattern ofFig. 1.6. When the Permian shelf invertebrate frame-builders have been of diHcrent seas had dried out completely the Pennian reef­ kinds; the present corals are only the most recent in complex type vanished, and there were no morc a series ofreef-building animals (Newell, 1972). reefS until the slO\v beginnings ofthe coral-algal reef The oldest known reef" ,He over 2000 Ma old, system that arose in the late Triassic. Coral reef) made up entirely of sediment-trapping and possibly lime-secreting cyanobacteria: the prokaryotic stro­ matolites. Some of these reef" reached considerable reef-flat dimensions. One is reported from the Great Slave Lake region in Canada as being over 450 rn thick, and separating a shallow-water carbonate platform from a deep-water turbidite-filled basin. There are no preserved metazoans in these reefs, however, and their ecological structure Inay have been very sim­ ple. 100 m With the rise of frame-building metazoans in the ! Lower Cambrian a new kind of reef community scale Ivortlcal and homolltall made its appearance. Stromatolitic reefs were Figure 1.7 Crest of a Permian algal reef. (Redrawn from Smith, invaded by the sponge-like archaeocyathids, grow- 1981.) Divisions of invertebrate palaeontology 19 have expanded and flourished since then, other than Thompson (1917) it has been understood that the during a catastrophic period in the early Cretaceous confounation of the parts of any organism is the it'om which there are no reefs known (though corals result ofinteracting forces, dictated by physicomath­ must have been living somewhere at that time). ematical laws, which have operated through When the corals recovered they were joined in growth. The central issue here (Thonras and Reif, many places by the peculiar rudistid bivalves, also 1993) is the balance perceived between 'aCCIdents of reef fonners, which at one period almost supplanted history and the prescription of physical laws, as corals as the dOlninant reefframe-builders. Yet these causes oforganic design'. too died out in the late Cretaceous, leaving corals Different marine invertebrate skeletons may be the undisputed and dominant reef-building inverte­ functionally convergent in the way they grow, brates. and the sanre kinds of growth patterns turn up There has been some decline in the spread of frequently in representatives of many phyla. This coral reefs and in the number of coral genera since is because there arc relatively few ways in which the beginning of the Tertiary. They arc now con­ an animal can grow and yet can produce a hard fined to the Indo-Pacific region and on a smaller covering. Invertebrate skeletons, by contrast with scale to the West Atlantic. This decline may still be those of vertebrates, are generally external, and this continuing, though the reef<; were not significantly narrows down the spectrum of growth possibilities affected by the Pleistocene glaciation. The tllture of still further. Some of these types of skeletons are as ''''orld reef communities, the most complex of all follows. l11arine ecosystems. rernains to be seen. External shells growing at the edges only by Functional morphology, growth and form accretion of new material along a particular mar­ The functions of particular organs in fossils cannot ginal zone of growth. Very often such growth be established by many of the nrethods available to results in a logarithmic spiral shell as in brachio­ zoologists, but it is still possible to go some way pods, cephalopods and in coralla ofcertain simple towards explaining how particular organs worked corals. when the animal that bore them was alive. Such External skeletons ofplates - di~iunct, contiguous functional interpretation is, however, limited and in or overlapping - normally secreted along a single many ways it is hard to go beyond a certain point, zone which may be but is not always marginal. A even when the function of a particular organ is good example is the echinoderm skeleton in known (section 11.3). which the plates once formed are permanently Palaeo!ltologists arc otten presented either with locked into place, though they may thereafter organs whose function is not clear and which have grow individually by accretion ofmaterial in con­ no real counterparts in living animals, or with fC1ssiis centric zones. of bizarre appearance which arc so modified from External skeletons all formed at the one tinie. the normal type for the taxon that they testif~l to The arthropod exoskeleton is most typical. extreme adaptations. Sorne attempts can be made Growth here is ditflcult for the skeleton has to be towards interpreting these morphologies in terms of periodically moulted, a process known as ecdy­ adaptation and mode oflitC, which in tum may lead sis. When the old exoskeleton is cast the arthro­ to a clearer understanding ofevolutionary processes. pod takes up water or air and swells to the next If these problems are to be tackled, then a coherent larger size, and the new cuticle which underlies methodological scheme is needed. One particular the old one then hardens. Growth is thus rapid system of approach, the paradigm nlethod of and episodic and is only possible during moulting. Rudwick (1961), has been much discussed, but it The disadvantage of this system is the vulnerabil­ lies largely outside the scope of this work and ity to damage and predation during moulting. so only some examples of its application are mentioned. Essentially, any kind of skeleton, internal or T\vo related aspects of palaeozoology which can external is 'highly constrained by geometric rules, be deduced from fossilized renrains alone are growth growth processes, and the properties of materials. and t(1nn. Following the classic work of d'Arcy This suggests that, given enough time and an 20 Principles of palaeontology

