TESTING FOR THE ALTERNATION OF MACROEVOLUTIONARY REGIMES IN PHANEROZOIC MARINE ANIMAL BODY SIZE

A major question in paleobiology centers on whether mass extinction occurs via the intensification of background processes versus via a shift into a separate macroevolutionary regime. In the former case, extinction selectivity patterns during background intervals should resemble those during mass extinctions; in the latter case, mass extinction events should differ from background extinction in degree or even direction of selectivity. Moreover, alternation of macroevolutionary regimes between background processes and mass extinctions is not limited to just extinction; origination dynamics are equally important to long-term evolutionary outcomes of pre- and post-mass extinction events. Thus, testing between these possibilities is a fundamental challenge with possible profound implications not only for understanding the origins of the modern biosphere but also for predicting the consequences of the current biodiversity crisis. The evolution of animal body size represents an ideal metric with which to test for the alternation of macroevolutionary regimes, as it scales with important aspects of organismal biology. Here, we test for the alternation of macroevolutionary regimes between background intervals and the “Big Five” mass extinction events using capture-mark-recapture approaches. We use body-size data for 10,127 genera of fossil marine solitary animal body sizes spanning 10 Linnaean classes with occurrences ranging from the Early Ordovician through the Pleistocene. Differences between background and mass extinction are more pronounced for origination than for extinction. Nine out of ten classes differ significantly in the size bias of origination between background intervals and recovery from mass extinction. By contrast, only six of the ten classes studied differ significantly in body-size selectivity of extinction between background and mass extinction. Thus, the differences in macroevolutionary regime between background and mass extinction may be more pronounced during recovery intervals than during mass extinction events themselves.