The Ecology of the Cambrian Radiation - Andrey Zhuravlev - Chapter 8

08-C1099 8/10/00 2:08 PM Page 171 PART II Community Patterns and Dynamics 08-C1099 8/10/00 2:08 PM Page 172 08-C1099 8/10/00 2:08 PM Page 173 CHAPTER EIGHT Andrey Yu. Zhuravlev Biotic Diversity and Structure During the Neoproterozoic-Ordovician Transition Diversity of 4,122 metazoan genera, 31 calcimicrobial genera, and 470 acritarch species are plotted for the Nemakit-Daldynian–early Tremadoc interval at zonal level. Generally congruent plots of diversity of metazoan genera, acritarch species, calcified cyanobacteria, and ichnofossils reflect Nemakit-Daldynian–early Botoman diversification, middle Botoman crisis leading to further late Botoman–Toyonian diversity decrease, and Middle-Late Cambrian low-diversity stabilization. All three sources of overall diversity (alpha, beta, and gamma diversity) contributed to the development of generic diversity at the beginning of the Cambrian. The apparent niche partitioning and several levels of tiering, observed in reefal and level-bottom communities, indicate that the biotic structure of these was already complex in the late Tommotian. A wide spectrum of communities was established in the Atdabanian. Ecologic, lithologic, and isotopic features are indicative of a nutrient-rich state of the oceans at the beginning of the Cambrian. The radiation of benthic and planktic filter andsuspensionfeedersconsiderablyrefinedtheoceanwatersandledtolessnutrient-rich conditions for later, more diverse, evolutionary faunas. The inherent structure of the biota, expressed in relative number of specialists and degree of competition, was responsible for its stability. Extrinsic factors could amplify crises but could hardly initiate them. AT THE END of the Neoproterozoic and beginning of the Phanerozoic, there was a rapid succession of distinct faunas and a diversity increase that involved the brief flourishing of the enigmatic Ediacaran fauna, subsequent expansion of the Tommo-tian small shelly taxa, and finally replacement by the more standard Cambrian and Ordovician groups. Discussions of Vendian to Cambrian diversification by Sepkoski (1979, 1981) treated the fauna of this interval as homogeneous. Most of the impor-tant Cambrian classes, including archaeocyaths, trilobites, inarticulate brachiopods (mainly lingulates in the present sense), hyoliths, monoplacophorans (now, princi- 08-C1099 8/10/00 2:08 PM Page 174 174 Andrey Yu. Zhuravlev pally,helcionelloids),stenothecoids,cribricyaths,volborthellids,eocrinoidsandsome other echinoderm classes, sabelliditids, soft-bodied and lightly skeletonized animals, and various Problematica, were assembled into the “Cambrian Evolutionary Fauna.” This fauna dominated the early phase of metazoan diversification. It attained maxi-mum diversity in the Cambrian and then began a long decline. Very few members of the Cambrian fauna participated in the Ordovician radiation or persist today. The Pa-leozoic Evolutionary Fauna began to radiate during the latest Cambrian and virtually exploded in the Ordovician. The Modern Evolutionary Fauna originated during the Cambrian Period but radiated in the Mesozoic. The three great evolutionary faunas were identified through Q-mode factor analy-sis of familial diversity through the Phanerozoic (Sepkoski 1981). The factors of fa-milialdatadifferedsignificantlyfromexpectationforstochasticphylogeniesandthere-fore reflected some underlying organization in the evolution of Phanerozoic marine diversity (Sepkoski 1991a). Smith (1988), noted that several important classes in the Cambrian Fauna—namely, Inarticulata, Monoplacophora, and Eocrinoidea—are paraphyletic, and he therefore suggested that the distinction between the Cambrian and Paleozoic faunas, and the apparently separate radiations of the Early Cambrian and the Ordovician, might be an artifact of taxonomy coupled with a poor fossil rec-ord in the Late Cambrian. He ably demonstrated that eocrinoids represent a poorly defined stem group for later pelmatozoans and cystoids (but see Guensburg and Sprinkle, this volume). In contrast, monoplacophorans and inarticulates are split into several holophyletic clades (class Helcionelloida, class Lingulata) (Gorjansky and Popov 1986; Peel 1991), the bulk of which further increase the distinction mentioned above. Thus, although taxonomic practice may contribute scatter to the pattern, the histories of Cambrian classes continue to remain distinct from members of the Paleo-zoic and Modern faunas. In addition, the Monte