extremely large number of evolutionary expen­ stratigraphy and chronostratigraphy, all ofwhich ments, the discovery by org;misIlls of "good" arc ways of ordering rock strata into meaningful designs - those that are viable and can be con­ units (Hedberg, 1976; Holland et al., 1978). structed with available materials - ",vas inevitable and in principle predictable ... the recurring designs we Lithostratigra phy observe arc attractors, orderly and stable configura­ Lithostratigraphy is concerned 'vvith the erection of tions of matter that must necessarily emerge in the units based upon the characters ofthe rocks and dif­ course ofevolution' (Thomas and Reif, 1993). ferentiated on types ofrock, e.g. siltstone, limestone In point of ElCt the potential available has been or clay. It is useful in local areas and essential in very well exploited by living creatures, though spe­ geological mapping, but there is ahvays the danger cific constraints seldom allow the production of that even in a small area rock units cut across time ideal organisms. Nature works as a tinkerer, rather planes. For instance, if a shoreline has been advanc­ than as an engineer Oacob, 1977; see also Chapter 2) ing in one direction a par6cular suite of sediments, - a point to be borne in mind in considering allliv­ probably of the same general kind, \'li]] be left in its ing and fossil organisms. wake. Though this bed will appear in the rock record as ;] single uniform layer, it will not all have been deposited at the one time; since it cuts across Stratigraphy time planes it is said to be diachronous. Such diachronism is common in the geological record. Sedimentary rocks have been built up by layer upon Furthermore, Blany suites of sediments arc laterally layer of sediment, which sornetimes has been much impersistent; different sedimentary Ell~ies rnay have the same for long periods of time but at other times existed at the same time within a small spacc- a has changed its character rapidly. The individual sandstone layer, for instance, passing into a shale layers \vithin sedimentary rocks arc separated by some distance away. Lithostratigraphy is thus only of bedding planes. These bedding planes are time hori­ real value within a relatively small region. zons, and the history of a rock sequence is reflected The divisions erected in lithostratigraphy arc in its layering. Each layer in the rock sequence must arranged in a hierarchial system: group, forma­ have been laid down on a pre-existing layer, so that tion, luember and bed. A bed is a distinct layer in the oldest rocks arc at the bottom of an exposed a rock sequence. A member is a group of beds sequence, the youngest at the top, unless the succes­ united by certain common characters. A formation sion has been tectonically inverted. This is the prin­ is a group of members, again united by characters ciple of superposition, recognized as long ago as with features in common. It is the primary unit of 1669 by the Danish scientist Nicolaus Stecnscn lithostratigraphy and is rnost usetll1 in geological (Steno). mapping. Hence it is formations that arc normally Stratigraphy is concerned with the study ofstrati­ represented by diHcrent colours on geological maps fIed rocks, their classification into ordered units and and cross-sections, and a formation is normally their historical interpretation. It bears not only upon defined tor its mapping applications. Finally, a group past geological events but also upon the history of ranks above a formation; it is composed of two or life and is perhaps the most basic part of all geology more tormations and is often used for simplifying (Harland and Anllstrong, 1990; Benton, 1995). stratigraphy on a small-scale map. Much of stratigraphy is concerned with chronol­ ogy: the geological record has to be divided up into Biostratigraphy time periods which are standardized, as far as possi­ J11 biostratigraphy the fossil contents of the beds arc ble, all over the world. One of the primary aims of used in interpreting the historical sequence. It is stratigraphy has been to produce an accurate based upon the principle of the irreversibility of chronology in which not only the order of events evolution. This means that at anyone moment in but also their dates arc known. Stratigraphical classi­ the Earth's history there was living a unique and fication is basic to all ofthis. special assemblage of animals, characteristic of that There are three principal categories of strati­ period and ofno other. As time went on these were graphical classification, lithostratigraphy, bio- replaced by others; each successive fossil assemblage Divisions of invertebrate palaeontology 21 is a pale reflection of the life at the time that the 4. Since fossil species could migrate following their enclosing sediments were deposited. Thus during own environment through time, there is always a the early Palaeozoic trilobites and brachiopods were possibility of diachronous faunas. The zone as the n10st con1mon fossils. By the Mesozoic tbe most defined in one area may not therefore be exactly abundant preservable invertebrates were the time-equivalent to that in another region. ammonites; they too became extinct, and snails and bivalves arc the commonest relics ofCenozoic time. In the example of a graptolite, therefore. for the This is how it appears on a broad scale. However, reasons outlined in (3) and (4), the total range or when the time ranges of individual f()ssil species arc biozone of a species is not likely to be preserved in examined it is evident that some of tbese lasted fc)[ any Olle area, and it is therefore hard to draw ideal only a traction of geological time, characterizing isochronous boundaries or time lines. very precisely a particular briefhistorical period. Ideally, zone f()ssils should have a particular com­ In any local area, once the sequence of fossil flU­ bination of characters to make them fully suitable nas has been precisely established through assiduous for biostratigraphy. These would be: collection and docmnentation from exposed sec­ tions, this known succession can be used for correla­ a wide horizontal distribution, preferably inter­ tion with other areas. Certain fossil species have continentaL been found to be particularly good stratigraphical a short vertical range so that they could be used to markers. They characterize short sections of the define a very precise part of the geological col­ geological succession known as zones. To take an llllln; example, ammonites are particularly good zone f()s­ enough morphological characters to enable them sils fc)r Mesozoic stratigraphy. The Jurassic period to be identified and distinguished easily; lasted some 55 Ma, and in the standard British suc­ strong, hard shells to enable them to be com­ cession there arc over 60 ammonite zones by which monly preserved; it is subdivided, so