Figure 20.5   Sepkoski’s three-phase coupled logistic model for diversifi cation of animal life in the sea. (a) The family-level diversifi cation curve for marine animals, showing the three evolutionary “faunas” from Fig. 20.4, each shaded differently. Numbers I to V are the fi ve big mass extinctions, in sequence from left to right, Late Ordovician, Late Devonian, end- Permian, Late Triassic and Quaternary-Cretaceous. (b) The handover from the Cambrian to the Paleozoic “fauna” involved a shift in equilibrium diversity (Dˆ ); equilibrium diversity is achieved when origination (ks) and extinction (ke) rates match. (c, d) The coupled logistic model gives a simple representation of the broad outlines of the progress of the three evolutionary “faunas” 1, 2 and 3 (c), and perturbations, shown by vertical arrows, may be added to correspond to the mass extinctions (d). (Based on information in Sepkoski 1979, 1984.)

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