INCREASED LONGEVITIES OF GENERA THAT ORIGINATED IN SUBSTAGES FOLLOWING MASS EXTINCTIONS

A growing body of evidence suggests that taxa originating in recovery intervals following mass extinctions were generally more widespread, ecologically and geographically, than those originating at other times. If, as some researchers have suggested, there is a direct relationship between the ecological/geographic breadth of a taxon and its evolutionary longevity, then one might expect taxa originating in post-mass extinction intervals to be unusually long-lived. Here, I present an initial investigation of this possibility by comparing the mean longevities of genera originating in successive substage-length bins throughout the Phanerozoic. The database for this analysis was Sepkoski's unpublished compendium of fossil marine genera, culled to exclude genera confined to a single substage. In all, some 16,689 genera were included in the analysis. A depiction of mean longevities for cohorts of genera originating in successive Phanerozoic substages reveals a series of distinct peaks corresponding to substages immediately following several mass extinction intervals, beginning with the Lower Induan, in the aftermath of the end-Permian event. The post-K/T peak, in the Lower Paleocene, is muted relative to earlier peaks because of "edge effects" associated with its proximity to the Recent. Thus, the pattern supports the hypothesis that recovery genera were, indeed, longer-lived on average than genera originating during other substages. Moreover, the results have proven impervious to several potential artifactual effects, tested by culling out genera that range to the Recent, genera that became extinct during mass extinctions, and genera that became extinct in substages following mass extinctions. Curiously, the pattern appears to be limited to the post-Paleozoic. Whether this indicates a difference in Paleozoic versus post-Paleozoic evolutionary dynamics or is, instead, an artifact of the analysis remains to be determined.