the zones are defined historical independence of facies, as would be expected periods which have :m average duration oflcss than from a free-swimming animal. a million years each. The practical problems in biostratigraphy are, All of these conditions are seldom fi..llfilled in fos­ however, very complex, and some parts of the geo­ sils used for zonation; perhaps the neritic ammonites logical succession are much more closely zoned than come closest to it, and it is not surprising that the others. The main problems are as follows. principles of really precise stratigraphical correlation were fi.rst worked out fully with these fossils, I. Many kinds of fossils, especially those of bottOITl­ notably by the German palaeontologist A. Oppel in dwelling invertebrates, are flcies controlled. They the 1850s. lived in particular environments only, e.g. liIne­ It was Oppel too who first recognized that there mud sea floor, reet~ sand or silty sea floor. They are various ways of using fossils in stratigraphy were often highly adapted fc)r particular condi­ which partially circumvent the difficulties nlen­ tions of temperature, salinity or substrate and arc tioned, and hence different types ofbiozones. There not found preserved outside this environm.ent. are four main kinds of biozones generally used This means that they can only be used for corre­ (Hedberg, 1976). Assemblage zones are beds or lating particular environments and thus arc not groups of beds with a natural assemblage of fossils. universaHy applicable. They may be based on all the fossils preserved 2. Some kinds offossils are very long-ranged. Their therein or on only certain kinds. They arc usually rates of evolutionary change were very slow. very much environmentally controlled and there­ They can only be used in a broad and general fore of usc only in local correlation. Range zones sense for long-period correlation and arc of very are perhaps of Inore general application. A range little use for establishing close subdivisions. zone usually represents the total range of a partiCll­ 3. Good zone fossils such as the graptolites are deli­ lady useful selected element in the fauna. One may cate and only preserved in quiet environments, therefore refer to the Psiloccras planorbis zone, based being destroyed in more turbulent conditiolls. upon the eponyInous ammonite that defines the 22 Principles of palaeontology

lowest zone of the European Jurassic, above which mere is as before: expansion and diversification fol­ is the Schlotheimia imgulata zone. Each range zone is lowed by extinction. Several such biomeres have always named after a particular species which occurs been detlned in the intensively studied Upper within it. Where there afe a number of zonally use­ Cambrian of North America. The pattern is invari­ ful species, or where the ranges ofindividual species ably similar and could probably be discerned in arc long, a more precise time definition may be other parts ofthe geological column as well. given by the usc of overlapping stratigraphical ranges. Such zones are therefore called concurrent Chronostratigraphy range zones. Acnle or peak zones are useful Chronostratigraphy is more tlr reaching than either locally. An acme zone is a body of strata in which bio- or lithostratigraphy but has its roots in both of the maximum abundance of a particular species is them. Its purpose is to organize the sequence of found, though not its total range. Such acme zones rocks on a global scale into chronostratigraphical may be narrow but are often useful as marker hori­ units, so that all local as well as \vorld\vide events zons in geological mapping. finally, an interval can be related to a single standard scale. l-lcnce it is zone is an interval between two distinct biostrati­ concerned with the age of strata and their time rela­ graphical horizons. It l11.ay not have any distinctive tions. To do this a hierarchical classification oftirne­ fossils, or indeed any fossils at all, being simply a equivalent units must be employed. The convenient way of referring to a group of strata conventional hierarchical system used is as shO\vn in bracketed bet\veen two named biostratigraphically Table 1.1. defined zones. Biostratigraphical units, unlike litho- and chronostratigraphical units, are not hierarchially Table 1.1 Conventional hierarchical correlation belween arranged, apart frOln in the case ofsubzones, which chronostratigraphical and geochronological units. arc local divisions where a zone can be divided Chronostratigraphical units Geochronological units more fInely in a particular region than elsewhere. A difterent kind ofstratigraphic concept, the bio­ Eonothem Aeon Erathem Era a ntere (Palmer, 1965, 19R4) was defined as a System o Period a regional biostratigraphic unit bounded by abrupt, Series a Epoch o non-evolutionary changes m the dominant clements Stage" Age of a single phylum. These changes are not necessar­ Chronozone Chron ily rclated to physical discontinuities in the sedimen­ aThese terms are in most common use. tary record and they may be diachronous. The biomere concept has proved rnost useful in studies of late Cambrian trilobite faunas, where a repeated Chronostratigraphical units relate quite simply to pattern of events is evident from the fossil record. In geochronological units; thus the rocks of the each biomerc the shelf sediments contain an initial Cambrian System were all deposited during the f~lUna oflow diversity and short stratigraphical range Cambrian Period. Most of these terms arc sclf­ (one or t\\TO species only). I Iowever, later faunas explanatory, but it should be recognized that they within the biomere become much more diversitled are all, at least in theory, worldwide in extent. and oflonger stratigraphic range and suggest, by this The Psiloceras planorbis chronozone is a time unit stage, 'sound adaptive plans' and the zenith of the equivalent to the time in which the said ammonite trilobite fauna. At the top of the biomere there is was in existence, even if it was confIned to certain often a rather specialized fauna of short-lived trilo­ parts of the world only. It is hard indeed, however, bites, then all the groups become extinct abruptly. to be able to delimit chronozones accurately, since The succeeding biomere begins in the same way most fossils were confined to certain