Decades of literature on large-scale taxonomic diversification and extinction patterns have hinged on compilations that record little more than first and last appearances of families or genera. Key examples include Sepkoski’s compendia of marine families (Sepkoski, 1984) and genera (Sepkoski, 1996) and the Fossil Record 2 database of marine and continental families (Benton, 1993). Numerous patterns of widespread scientific and public interest have been identified on the basis of the older compilations, such as the identity of the five largest mass extinctions (Raup and Sepkoski, 1982; Raup, 1986), a gradual decline of extinction rates throughout the entire Phanerozoic (Raup and Sepkoski, 1982), and possible cycles in extinction rates (Raup and Sepkoski, 1984). A complete reevaluation of these hypotheses is now made possible by the maturation of the Paleobiology Database, a relational, web-based, and much more detailed resource created by and for the paleontological community (Alroy et al., 2001).

Arguably, the most enduring and biologically important question these data can answer is whether global biodiversity is saturated (Sepkoski, 1978, 1979, 1984). If so, then ecological interactions, such as competition and predation, must control rates of speciation and extinction (MacArthur, 1969; Rosenzweig, 1975; Walker and Valentine, 1984). Speciation rates must be lower or extinction rates must be higher than they would be without these interactions. Diversity curves should increase logistically as they approach the saturation point (Sepkoski, 1978) instead of exponentially (Benton, 1995). Increases in the diversity of major taxonomic groups should be balanced by decreases in the diversity of other groups (Sepkoski, 1979). Most importantly, any recovery from a mass extinction, such as the current one, should eventually bring diversity back to the saturation point. Of course, the recovery will be rapid only in geological terms, the saturation point may change, and the extinction may fundamentally reorganize the global biota both taxonomically and ecologically, as seen in the wake of major perturbations, such as the end-Permian crisis (Erwin, 2001).

Past predictions about recovery have been hampered by limited direct evidence for saturation in the fossil record. Sepkoski (1978, 1979, 1984) did argue in detail that turnover rates have constrained the global diversity of all marine animals over the entire Phanerozoic. Some studies of particular taxonomic groups over specific parts of the Phanerozoic also suggested density-dependent dynamics (Mark and Flessa, 1977; Wagner, 1995; Alroy, 1996, 1998; Connolly and Miller, 2002). However, both earlier (Flessa and Levinton, 1975) and later (Benton, 1995) workers argued that Phanerozoic diversity was not constrained. Even though this view is very inconsistent with such well-documented patterns as rapid rebounds from mass extinctions (Kirchner and Weil, 2000b; Erwin, 2001; Foote, 2003), a basic logistic model assuming a single equilibrium point (Sepkoski, 1978) failed