Understanding mechanisms for the end-Permian mass extinction and the protracted Early Triassic aftermath and recovery

Modern study of the end-Permian mass extinction in the marine realm has involved intensive documentation of the fossil content, sedimentology, and chemostratigraphy of individual stratigraphic sections where the mass extinction interval is well preserved. These studies, coupled with innovative modeling of environmental conditions, have produced specific hypotheses for the mechanisms that caused the mass extinction and associated environmental stress. New paleobiological studies on the environmental distribution and ecological importance of brachiopods, benthic molluscs, and bryozoans support the hypothesis that stressful ocean conditions—primarily elevated H2S levels (euxinia) but also heightened CO2 concentrations—were the prime causes of the end-Permian mass extinction. These studies also further demonstrate that both the Late Permian interval preceding this mass extinction and the Early Triassic interval that followed were times of similar elevated environmental stress. In the low-diversity Early Triassic biosphere, huge numbers of benthic molluscs, in particular four cosmopolitan genera of bivalves, typically covered the seafloor. That a few marine genera thrived during this time indicates a greater than usual tolerance to some combination of marine anoxia, as well as elevated CO2 and/or increased H2S concentrations. Research focusing on experiments with modern organisms similar to those that died, as well as those that thrived, in microcosms where levels of O2, CO2, and H2S can be experimentally manipulated will enable an even more detailed understanding of the nature of this greatest Phanerozoic biotic crisis.