gcographical as its predecessor, often with trilobites of similar regions or provinces, as arc most of the animals liv­ appearance to those at the base ofthe first one. Thc)' ing today. There arc relatively few well-established may have rnigratcd in ii-om a stock of more slowly cllronozoncs, or 'world instants' as they have been evolving trilobites in an outlying, possibly deeper­ called, and so 'chronm:one', though it has a real water area. The later development of the ne\v bio- mcamng, is not a term applicable to most practical Bibliography 23 stratigraphy. A stage, on the other hand, is a group of successive zones having great practical usc, espe­ cially since it is normally the basic working time unit of chronostratigraphy, the narrowest that can Books, treatises and symposia actually be used on a regional scale. Agel', D.V. (1963) Pri/lciples of Palaeoerol(~!;y. McGraw­ It is usually at the stage level that rocks of widely Hill, New York. (Useful basic text) different £:lcies can be correlated. As an example Barrington, E..J.W. (1967) lt11Jertebrate Strt/c'ture and there are some ditticulties in making precise zonal Fl1nction. Nelson, London. (Invaluable zoology text) correlations between Ordovician trilobite-brachio­ Benton, M.]. (1995) The Fossil Record 2. Chapman & pod tnmas and time-equivalent faunas with grapto­ Hall, London. (Invaluable summary of stratigraphical lites. Graptolites arc rarely preserved in the siltstones distribution ofall known fossil genera) and limestones favoured by the shelly fossils, and the Boardman, R.S., Cheetham, A.I-l. and Powell, A.]. (cds) (1987) Fos.iil Invertebrates. Blackwell, Oxford. latter, being benthic, could not inhabit the stagnant (Exceptionally Llsdid multi-author text) muds in which the graptolites were best preserved. Bosenee, D.W.]. and Allison, P.A. (1995) (cds) f\;!Clrine In some areas, of course, the faunas do alternate in pabeoC11vironmental analysis from .fi)ssils. Geological vertical sequence since the sites of deposition of Society ofLondon Spccial Publication No. 83. (12 usc­ these two t\Cies fluctuated with oscillating shore­ ful papers) lines, but though precise zone-to-zone correlations Boucot, A. (1975) Evolution dnd Extinction Rate Controls. are possible at some levels it is tl)Und in practice that Elsevier, Amsterdam. (Advanced text, mainly dealing Ordovician graptolite zones correlate best with with brachiopod distribution) stages dcflned on shelly fossils. I3oucot, A.]. (1981) Principles of Benthic Marine Fossils give a relative chronology which can be Palaeoecology. Academic Press, New York. (Valuable if idiosyncratic) used as the primary basis of chronostratigraphy. Brenchlcy, P. and I Iarper, D.A.T. (1997) Ecosystems, Nevertheless, it is otten hard to correlate precisely Ecology and El/o111tion. Chapman & I !all, London. beds of equivalent age 111 widely separated areas. (New, vigorous and readable text) The fossil sequences, though well documented with Briggs. D.E.G. and Crowther, D. (1990) Pdlacobiology: a anyone area, may contain very few e1enlents in Sywl1esis. Blackwell, Oxford, 583 pp. (The best and common, if indeed any at all, since they belong to most Llseful palaeontological compendium ofall) diflerent Ewnal provinces which are hard to corre­ Briggs, JC (1974) lV!aril1e Zoogeography. McGraw-Hill, late. Sometinles, however, the boundaries of such New York. (Delimitation offaunal provinces) provinces may have oscillated to and tJ-o. There may Bruton. D.L. and lIarpcr, I).A.T. (1990) Micro­ therefore appear elements of adjacent faunal prov­ computers m palaeontology. Contributions of the mces in vertical succession, thus [1cilitating strati­ fYalaeontolo.gical J\lhtSfltl11, Oslo 370,1-105. (8 papers) Cowen, R. 1994. History of Liti~, 2nd edn. Blackwell, graphical correlation. At most stratigraphical horizons Oxford. (Eminently readable) there are usually some ubiquitous worldwide fl)ssils, Dodd. JR. and Stanton, R.]. (1990) Palaeoewlogy, so that intercontinental correlation is not impossible. Coucepts {wd Applications, 2nd edn. Wiley, New York, In chronostratigraphy the relative sequence given 502 pp. (Interesting approach) by the f()ssils is supplemented and enhanced by Ekman, S. (1953) ZOo,!;eo.graphy of the Sed. Sidgwick and absolute dates which can be affixed at certain points Jackson, London. (An older but still usdi.d text on wherever appropriate rocks occur. These are usually marine life zones and taunal provinces) lavas bracketed bet\veen fossiliferous sediments, and Eldredge, N. and Cracraft, J (1980) Phylogenetic Patterns their occurrence is not too common. It is most and the Evolntionary Process. Columbia University Press, unlikely, therefore, that radiometric dating will New York. (Explains cladistic methods) supersede palaeontological correlation; the two arc Fairbridge, R.W. and Jablonski, D. (eds) (1979) The Encyclopedia of Earth Sciences, Vol. VII, The Encyclopedia entirely complementary, and the great success of of Paleontology. Dowden, Hutchinson and Ross, chronostratigraphy, in spite of its limitations, owes Stroudsberg, Penn. (Invaluable reference) much to both. The use ofautomatic data-processing Goldring, R. (1991) Fossils in the Field: Information and retrieval systems is growing and may (Hughes, Potential and andlysis. Longt11.an, Harlow. (How 1989) compensate for some of the constraints in palaeontological information is gathered in the field) present stratigraphical practice. Gould, S.J (1990) I1/onderfiti Life: the BI1~;;ess Slwle and the 24 Principles of palaeontology

