PHANEROZOIC DIVERSITY IN THE MARINE REALM: AN ANALYSIS OF ORDERS AND EVOLUTIONARY FAUNAS

The history of marine animal diversity involves numerous episodes of radiation and extinction, but observed rates and patterns of evolutionary change depend on taxonomic level and may be distorted to varying degrees by the incompleteness of the fossil record. A branching model of cladogenesis incorporating empirical rates of fossil preservation was consequently used to estimate the true durations of marine animal orders. Marine ordinal diversity through geologic time is consistent with the logistic model of population growth, exhibiting a rapid exponential increase in the number of orders during early Paleozoic time followed by approximately constant diversity over the next five hundred million years. Goodness-of-fit between observed and corrected ordinal diversity is remarkably high at the 50% confidence level (r² = 0.966), with only minor deterioration at the 95% confidence level (r² = 0.900). Each of these orders can be assigned into the great evolutionary faunas, aggregations of classes that reached maximum diversity at similar times and have successively dominated the marine fossil record. Analysis of the Cambrian Fauna at the 50% confidence level (r² = 0.980) indicates higher early ordinal diversity in the corrected curve, probably reflecting both genuine range extensions and poor stratigraphic constraints. The Paleozoic Fauna (r² = 0.966) radiated rapidly, stabilizing in mid-Paleozoic time with over forty orders followed by a steady four hundred million year post-Devonian decline. Unlike the patterns in families and genera, no massive drop in ordinal diversity is seen during the terminal Permian mass extinction. Slow expansion characterizes the Modern Fauna (r² = 0.993), which achieved maximum diversity in the late Mesozoic Era with over seventy orders and declined slightly thereafter in Cenozoic time. A smaller sample consisting of bivalve and gastropod mollusks also shows a strong correlation (r² = 0.984) between observed and corrected curves, indicating that the fidelity of the fossil record extends to the diversity histories of well-skeletonized invertebrate classes. These results therefore suggest that the fossil record largely preserves the shape of marine diversity at the ordinal level.