Walk any beach today and the shells in the sand almost all belong to the same handful of groups: clams, snails, the occasional sea urchin spine. Go back 252 million years, and the seafloor would have looked almost unrecognizable, carpeted instead with brachiopods and crinoids, or “sea lilies,” anchored in place and filtering the water for food. Then, in a geological instant, that world ended. A new study from Stanford University has pinned down, with unprecedented physiological detail, why one set of animals died and the other survived to inherit the oceans.
The event in question is the end-Permian mass extinction, nicknamed the “Great Dying,” which wiped out roughly 96 percent of marine species and 70 percent of land animals around 252 million years ago. It remains the single most destructive biological catastrophe in the planet’s history, dwarfing even the asteroid impact that ended the age of dinosaurs. But the Great Dying was never an equal-opportunity killer. Brachiopods and crinoids, which had dominated the seafloor for some 280 million years, were nearly erased. Mollusks — the ancestors of today’s clams, snails, and oysters — took a beating too, but only about half their species vanished, and they went on to become the ecological backbone of modern oceans.
The new study, published July 6 in Proceedings of the National Academy of Sciences, is the first to directly test why that split happened by measuring the actual physiology of living relatives of both groups. “With this study, we essentially wanted to solve the mystery of why, when you go to the beach, you collect the shells of clams and snails rather than those of brachiopods,” said lead author Jose Andres Marquez, a former doctoral student in the Stanford lab of senior author Erik Sperling. “Our findings show that, across different organism groups, extinctions happened at much higher rates for those more vulnerable to increases in water temperature and decreases in oxygen availability.”
The mechanism the team zeroed in on is what scientists call temperature-dependent hypoxia: as ocean water warms, animals’ metabolisms speed up and their oxygen demand climbs, even as warmer water itself holds less dissolved oxygen to supply that demand. Whether a species can keep pace comes down largely to its body plan and lifestyle.
Brachiopods, crinoids, and their Paleozoic-era neighbors were mostly slow-metabolizing, bottom-dwelling filter feeders — built for efficiency, not speed. The animals that eventually took over, including bivalves, gastropods, and fish, tend to be far more mobile and predatory, with the faster metabolisms and greater muscle and gill capacity that lifestyle demands. Sperling put the physiological contrast in more relatable terms: “This is why we eat clam chowder and we don’t eat brachiopod chowder. Brachiopods have almost no meat.”
To measure that contrast directly, the researchers traveled to field sites including the San Juan Islands of Washington state, one of the few places brachiopods still persist in meaningful numbers, and brought specimens back to chambers in Sperling’s Stanford lab that track how much oxygen an organism consumes as water temperature rises. The results showed the Paleozoic-style animals could tolerate lower oxygen levels than their modern counterparts at cool temperatures, but their oxygen needs shot up disproportionately fast as the water warmed — a mismatch modern fauna, with their more athletic physiology, were better equipped to handle.
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The new experiments extend a 2018 study in Science, also involving Sperling and Stanford co-author Jon Payne, that first showed ocean warming and deoxygenation could explain the geographic pattern of the extinction, with the hardest-hit species concentrated outside the tropics. That earlier model, however, relied on physiological data drawn mostly from modern species already well-studied by other researchers — commercially important fish and crustaceans — leaving a gap around the extinct-prone Paleozoic groups themselves. “In our new study, we filled in this gap about the physiology of the Paleozoic fauna to see if we could explain not only the biogeography of the extinction but the taxonomic selectivity of the extinction,” Sperling said.
With that gap closed, Sperling considers the case essentially settled: “This study is really the final nail in the coffin for what caused the Permian–Triassic mass extinction. The biggest mass extinction of all time started from a world that is very similar to today in having a relatively cool, relatively well-oxygenated ocean, and then there was a giant injection of carbon dioxide into the Earth system. Understanding how Earth and Earth’s biota responded back then could inform us of what’s to come.”
Ocean acidification, driven by the same carbon dioxide surge, has also been implicated by other researchers as a contributing stressor of the era, particularly for shell-building organisms. The new findings don’t rule that out, but the team’s data suggest it was a secondary factor compared to the one-two punch of warming and oxygen loss.
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The scale of the ancient disaster puts modern warming in sobering perspective, even as it underscores real differences in pace. Ocean temperatures rose an estimated 8 to 12 degrees Celsius over thousands of years to trigger the Great Dying; current projections put human-driven warming at 1.5 to 4 degrees Celsius above pre-industrial levels by 2100 — a smaller temperature swing, but compressed into a couple of centuries rather than millennia. “The bad news is, we are on track for Permian-Triassic levels of warming in worst-case scenario projections,” Sperling said. “But the good news is, we’re still at the point where we can change things and do something about it.”
That framing is central to why a 252-million-year-old extinction matters now. If physiological vulnerability to warming and low oxygen predicted who lived and died the last time Earth’s oceans underwent rapid, carbon-driven change, the same traits — mobility, metabolic rate, gill and muscle capacity — may again sort winners from losers as today’s oceans warm and lose oxygen. The Stanford team plans to expand its physiological survey to more marine groups to better untangle the combined effects of warming, deoxygenation, and acidification, the same trio of stressors now converging in the world’s oceans.
Endnotes
Funding: U.S. National Science Foundation, NASA, the Palaeontological Association, and the Stanford Woods Institute for the Environment.
Stanford University, “Researchers confirm cause of Earth’s biggest mass extinction,” news release via EurekAlert!, July 9, 2026.
J.A. Marquez et al., “Differences in physiological tolerance to global warming caused the Permian–Triassic transition between the Paleozoic and Modern faunas,” Proceedings of the National Academy of Sciences, July 6, 2026, DOI: 10.1073/pnas.2533086123.
J.L. Penn, C. Deutsch, J.L. Payne, E.A. Sperling, “Temperature-dependent hypoxia explains biogeography and severity of end-Permian marine mass extinction,” Science 362, eaat1327 (2018), DOI: 10.1126/science.aat1327.
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