nature C!f histOly. Hutchinson Radius, London. Moore, R.C. Teichert, C., Robison R.A. and Kaesler, (Comments on taxonomy) R.C. (successive editors from 1953). Treatise 011 Gray, J. and BOllcot, A. (eds) (1979) Historical Bio~eo­ 11I1/crtebrate Paleontology. Geological Society of graphy: Plate Tectonics af/(} the Chal1ginc~ Hill/iron/lien I. American and the University Kansas Press, Lawrence, Oregon State University Press, Corvallis, Ore. (Several Karl. (The standard reference work on invertebrate fos­ papers on faunal provinces) sils - each phylull1 treated in separate volumes). Hallam, A. (1973) Atlas of Palaeobi(~Reography. Elsevier, Murray, J-W. (ed.) (1986) Atlas ~f Invertebrate /\!lacr~lo5sils. Amsterdam. (48 original papers on distribution offossil Longman, Harlow, fi)r the Palaeontolot,>1cal Association. faunas) (Excellent photographic coverage ofmain groups) Hallam, A. (1990) ."1 n Outline or Phanerozoic Bio}!co,graphy. Paul, CR.C. (1980) The Natural History of Fossils. Oxford University Press, Oxford. W cidenfeld and Nicolson, London. (Simple but inter­ I brIand, w.n. (cd.) (1976) The Possil Record - a esting) Symposium with Documentation. Geological Society of Pivcteau, J. (cd.) (1952-1966) Trait?! de paleolltologie. L.ondon, London. (Contains time-range diagrams ofall Masson, Paris. (The French 'Treatise', slightly older tl.)ssil orders and charts showing diversity fluctuations than the American Treatise 011 Invertebrate Paleontology, through time) but ofvery high quality) Harland, W.B. and Armstrong, R.L. (1990) J'1 Geoh~gi((ll Raup, n.M. and Stanley, S.M. (1978) Principles of Time Scale. Cambridge University Press, Cambridge, Pale(mtoh~ifY, 2nd ecin. Freeman, San Francisco. 253 pp. (Essential and up~to-date) (Excellent textbook emphasizing approaches and con­ Hedberg, H.D. (1976) International Srratigraphir Guide. cepts, but not morphological or stratigraphical details) Wiley. New York. (Official guide to stratigrapillcal Ridley, M. (19K)) Evolution and Classification. The procedllre) Rlfomwtioll (~r Cladism. Longman, Harlow, 201 pp. Hedgpeth, J. W. and Ladd, ].S. (1957) Trearise on Marinc (Clear treatment of different approaches to classifica­ b'cology and P(/lacoe(OI<~'!,y, Vols 1 and 2, Memoirs ofthe tion) Geological Society of America No. 67. Geological Ryan, P., Harper, D.A.T. and Whalley, ].S. (1995) PAL­ Society of America, Lawrence, Kan. (Standard text, S'!:!!. T: [her '5 manual and case histories. S'latistice; j{)r with papers and annotated bibliography on ecology of palaeontoh~l!.ists . 71 pp. and disc. Chapman & Hall, aJlliving and fossil phyla) London and Palaeontological Association. Hennig,W. (1966) Phylo/fcnetic Systematics. University of Sch;ifi:.'r, W. (1972) Ecology and Palaeoecology oj' A1arine Illinois Press, Urbana, Ill. (Original chdisrn; second bwironmcnts (translated edn) (cd. G.Y. Craig). Oliver edition published in 1979) and Boyd, Edinburgh. (Standard work on Recent Hughes, N.F. (cd.) (1975) Otg(misms and Contillcnls North Sea environments with applications for palaeo­ Through Tilllc, Special Papers in Palaeontology No. 17. ec0106')') Academic Press, London. (23 original papers on distri­ Schopf, TJ.M. (ed.) (1972) Afodcls in Paleobiology. bution offJ.unds; many charted on global maps) Freeman, Cooper, San Francisco. (10 original papers, I Jughes. N.F. (1989) Possils as Injonnation: New Rcwrdinx many significant) and Srratal Correlation Tedi1liq~tes. Cambridge University Scott, R.H. and West, R.R. (cds) (1976) Structure and Press, Cambridge, 144pp. Classification of Palcocomnlunities. Dc}\vdcn, Hutchinson Joysey, K.A. and Friday, A.E. (1982) Prohlems biogeography. Geological work ofseveral papers on palaeontological taxonomy) Society Memoir No. 12, pp. 1-240. (Many valuable Teichert, C (1975) Tmltisc 011 Iuvcrtcbrate Paleontology, papers including Palaeozoic \vorld maps) Part VvT (Suppl. 1), Trace Fossils and Problematica. Middlemiss, FA., Rawson, P.F. and Newall, G. (cds) Geological Society ofAmerica, Lawrence, Kan. (1971) Faunal PYOI/;lIces in Space alld Time. Seel House Thompson, d' Arcy W. (1917) 011 Growth and Form. Press, LiverpooL (13 original papers on faunal distribu­ Cambridge University Press, Cambridge. (individ­ tion) ualistic classic work on physical laws determining Bibliography 25

growth; an abridged edition ofthis book was edited by Kautfmann, E.G. and Scott, R.W. (1976) Basic concepts J.T. Bonner in 1961 and also published by Cambridge of community ecology and palaeoecology, in Structure University Press) aud Classificatioll of Palaeocommunities (cds R. W. Scott Thorson, G. (1971) Lire in the Sea. World University and R.l~. West). Dowden, Hutchinson and Ross, Library, London. (Simple, well illustnted text; deals Stroudsberg, Penn., Pl'. 1-28. with commLltlity structure) Lockley, M. (1983) A review of brachiopod dominated Valentine, J.W. (1973) Ellolutionary Palaeoecolo,~y (~f the palaeocommunities trom the type Ordovician. j\i[arine Biosphere. Prentice-Hall, Englewood Cliffs, NJ Palaeontology 26, 111-45. (Terminology for palaeo­ (Valuable text on evolution and ecology) communitv structure) Willmer, P. (1990) Inllertebrate Relationships. Cambridge Moore. J, and Willmer, P. (1997). Convergent evolution University Press, Cambridge, 400 Pl'. (Modern, very in invertebrates. Riological Reviews 72, 1-60. readable and up-to-date treatment) Newell, N.D. (1972) The evolution of reefs. Scientific AI/lerican 226, 54~65. (Infc.JrInative short paper) Palmer, A. R. (1965) Biomere - a new kind of strati­ Individual papers and other references graphic unit. JOllrnal of Palaeontology 39,149-53. Palmer, A.R. (1984) The biomere problem: evolution of Craig, G.Y. (1954) The palaeoecology of the Top I-Josie an idea. Journal of Paleotltology 39, 149-53. Shale (Lower Carboniferous) at a locality near Kilsyth. Petersen, (~.CJ (1918) The sea bottom and its produc­ Quarterly Journal of the Geological Society or London 110, tion of fishfood. Reports of the Danish Biological 103-19. (Classic palaeoecological study) Station 25, 1~62. (The first definitive summary ofben­ Fortey, R.A. and JdIeries, R. (I f)81) Phylogeny and thic communities) systematics - a compromise approach, in Problems of Pickerill, R.. K. and Brenchley, P. (1975) The application Phylogenetic Reconstruction (eds K.A. Joysey and A.E. of the cOInIllunity concept in palaeontology. J\/Iaritime Friday). Academic Press, New York, Pl'. 112~47 Scdilllerits 11, 5-8. (Terminology and application) (Wh~re cladistics is and is not useful) Rudwick, MJS. (1961) The feeding mechanism of the Fiirsich, F.T. (1'177) Corallian (Upper Jurassic) marine Permian brachiopod Prorichthof/:nia. Palaeontolt~RY 3, benthic associations from England and N orrnandy. 450~ 71. (paradigm approach) Palaeoutolopy 20, 337--·85. (Palaeoecol0!o.'y, mainly of Smith, D.B. (1981) The Magnesian Limestone (Upper bivalve communities) Permian) reef complexes of north-east England, in Gingerich, P.D. 1'1'10. Stratophenetics, in Palaeobio"~RY; a Ellropean Relf A10dels (cd. D.F. Toomey), Society of Synthesis (eds D.E.G. Briggs and P.R. Crowther). Economic Paleontologists and Mineralogists Special Blackwell, Oxfcwd, 437-42. Publication No. 30, Tulsa, Oklahoma, SEPM, PI'. Harper, D.A.T. (1989) Brachiopods from the Upper 161-8. Ardmillian succession (Ordollicllw) of the Gin;au District, Thomas, R.D.K. and Reif; W.