Carlo simulations did not reveal a significant bias produced by paraphyletic taxa (Sepkoski and Kendrick 1993). A dis-tinct pattern is observed in the stratigraphic distribution of fossils treated as earliest pelecypods,rostroconchs,andgastropods:theirfirstrepresentativesdisappeareddur-ing the middle Botoman extinction event, but the classes apparently diversified at the very end of the Cambrian and Ordovician. Such a pattern emphasizes the distinction between elements that contributed to the Cambrian and Ordovician radiations. Further investigations by Q-mode factor analysis, performed on generic diversity data, recognized at least three evolutionary faunas at the start of metazoan diversifica-tion—the Ediacaran, Tommotian, and Cambrian sensu stricto faunas—and archaeo-cyathsreceivedtheirownfactor(Sepkoski1992).TheTommotianEvolutionaryFauna factor received maximum loadings from the Nemakit-Daldynian, Tommotian, and early Atdabanian, and the fauna included orthothecimorph hyoliths, helcionelloids, paragastropods, sabelliditids, and a variety of short-ranging Problematica that origi-nated during this time interval. Finally, the restricted Cambrian Evolutionary Fauna factor received maximum loadings from the late Atdabanian through Sunwaptan; it consisted of trilobites, bradoriids, and some other arthropods, lingulates, and echino- 08-C1099 8/10/00 2:08 PM Page 175 BIOTIC DIVERSITY AND NEOPROTEROZOIC-ORDOVICIAN TRANSITION 175 derm classes. This latter assemblage actually represents a mixture of members of the Cambrian sensu stricto, Paleozoic, and Modern faunas. Metazoans of all taxonomic levels from genus to class exhibit, in general, congru-ent diversity patterns through the Cambrian-Ordovician (Sepkoski 1992). The major Cambrian radiation of large metazoans with mineralized skeletons was accompanied by a continued radiation of soft-bodied burrowing infauna in both nearshore silici-clastic and carbonate shelf settings expressed in increased diversity of trace fossils and intensity of bioturbation from the Vendian through Early Cambrian; thereafter there was little change in the Early Paleozoic (Crimes 1992a,b, 1994; Droser and Bottjer 1988a,b). The same pattern is repeated broadly by calcified cyanobacteria (Sepkoski 1992; Zhuravlev 1996) and acritarchs (Rozanov 1992; Knoll 1994; Vidal and Moczydiow-ska-Vidal 1997). Preliminary data on calcified cyanobacteria and algae allowed Chu-vashov and Riding (1984) to establish three major marine Paleozoic floras—the Cambrian, Ordovician, and Carboniferous floras. Quantitative and taxonomic analy-ses of these entities are needed. However, the diversity pattern of their Cambrian Flora is congruent with that of the Early Cambrian Biota, as has been shown by quan-titative data (Zhuravlev 1996). This flora was dominated by calcified probable bac-teria(e.g.,Girvanella,Obruchevella,Epiphyton,Renalcis,Acanthina,Bija,Proaulopora),to which a few problematic calcified algae were added during the Middle to Late Cam-brian (see Riding, this volume). Some elements of this flora have a discontinuous record to the Cretaceous. In contrast, the Ordovician Flora, which diversified in the Middle Ordovician, contained a large variety of calcified green and red algae and new groups of calcified cyanobacteria. Thus, all patterns are remarkably similar as indicated in figure 8.1A–D. DIVERSITY ANALYSIS New and revised biostratigraphic data for the Cambrian permits quantitative analy-sis of changes in biotic diversity, which I accept here as simple taxonomic diversity. Global generic diversity data are calculated on the basis of my literature compilation ofstratigraphicrangesandpaleogeographicdistributionsofgenerafromtheNemakit-Daldynian to Tremadoc for all groups (4,122 genera), with the exception of spicular sponges (figure 8.1A). These data are calibrated by Russian (Siberian) stage and zonal scales for the Early and early Middle Cambrian, North American (Laurentian) stage and zonal scales for the late Middle and Late Cambrian, and Australian Datsonian as the terminal Cambrian interval (from the base of the proavus Zone to the base of the lindstromi Zone). The global correlation of these stratigraphic units is given by Zhu-ravlev (1995) for the Early Cambrian and by Shergold (1995) for the Middle and Late Cambrian (Zhuravlev and Riding, this volume: tables 1.1 and 1.2). Asisalreadywellknown,theNeoproterozoic–EarlyCambrianmetazoanexplosion was relatively rapid, spanning a period of about 20 m.y. from the Nemakit-Daldynian ... - tailieumienphi.vn