Biotic Diversity and Structure During the
Andrey Yu. Zhuravlev
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 and suspension feeders considerably refined the ocean waters and led to less nutrientrich 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 Tommotian 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 important Cambrian classes, including archaeocyaths, trilobites, inarticulate brachiopods (mainly lingulates in the present sense), hyoliths, monoplacophorans (now, princi-