-E. (1'193) The skeleton Scotland, Part 2, 79··-128. Palaeontographical Society space: a flnite set of organic designs. Evolution 47, Monographs (taxonomic procedure referred to in text) 341-60. Harper, D.A.T. and Ryan, P. (1990) Towards a statistical Thorson, G. (1957) Bottom communities, in Treatise on system for palaeontologists. Journal of the Gelllogical A1arille bcology alld Palaeoecology, Vol. 1 (eel. JW. Society ofLondon 147, 935-48. (Essential reading) Hedgpeth), Memoirs of the Geological Society of l!emy, J-L. (1984) Analyse c1adistique et Trilobites; un America No. 67, Geological Society of America, point de vue. Lethaia 17,61-6. (Use and hrnitations of Lawrence, Kan., Pl'. 461~534. (Standard work, invalu­ c1adism) able, well illustrated) Holland, C.H., Audley-Charles, M.G., Bassett, M.G. et Ziegler, A.M., Cocks, L.R.. M. and Bambach, R.K. al. (1978) A guide to stratigraphic procedure. (1968) The composition and structure of Lower Geological Sociery of London Special Report No. 10, Silurian marine communities. Lethaia 1, '1-27. (The Pl'. 1-43. (Invaluable modern rreatment) first major work on Lower Palaeozoic communities; Jacob. F. (1977) Evolution and tinkering. Scietlce t 96, very 'vell illustrated) 1161-~7. Evolution and the fossil record

molecular biology arc transforrning and adding to this synthesis, and new views on the nature of the gene seem to be changing our whole conception of Amongst all the sciences concerned with organic how organisms evolve. evolution it is only palaeontology that has the In the f()llowing text, therefore, I present firstly a unique perspective of geological time. It is the rock simplified account of classic neo-Darwinian evolu­ record alone that provides an historical perspective tionary theory, aimed particularly at students of for the study of evolutionary events, and this time Earth science who may only have a limited back­ dimension could never have corne from any other ground in biology. This includes some information source. Accordingly, the input of palaeontology to on the impact of neyv information from molecular evolution theory has been in understanding the his­ genetics all evolution theory. This section is not tory of life, in interpreting patterns of evolution intended to be comprehensive, nor does it pretend (e.g. adaptive radiations) and lines of descent, and, to be a guide to all recent developments and dis­ importantly, in assessing rates of evolution. Now it coveries. Fuller treatments are readily available eJse­ has to be admitted that the fossil record is incom­ \vhe1'e viz., Simpson (1953), Maynard Smith (1<)75, plete, and to interpret it can in some instances be 1982), Mayr (1963, 1(76), Dawkins (1986), 'like trying to rcad a diary with half the pages Dobzhansky ct a!. (1977), Gamlin and Vines (1987), missing' (Sheldon, 19KK) . Yet it still remains an Bonner (1988), Endler and l\1cLellan (1988), immensely rich source of primary data, and can Hoffrnann (1989), Carnpbel1 and Schopf (1994), give, if resotved finely enough, a fair picture ofevo­ Maynard Smith and Szathmary (1995), Futuyma lutionary events that actually took place, however (1996), Strickberger (1996), Ridley (1996) and long ago. others listed in the bibliography, while Valentine The student of palaeontology needs to have some (1973), Hallam (1977), Stanley (1979), Cope and background in biological evolutionary theory, other­ Skelton (1985), Levinton (1988) and Skelton (1993) wise he will be in the position of 'the man standing arc more directly concerned with evolution and by the roadside and watching the cars go past but the fossil record. In the second part of this chapter without any idea ofhow their engines work', to use there is a more extended account of \vhat the fossil Brouwer's (1973) graphic simile. f;or the fi)ssil r<:'cord can tell us about the nature of evolutionary record cannot give much information on the mech­ change. anism of evolutionary change; this has to be pro­ vided by genetics, cytology, molecular biology and population dynamics, building upon the original conceptions of Charles Darwin. Many years ago the first real multidisciplinary amalgam ofdata was pub­ lished as EfJo!ution.' the i\1odern SYlzthesis (Huxley, 1942). From this highly successful atternpt at weld­ The theory ofevolution links together a multiplicity ing together Illt~}rmation trom various sources, the of biological phenomena and is underpinned by all nco-Darwinian synthesis takes its name - and this is the evidence of the geological record. It remains a a useful starting point. But recent developments in theory, not a proven [let, tor the immense timescale Darwin, the species and natural selection 27 over which evolutionary changes have taken place few species to small areas only where each had does not permit their direct observation. There is, become adapted to its own environment. however, no other theory which encompasses so Darwin was particularly interested in selective m.uch and accords with the evidence ofcomparative breeding of animals and plants under domestication. anatomy, biochemistry and physiology, of genetics He realized that the present great variety ofdogs and and cytolot,'Y, and of the relationships between cattle had been produced in only a few thousand organisms perceived by taxonomy. Whereas the years from only one or at most a few ancestral types. evidence in some of these fields is circumstantial, He concluded that a great potential for 'descent when brought together and interpreted it builds up with modification' must exist in all animals and to a theory of formidable consequence, and as plants which could be speeded up by such artificial Dobzhansky has said, 'Nothing in biology makes breeding. This led on to the belief that similar sense except in the light of evolution.' processes, though probably on a slower time scale, The recent facile attempts to discredit evolution had operated in the wild state; in other words 'nat­ by self-styled 'creation scientists' have been so elo­ ural selection'. Thus breeding experiments are the quently dispatched by Dott (1982), Kitchel' (1982) foundation of classical genetics, where the mecha­ and Stanley (1982) that no further comment is nisrn ofheredity is understood in tenns of its eHects. needed here. Darwin was also concerned with palaeontology Although Charles Darwin is generally regarded as but he f()Und that the fossil record was somewhat of the flther ofevolution theory (Darwin, 1859), there a disappointment in supporting the case for evolu­ were many pre-Darwinian scientists who postulated tionary change. He had hoped to find evidence that animals and plants had changed over long peri­ of gradational change between animal species, of ods oftime and that new types of,species' had arisen 'inilnitely numerous transitional links' connecting £i'om pre-existing ones. These early workers and ancestors to descendants and of stratigraphically Darwin himself identified many different points arranged series showing 'descent with modification'. which could be regarded as evidence for the origin In fact, he did not find what he had expected. of rnodern species from pre-existing and more Darwin assumed that the imperfections of the rock primitive ancestors. All these are accepted today, record and the limitations ofknowledge at that time though rdined and added to. Taxonomy, as always, were the bctors responsible. He was indeed partially lies at the heart of evolutionary thought, and closely correct, but to this point we shall return later. intertwined with it is comparative anatomy. The Of the pre-Darwinian evolutionists the most five-fingered, or pentadactyl, limb of higher verte­ prominent was the French naturalist Jean-Baptiste brates, for example, has been modified in a variety Lamarck (1774-1829), who proposed long before ofways. The grasping hand ofprimates, the flippers Darwin that all living organisms had originated from of marine mammals, the wings of birds and bats; primitive ancestors and that in the slow process of the hootS of horses - they all look dissim.ilar, but such changes had becolTle adapted for living in partic­ arc all variants on a common theme. Likewise the ular environments..rhe concept ofsuch adaptation diversity in structure and function of the beaks of originated with Lamarck. He appreciated that in the Galapagos finches (Ceospiza), which Darwin order to live, animals have to be eHiciently adapted to encountered during the voyage of the Bc(~gh> in all the physical and biological parameters ofthe envi­ 1833, Icd him to say 'Seeing this gradation and ronments they inhabit. In a sense, of course, an ani­ diversity in structure in one small, intimately related mal is 'all adaptation'; it has to be anatomically, group of birds, one might really tancy that from an physiologically and trophically adapted to its environ­ original paucity of birds in this archipelago, olle ment, and it is critical to evolution theory to under­ species had been taken and modified for difIt'rent stand how such adaptations came to be. ends'. Geographical distribution was seen as impor­ While appreciating the importance of adaptation, tant to evolutionary thought in other ways too. however, Lamarck linked his insight with some Thus the existence of 'relict' and isolated species concepts no longer believed to be tenable. He (e.g. lungfish) in diHt~rent parts of the world surely believed that adaptation had come about through indicated an original widespread ancestral type, a some kind of internal driving force, a 'vital spark' subsequent population collapse, and restriction of a which made animals becolTle more complex. He felt 28 Evolution and the fossil record

that ne-vv organs nlust arise from nc\'i7 needs and that the nature of variation and heredity \vas largely these 'acquired characters' were inherited, as in his unknown, so that his views on this were speculative classic postulate that the neck of the giraffe had and insuffIcient f()r the th(01)7 to be seen to vvork. become longer in response to a 'need' to reach The pioneering work of the Austrian monk leaves up on the tree. The theory of inheritance of Gregor Mendel in 1865, and of the later school of acquired characters is not highly regarded no\vadays T.H. Morgan which began in 1910, laid the foun­ and is generally lllltestable (although there is dation for genetic experiment and theory. It was this somc evidence of a kind of genetic feedback from that supplied the necessary understanding of hered­ the environment operating to produce apparent ity essential to the amplification ofDarwin's theory. 'Lunarckian changes'). Darwin, on the other hand, provided a logical and testable theory ±()[ evolution­ ary change: one that has stood the test of time and Inheritance and the source of variation provided a starting point for later developments. The flill title ofDanvin's major work of J859 -vvas In the cells ofall eukaryotes (all organisms except for On the origin (?( .Ipecfes by nmms C!f natural seleaion, or viruses, bacteria and cyanobacteria) there is a the preservation (!ffauollred races in the stnl~~le jc)r life. nucleus (Fig. 2.1) containing elongated thread-like The main points ofthe theory are straightforward. bodies, the chrOnl.OSOllleS, vvhich are made ofpro­ tein and deoxyribonucleic acid (DNA). 1. Animal species reproduce more rapidly than is needed to maintain their numbers. Animal popu­ smooth endoplasmic reticulum lations, however, though fluctuating, tenel to remain ~table. (Here he was influenced by the Englishman Malthus, who had \vritten on this subject some years earlier.) vacuole mitochondrion 2. There must therefore be competition within and between species in the 'struggle for existence', for f(Jod, for living space and (within members ofthe Goigi apparatus same specie~) for mates, ifthe characters that indi­ ~/ viduals bear are to be transmitted to the next gen­ eration. 3. Within species all animals vary, and this variation is inherited. 4. 1n the struggle tlX life those individuals best fitted to survive in a particular environment are the ones to live and to reproduce. The others are weeded out in the intense competition. The f~1Vourablc characteristics that make such survival possible arc inherited by fllture generations, and the accumulation of different favourable charac­ ters leads to the separation ofspecies well adapted to particular cnvironrnents. This is what Darwin called 'natural selection'.

