PERIODICITY AND THE CAUSES OF MAJOR EXTINCTIONS

Striking similarities in the rise and fall of marine invertebrates, as well as reef faunas restricted to shallow tropical seas, suggest that geographically widespread environmental stress is a prime agent of extinction in the fossil record. To investigate the relationship between ecology and periodic mass extinctions, percent extinction was calculated for primarily post-Paleozoic marine genera spanning the last 250 MY (mid-Permian to Neogene time), heavily culled to eliminate the Pull of the Recent and the effects of Lagerstätten and taxonomic oversplitting. Remaining genera were then sorted by locomotion, life habit, and feeding mode. Spectral analyses, which circumvent the limitations of traditional Fourier-based methods, were performed on these categories and corrected for biases using Monte Carlo simulations. Actively mobile, pelagic, infaunal, and carnivorous genera display prominent peaks generally consistent with a 26 MY periodicity. These include so-called “elastic taxa”, such as mammals and ammonites, which undergo rapid post-crisis radiations and are thus expected to be particularly susceptible to extinction. Sessile, epifaunal, and suspension feeding genera show no such features, but the small sample sizes of herbivorous grazers (mainly echinoids and gastropods) and deposit feeders preclude unambiguous interpretation. While the preceding results fail to establish significance at the 95% level according to an unrealistic null hypothesis of independent random data, a series of runs tests demonstrate that an autocorrelated model is a more appropriate fit for each ecological grouping. Alternative manipulations of the geologic column may indeed confirm the existence of periodicity across the entire gamut of adaptive zones, thereby supporting a common physical cause of major extinctions in the history of life. If the patterns of extinction selectivity are real, however, periodicity could instead be explained as a consequence of internal evolutionary dynamics, such as delayed recovery following biotic crises.