Taxonomic composition and environmental distribution of post-extinction rhynchonelliform brachiopod faunas: Constraints on short-term survival and the role of anoxia in the end-Permian mass extinction

Marine taxonomic losses during the end-Permian mass extinction were driven by physiological stresses from ocean warming, acidification, and anoxia that ultimately resulted from CO2 release from Siberian Traps flood volcanism. Despite abundant proxy evidence for anoxia, its role is not well resolved because the timing and selectivity of the extinction are better explained by warming and ocean acidification. We studied the taxonomic composition and spatial and temporal distribution of brachiopod-rich post-extinction faunas, which contain short-lived Permian survivors that lived at a key time during and immediately after the peak of the extinction, to elucidate the controls on survival and the role of anoxia. Holdover brachiopods primarily belong to extinct families and orders, not to long-term survivors, and their probability of short-term survival was a function of pre-extinction metapopulation size. Although short-term survival appears to have been stochastic, likely because of intraspecific variation in tolerance within larger metapopulations, opportunistic and possibly dysaerobic-tolerant genera thrived locally. Rhynchonelliform brachiopod distribution was patchy, both environmentally and temporally. They were more abundant in shallow-water settings, consistent with an oxygenated habitable zone, and their local demise often corresponded with the local development of low-oxygen conditions. Thus, although warming and acidification may have been the primary triggers of taxonomic loss, the addition of spatially and temporally variable anoxic conditions exacerbated physiological stress and contributed to ultimate extinction of short-lived survivors. The combination of the three stresses warming, acidification, and anoxia-which act synergistically to negatively affect respiratory physiology of marine invertebrates, may explain the severity of the end-Permian extinction and provides a sobering analogue for modern ocean acidification and anoxic dead zones. (C) 2013 Elsevier B.V. All rights reserved.