All this seems logical enough, though Danvin's early critics, Mivart for instance, argued that Darwin had not really shmvn how favourable characters were actually accumulated, only how those animals less fitted to their environment failed to survive. In this they were not unsound, for the most serious weak­ ness ofthe theory, as presented by Danvin, \vas that Figure 2. I Eukaryote cell structure, with organelles. Darwin, the species and natural selection 29

The DNA molecule forms a long, twisted, spiral that genes themselves are made of two kinds of ladder of which the 'uprights' consist of alternate components arranged in series. These are exons, blocks of phosphoric acid and the pentose sugar which code for proteins, and introns, which do deoxyribose, whereas the 'rungs' are matching pairs not. When the nuclear DNA is transcribed to of relatively small units, the nucleotide bases. These nuclear IZNA it retains the organization of the orig­ bases, in DNA, arc adenine, guanine, thymine and inal DNA, but when nuclear RNA is reprocessed to cytosine. They are attached to the pentose sugar messenger RNA the introns are lost and the exons units and project inwards, linking up together in are spliced together. This does not always take place pairs. Adenine can only pair with thymine, and in the original order, however, and such exon shut:" cytosine with guanine. fling may be the basis of rapid evolution of proteins In the chromosome the DNA strands are themselves in new combinations. Provided that arranged in discrete units, the genes, which are these are fLmctional, new gene systems may arise strung together along the length of the chromo­ through comparativcIy few such shuffling events, some, and of which there may be several hundred and in short periods of time. This is a newly under­ per chromosome. The genome is a term used to stood phenomenon, but may have important conse­ describe the vvhole genetic complex. These genes quences tor evolution theory.) arc the primary units of heredity, carrying the When messenger RNA arrives at the surface of a genetic code, which is involved in producing pro­ ribosome it does not form protein directly, but teins and in directing the development and func­ through yet another imen11ediate molecule, transfer tioning of the whole organism. All the necessary RNA, and when this complex process is complete, information for these ends is carried in the DNA, the result is a protein sequence, coded for by the and what is important is the sequence of the four nuclear DNA, the transfer RNA meanwhile return­ nucleotide bases along its length. Though these ing to the cytoplasm. f(xm a kind of alphabet consisting only of four The understanding ofprotein synthesis has been a letters, the number of possible combinations in m,~or triumph for molecular biology, but the mole­ which they can exist, coding tor specific proteins, is cular pathways that lead from genes to actual organs enormous. This sequence of nucleotide bases and characters are very complex, and at present determines the sequence in which any of the 20 largely unknown. How the proteins and other com­ amino acids found in living organisms are strung pounds which are produced are organized and Imilt together to make proteins. It has been shown that up into functional organs and whole bodies remains combinations ofthree bases acting together code for one of the main tasks for molecular biology for the particular an11110 acids, and there are 1nore than fi-Iture. Some progress has already been made; it is enough possible combinations in this 'base-triplet' now known that S0111e genes arc 'structural', and system to make all the biogenic amino acids, and to concerned only with the synthesis of materials, link them up in the right order to make a specifre others are 'regulator' genes, which control and protein. organize the compounds and direct their building. Proteins are synthesized, not in the nucleus, but Such genes release chemical products which start a at the ribosomes in the cellular cytoplasm, and this whole host of complex reactions. In some kinds means that all the information has to be transferred of development the genes are switched on and off out of the nucleus to these sites ofprotein synthesis. in particular sequence, releasing products which For this to take place the nuclear DNA tlTSt pro­ react together in synthesizing complex molecules. duces a single-stranded copy ofitself, nuclear RNA, Structural genes can be activated and deactivated but with uracil replacing thymine and ribose sugar when needed; evidently the initial stimulus for the replacing deoxyribose. Following this process of switching on of stmctural genes is given when a 'transcription', a further molecule (messenger sensor gene receives an appropriate stimulus. RNA) is formed, which moves out of the nucleus, Simple organisms such as bacteria and tllllgi have presumably through pores in the nuclear membrane, sets ofadjacent genes known as operons, coding for and attaches itself to the ribosome. (This is not a a particular metabolic pathway. These can be simple process, however, for at this stage the genes switched otT and on together. In higher organisms, themscIves arc moditled. It has been shown recently however, genes which control different parts of a