A Stanford-led study explains why clam shells, not brachiopods, fill beaches today. The answer traces back to a selective extinction 252 million years ago. Warmer, low-oxygen seas changed marine life forever.
New research from Stanford University has finally clarified why beachgoers almost always find clam, mussel, and snail shells on the sand, but rarely encounter brachiopod shells—despite brachiopods once dominating the world's oceans. The study, published in Proceedings of the National Academy of Sciences, connects this modern-day pattern to a catastrophic event 252 million years ago known as the Permian-Triassic extinction, or the “Great Dying.”
During this mass extinction, about 96% of marine species and 70% of land animals vanished. The Stanford team’s findings indicate that the extinction was not random. Instead, it disproportionately affected species with slower metabolisms and limited ability to cope with rising ocean temperatures and falling oxygen levels. Brachiopods, which had thrived for nearly 280 million years, were especially vulnerable due to their anatomy: they are slow-moving filter feeders with minimal muscle mass and underdeveloped gills.
To test their hypothesis, researchers collected living brachiopods from the San Juan Islands in Washington State, where these ancient animals still survive. They compared the oxygen consumption of brachiopods and other representatives of both ancient (paleozoic) and modern marine faunas under varying water temperatures. The results showed that while brachiopods could tolerate low oxygen in cold water, their oxygen needs rose sharply as temperatures increased—far more than in modern bivalves and gastropods. This combination of heat and oxygen shortage proved fatal for many ancient species.
Lead author José Andrés Márquez explained that extinction rates were highest among groups most sensitive to warming and oxygen loss. The study’s senior author, Erik Anders Sperling, noted that the anatomical differences between brachiopods and modern bivalves explain why the latter now dominate ocean floors and, by extension, the shells found on beaches. Today, only about 400 brachiopod species remain, compared to an estimated 10,000 to 15,000 bivalve species.
This new research builds on a 2018 Science study that first highlighted the role of warming and oxygen loss in the Great Dying. However, the earlier work relied mainly on data from modern commercial species, such as fish and crustaceans, which are not closely related to the groups most affected by the extinction. The current study fills this gap by directly measuring the physiology of living paleozoic-type animals, allowing for a clearer understanding of why some groups disappeared while others survived.
While ocean acidification—caused by increased atmospheric CO2—may have contributed to the extinction, the Stanford team found it was not the decisive factor. Their data point to warming and oxygen loss as the main drivers. The researchers now plan to expand their experiments to more marine groups to better separate the effects of temperature, oxygen, and acidification in today’s changing oceans.
The context of this ancient crisis is uncomfortably relevant. The Permian extinction was triggered by massive volcanic eruptions that released huge amounts of CO2 and methane, causing global temperatures to rise by 8 to 12 degrees Celsius over thousands of years. Modern projections suggest ocean temperatures could increase by 1.5 to 4 degrees Celsius by 2100—a much faster rate. According to Sperling, the 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.
Understanding how past ocean crises shaped marine life is not just academic. The selective extinction of brachiopods and the rise of bivalves offer a stark warning about the vulnerability of slow-adapting species to rapid environmental change. This echoes the risks highlighted in other recent disasters, such as 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 lessons from this research are particularly relevant. The fate of ancient brachiopods underscores the importance of monitoring how rising temperatures and declining oxygen levels could reshape marine biodiversity in the coming decades. As the Stanford team continues its work, the hope is that understanding the past will help inform strategies to protect today’s oceans from a similar fate.
