Earth Science
Why Clam Shells Fill Beaches and Brachiopod Shells Don't — The Great Dying Solved at Last
If you have ever walked along a beach and wondered why the shells at your feet are almost always from clams and snails rather than the ancient-looking brachiopods that once blanketed the seafloor, you were asking a question that has puzzled paleontologists for decades. Now a team led by Stanford University has provided the most complete answer yet: it comes down to which animals could handle the heat and low oxygen of Earth's worst extinction event.
The Permian-Triassic extinction event 252 million years ago — known as the "Great Dying" — wiped out approximately 96% of marine species and 70% of terrestrial vertebrates. But the extinction was strikingly selective. Before the catastrophe, the ocean floor was dominated for 280 million years by brachiopods, animals that superficially resemble clams but belong to a completely different lineage. Today, brachiopods have been reduced to roughly 400 surviving species, while clams, snails, and other mollusks number in the tens of thousands. The study, published in Proceedings of the National Academy of Sciences, explains why.
Metabolism was the deciding factor
The researchers combined biological data from both the groups that perished and those that survived the Great Dying. The pattern was clear: animals with slower metabolisms that were less able to cope with warm, oxygen-poor water suffered far higher extinction rates. Brachiopods, which are sedentary filter feeders with minimal energy demands, could not adjust when rising ocean temperatures and falling oxygen levels stressed their metabolic systems. In contrast, mollusks and other more active animals — fish, starfish, sea urchins, mobile snails — naturally have faster metabolisms that gave them greater physiological flexibility under the same conditions.
Those harsh ocean conditions were triggered by massive volcanic eruptions in what is now Siberia, which pumped enormous volumes of carbon dioxide and methane into the atmosphere, causing rapid global warming and depleting seawater of oxygen.
What this means for today
The study's senior author, Professor Erik Sperling of the Stanford Doerr School of Sustainability, notes that the environmental conditions before the Great Dying were remarkably similar to the relatively cool, well-oxygenated oceans that existed before humans began burning fossil fuels at scale. "The biggest mass extinction of all time started from a world that is very similar to today," Sperling said. "Understanding how Earth and its biota responded then could inform us of what is to come."
Confirms a long-standing hypothesis. While scientists suspected that oxygen and temperature played roles in the Permian-Triassic extinction, this study provides the strongest evidence yet by directly comparing survivors and victims across multiple animal groups in a single analytical framework.
Modern climate warning. The finding is sobering because today's oceans are also warming and losing oxygen due to climate change, though at a slower pace than during the Great Dying. The animals most vulnerable in the ancient extinction — slow-moving, bottom-dwelling filter feeders — are similar to many species in modern marine ecosystems that are already showing signs of stress.
Explains why we eat clam chowder, not brachiopod chowder. As Sperling put it, brachiopods "have almost no meat" — their limited muscle mass is a direct consequence of the low-metabolism lifestyle that made them vulnerable to extinction. The ecological shift was as permanent as the one that replaced non-avian dinosaurs with mammals: brachiopods never regained their former dominance.