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According to when they attained high diversity, major taxa of marine animals have been clustered into three groups, the Cambrian, Paleozoic, and Modern Faunas. Because the Cambrian Fauna was a relatively minor component of the total fauna after mid-Ordovician time, the Phanerozoic history of marine animal diversity is largely a matter of the fates of the Paleozoic and Modern Faunas. The fact that most late Cenozoic genera belong to taxa that have been radiating for tens of millions of years indicates that the post-Paleozoic increase in diversity indicated by fossil data is real, rather than an artifact of improvement of the fossil record toward the present.
Assuming that ecological crowding produced the so-called Paleozoic plateau for family diversity, various workers have used the logistic equation of ecology to model marine animal diversification as damped exponential increase. Several lines of evidence indicate that this procedure is inappropriate. A plot of the diversity of marine animal genera through time provides better resolution than the plot for families and has a more jagged appearance. Generic diversity generally increased rapidly during the Paleozoic, except when set back by pulses of mass extinction. In fact, an analysis of the history of the Paleozoic Fauna during the Paleozoic Era reveals no general correlation between rate of increase for this fauna and total marine animal diversity. Furthermore, realistically scaled logistic simulations do not mimic the empirical pattern. In addition, it is difficult to imagine how some fixed limit for diversity could have persisted throughout the Paleozoic Era, when the ecological structure of the marine ecosystem was constantly changing. More fundamentally, the basic idea that competition can set a limit for marine animal diversity is incompatible with basic tenets of marine ecology: predation, disturbance, and vagaries of recruitment determine local population sizes for most marine species. Sparseness of predators probably played a larger role than weak competition in elevating rates of diversification during the initial (Ordovician) radiation of marine animals and during recoveries from mass extinctions. A plot of diversification against total diversity for these intervals yields a band of points above the one representing background intervals, and yet this band also displays no significant trend (if the two earliest intervals of the initial Ordovician are excluded as times of exceptional evolutionary innovation). Thus, a distinctive structure characterized the marine ecosystem during intervals of evolutionary radiation—one in which rates of diversification were exceptionally high and yet increases in diversity did not depress rates of diversification.
Particular marine taxa exhibit background rates of origination and extinction that rank similarly when compared with those of other taxa. Rates are correlated in this way because certain heritable traits influence probability of speciation and probability of extinction in similar ways. Background rates of origination and extinction were depressed during the late Paleozoic ice age for all major marine invertebrate taxa, but remained correlated. Also, taxa with relatively high background rates of extinction experienced exceptionally heavy losses during biotic crises because background rates of extinction were intensified in a multiplicative manner; decimation of a large group of taxa of this kind in the two Permian mass extinctions established their collective identity as the Paleozoic Fauna.
Characteristic rates of origination and extinction for major taxa persisted from Paleozoic into post-Paleozoic time. Because of the causal linkage between rates of origination and extinction, pulses of extinction tended to drag down overall rates of origination as well as overall rates of extinction by preferentially eliminating higher taxa having relatively h