Symmetric waxing and waning of marine invertebrate genera

Occurrence data from the Paleobiology Database are used to analyze the waxing and waning of genera over time. Irrespective of whether we tabulate species richness, frequency of occurrence, geographic range, or other measures, the average rise and fall of genera is remarkably symmetrical. Genera tend already to be in a state of decline when they become extinct. Genera that last appear in the major mass extinction stages, however, are more frequently truncated while they are holding steady or even increasing. This need not imply that mass extinctions are qualitatively different from other events; it is consistent with the expected effects of simply increasing the magnitude of extinction. For reasons that are not completely clear, post-Paleozoic genera show less of a rise and fall on average and tend to be less symmetrical than do Paleozoic genera.