TEMPORAL AND ENVIRONMENTAL DISTRIBUTION OF MIXED FAUNA SURVIVORS AFTER THE END-PERMIAN MASS EXTINCTION

Although the main pulse of end-Permian mass extinction was a synchronous, geologically abrupt event that eliminated 79% of marine invertebrate genera, a small number of Permian brachiopod taxa persisted for up to one million years in post-extinction “mixed faunas.” The environmental context of these mixed faunas constrains the nature of refugia where taxa with poorly buffered respiratory physiology, such as brachiopods and crinoids, could survive the stresses from hypercapnia and hypoxia around the Permian-Triassic boundary. Survival mechanisms during the end-Permian extinction also have broader implications for survival during other physiologically-driven crises, such as the end-Triassic extinction or 21st-century anthropogenic perturbations. The duration of mixed faunas is best constrained in the lower Yangtze River sections of south China, where there is evidence for an abrupt epilogue extinction at the end of the lowermost Griesbachian parvus zone. The epilogue extinction is not globally synchronous, however, as surviving brachiopods were restricted to the Changhsingian (although after the main end-Permian extinction) in western Tethys and peri-Gondwana but persisted until the upper Griesbachian in Oman. Field evidence from western Tethys (Italy and Hungary) and eastern Panthalassa (western United States) indicates that surviving brachiopods were most dominant in the shallowest facies, consistent with the hypothesized shallow-marine “habitable zone” refugium. In other regions, mixed fauna occurrences correspond with proxies suggestive of oxygenated conditions and their diachronous extinction matches the diachronous development of anoxia in many Permian-Triassic boundary sections. These results support the importance of anoxia, which would have exacerbated the primary physiological stresses from hypercapnia and ocean acidification, in the final stages of the end-Permian extinction and suggest that shallow-marine habitats may serve as refugia during other physiologically-driven crises.