DIVERSIFICATION OF MARINE LIFE THROUGHOUT PHANEROZOIC TIME: UNBRIDLED EXPONENTIAL INCREASE PUNCTUATED BY MASS EXTINCTIONS

Individual marine animal taxa, such as ammonoids, bivalves, brachiopods, gymnolaemate bryozoans, ostracods, and stenolaemate bryozoans, have been characterized by intrinsic rates of origination and extinction that persisted from Paleozoic into post-Paleozoic time. Because of fortuitous causal linkages, rates of origination and extinction have been highly correlated among taxa; groups with high turnover rates have been struck especially hard by mass extinctions because these crises have entailed the intensification of background rates of extinction (Stanley, 1979). The so-called Paleozoic and Modern faunas have experienced characteristic rates of diversification because of the robustness of the intrinsic rates of their component taxa. Members of the Paleozoic Fauna were characterized by high rates of turnover and therefore experienced especially heavy losses in the great terminal Permian mass extinction. This is why they constitute the Paleozoic Fauna; they are not unified by shared ecological traits.

When realistic rates are used to simulate the Phanerozoic diversification of the Paleozoic and Modern faunas as logistic increase, the resulting curves look nothing like actual diversification curves. Because competitive exclusion is uncommon in the ocean, exponential, rather than logistic (damped) increase is to be expected for marine animals. A simple simulation of exponential increase punctuated by mass extinctions mimics the actual Phanerozoic patterns of diversification for the Paleozoic and Modern faunas with remarkable fidelity. This simulation entails just a few rates of exponential increase: characteristic rates for the Paleozoic and Modern faunas. Because all rates of exponential increase and impacts of mass extinction are empirically derived, this simulation amounts to a description, rather than a model, of the pattern of Phanerozoic diversification. The implication is that we have no evidence that there is any limit for the proliferation of life in the ocean.