A Stanford University-led study has pinpointed differences in how marine animals handle oxygen scarcity as water temperatures rise. Published on 6 July 2026 in the Proceedings of the National Academy of Sciences, the research offers a clear physiological explanation for the uneven survival seen during the end-Permian mass extinction.
Around 252 million years ago, rapid global climate change warmed the oceans, stripped them of dissolved oxygen and increased acidity. Brachiopods and other members of the Paleozoic fauna experienced much higher extinction rates than bivalves, gastropods and the groups that now dominate modern seas.
The team combined direct respirometry experiments on living representatives of these ancient lineages with biogeographic data and trait-based modelling of metabolic oxygen balance. They found that species from the Paleozoic fauna show a stronger temperature dependence of hypoxia than those from the Modern fauna.
Simulations driven by this physiological contrast predict greater loss of aerobic habitat for the older groups. Those predictions match the observed intensity and selectivity of the extinctions.
Temperature-dependent hypoxia as the primary kill mechanism
The study concludes that temperature-dependent hypoxia accounts for the magnitude, biogeography and taxonomic selectivity of the end-Permian event. Differences in average physiological tolerances across taxonomy and functional ecology, rather than direct competition, appear to have driven the permanent replacement of Paleozoic dominance by Modern fauna in marine ecosystems.
This work adds substantially to the physiological data available for understudied yet ecologically important marine taxa. It moves beyond fossil occurrence records alone by testing living animals under controlled conditions that mimic the ancient stressors.