WAS THE END-PERMIAN MASS EXTINCTION UNUSUALLY SELECTIVE IN MARINE ANIMALS? A TEST OF THE KNOLL ET AL. MODEL

Extinction selectivity – variation in extinction intensity among clades, functional groups, etc. – can be used to test the plausibility of extinction kill mechanisms, since different environmental perturbations should impose unusual levels of extinction risk on different types of species. In a series of influential papers, Knoll and colleagues observed that heavily calcified marine animals were preferentially victimized in the end-Permian mass extinction and proposed that this pattern resulted from poor physiological buffering against increased pCO₂ and ocean acidification, which has potential implications for extinction risk in the modern oceans. However, in some subsequent analyses, it has not been entirely clear that the pattern of extinction risk in the end-Permian is actually distinct from background patterns of risk. If the types of species that fared poorly during the mass extinction also experienced higher extinction intensity during normal times, then the proposed link between selectivity and CO₂ release would be less well supported.

Here, we compare patterns of extinction selectivity at the class level during the end-Permian extinction to background patterns during other Paleozoic time intervals using an extension of the analytical framework developed by Bush et al. (2020, Paleobiology). The results suggest that the end-Permian extinction did have unique patterns of selectivity that are consistent with a modified version of the Knoll et al. model. Specifically, animals without specialized respiratory and circulatory systems (e.g., Anthozoa) fared worse than predicted by background patterns, while animals with closed circulatory systems (vertebrates, cephalopods) fared better than expected. However, patterns of extinction selectivity among animals with open circulatory systems were similar to background patterns, regardless of whether they were classified as “buffered” or “unbuffered”. For example, brachiopods had higher extinction intensity than bivalves or gastropods, but no more so than would be expected based on background intervals. This analysis supports the contention from Knoll et al. that basic features of animal body plans have played key roles in their responses to earth system evolution.