New research from Stanford University helps explain why beachgoers almost always find shells from clams and mussels, with snails also common, but rarely encounter brachiopod shells. Brachiopods once dominated the world's oceans. Published in Proceedings of the National Academy of Sciences, the study links this modern pattern to the Permian-Triassic extinction about 252 million years ago, known as the “Great Dying.” Widely regarded as the most severe of the major biological extinctions of the Phanerozoic, the crisis reshaped marine ecosystems and shifted the balance between brachiopods and bivalves.
During this mass extinction, an estimated 95-96% of marine species and about 70% of land animals vanished; estimates vary information from the groups and measures used. The Stanford team’s findings suggest the extinction was selective, disproportionately affecting species with slower metabolisms and less ability to withstand rising ocean temperatures as oxygen levels fell. Brachiopods, which had thrived for nearly 280 million years, were especially vulnerable because of their anatomy. They are slow-moving filter feeders with relatively little muscle mass and less-developed gills.
The leading geological explanation for the crisis centers on massive eruptions in the Siberian Traps. Volcanic gases contributed to rapid global warming, while carbon dioxide entering the oceans altered seawater chemistry and lowered pH. As water warms, it holds less oxygen; changes in circulation and biological demand can intensify that loss. In some marine regions, oxygen depletion progressed to anoxia or euxinia, conditions where oxygen is absent and sulfide can accumulate. These stresses varied in timing and severity from one region to another, making the extinction a cascade of interacting environmental pressures rather than a single, uniform event.
To test their hypothesis, researchers collected living brachiopods from the San Juan Islands in Washington State, where these ancient animals still survive. They compared oxygen consumption in brachiopods with that of other representatives of ancient Paleozoic and modern marine faunas at varying water temperatures. The results showed that brachiopods could tolerate low oxygen in cold water, but their oxygen needs rose sharply as temperatures increased, far more than those of modern bivalves and gastropods. The basic is the interaction between temperature and oxygen availability: as water warms, animals’ oxygen demands can rise even as the environment supplies less. For many ancient species, the combination proved fatal.
Lead author José Andrés Márquez explained that extinction rates were highest among groups most sensitive to warming and oxygen loss. Senior author Erik Anders Sperling said anatomical differences between brachiopods and modern bivalves help explain why the latter now dominate ocean floors, and why their shells are so common on beaches. Today, only about 400 brachiopod species remain, compared with an estimated 10,000 to 15,000 bivalve species. Brachiopods were among the major marine groups hit at the end of the Permian; bivalves and gastropods became far more prominent in later marine ecosystems.
This research builds on a 2018 Science study that first highlighted warming and oxygen loss in the Great Dying. The current study extends that physiological comparison to living representatives of ancient marine groups, helping explain why some lineages were more vulnerable than others. Its findings support a broader picture: temperature and oxygen together shape how much marine habitat animals can use for aerobic activity.
Ocean acidification, caused by increased atmospheric CO2, may also have contributed to the extinction, but the Stanford team found it was not decisive in its analysis. The data point to warming and oxygen loss as the basic drivers. Acidification was part of the wider upheaval: changes in seawater carbonate chemistry can make it harder for organisms to build calcium-carbonate shells or skeletons. Researchers plan to expand their experiments to more marine groups, helping separate the effects of temperature and oxygen from those of acidification in today’s changing oceans.
The ancient crisis has present-day relevance. Massive volcanic eruptions triggered the Permian extinction, releasing huge amounts of CO2 and methane and driving global temperatures up by 8 to 12 degrees Celsius over thousands of years. Modern projections suggest ocean temperatures could rise by 1.5 to 4 degrees Celsius by 2100, at a much faster rate. Sperling said worst-case scenarios for the coming decades could approach the conditions that led to the Great Dying, though there is still time to alter this trajectory. The ancient record also shows why comparisons must account for regional differences: marine environments experienced changing conditions at different times and with varying severity.
The selective extinction of brachiopods and the rise of bivalves warn of the risks rapid environmental change poses to slow-adapting species. That concern echoes other recent disasters, including the deadly wildfire in Almería, where environmental shifts had tragic consequences for local residents, as detailed in this report on the Los Gallardos evacuation.
For Spain, with its extensive coastline and reliance on healthy marine ecosystems, the research has particular relevance. The fate of ancient brachiopods underscores the need to monitor how rising temperatures and declining oxygen levels could reshape marine biodiversity in the coming decades. The Stanford team plans to continue its work, aiming to inform strategies to protect today’s oceans from a similar fate.
