SELECTIVITY OF THE END-PERMIAN MASS EXTINCTION

The Permo-Triassic mass extinction eliminated nearly 80% of marine invertebrate genera but did not impact all groups equally, in large part because taxa with differing physiological tolerances, geographic and bathymetric distribution, life habits, and/or body sizes responded to the extinction forcing mechanisms in different ways. Although metabolically active and lightly calcified members of the Modern fauna were more likely to survive, the end-Permian extinction was not selective for body size among brachiopods, bivalves, or gastropods. Small gastropods and brachiopods were more likely to become extinct, but the pattern was reversed among bivalves and none of the results is statistically significant, suggesting that within each clade neither large heavily-calcified nor small lightly-calcified genera were more susceptible. Several lines of evidence point to more rapid post-extinction recovery in the high-latitude Boreal realm; however, taxonomic loss during the extinction was no lower in that region. There was no spatial variability in extinction severity among bivalves or gastropods, whereas the few surviving genera of rhynchonelliform brachiopods found in earliest Triassic “mixed faunas” were predominantly taxa that were abundant in South China prior to the extinction. The replacement of temperate Gondwanan taxa by tropical Chinese brachiopods in southern mid-latitudes during the earliest Griesbachian implies that short-term survivorship may have been related to abrupt warming during the extinction event. Those initial survivors do not belong to later Triassic clades, however, suggesting that long-term survivorship was mediated by a different suite of selective pressures. Successful survivors tended to live in inner shelf environments during the late Paleozoic, whereas extinct groups were more diverse in offshore settings, consistent with expected bathymetric distribution of deleterious effects from upwelled euxinic water.