Roughly 252 million years ago, the Permian-Triassic extinction event — often called the "Great Dying" — wiped out about 96% of marine species and 70% of land animals. Yet the devastation was not evenly distributed. Before the extinction, ancient seafloors were dominated for 280 million years by brachiopods (shellfish resembling clams) along with sea lilies (crinoids) and other bottom-dwelling animals. After the catastrophe, those once-dominant groups were nearly eliminated. In contrast, only about half of mollusks — including clams and snails — disappeared. The survivors went on to dominate Earth's oceans, a pattern that continues today.
Now, a landmark study from Stanford University, published July 6 in the Proceedings of the National Academy of Sciences, has finally solved the mystery of why. The answer, it turns out, comes down to metabolism.
"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. "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 researchers conducted years of fieldwork, collecting living brachiopods in Washington state's San Juan Islands and measuring how much oxygen each organism consumed under different water temperatures. The experiments revealed that Paleozoic animals — slow-moving, bottom-dwelling filter feeders — could survive in lower oxygen conditions than many modern species. But once temperatures rose, their slow metabolisms could no longer keep up. Their oxygen demands increased much faster than those of more active modern marine animals like fish, mobile snails, and clams.
"This is why we eat clam chowder and we don't eat brachiopod chowder," said senior author Erik Sperling. "Brachiopods have almost no meat."
The study carries urgent implications for today. The environmental conditions before the Great Dying resembled the relatively cool, oxygen-rich oceans that existed before human activities began rapidly altering Earth's climate. Massive volcanic eruptions pumped enormous CO₂ and methane into the atmosphere then — just as fossil fuel emissions are doing now, albeit at a slower rate.
"The bad news is, we are on track for Permian-Triassic levels of warming in worst-case scenario projections," Sperling warned. Temperatures rose 8–12°C over thousands of years to cause the Great Dying. Today, over just 100–200 years, temperatures are projected to rise 1.5–4°C above pre-industrial levels by 2100. "But the good news is, we're still at the point where we can change things and do something about it."
Funding was provided by the U.S. National Science Foundation, NASA, the Palaeontological Association, and the Stanford Woods Institute for the Environment.
