It is the kind of question you might ask on the beach: why do we collect the shells of clams and snails, but almost never those of brachiopods? An international team led by Stanford University now has an answer that reaches well beyond fossil hunters.
About 252 million years ago, at the boundary between the Permian and Triassic periods, Earth suffered the largest mass extinction in its history. Roughly 96 percent of marine species and about 70 percent of land animals vanished – researchers call it the "Great Dying." The catastrophe was triggered by immense volcanic eruptions that released vast amounts of carbon dioxide and methane, heating both the oceans and the atmosphere.
For a long time, ocean acidification was seen as the main culprit. The new study, published in the Proceedings of the National Academy of Sciences, shifts the focus: what mattered most was the combination of heat and oxygen loss. For the first time, the team modeled how well individual animal groups could physiologically cope with warm, poorly oxygenated water – and matched that against the actual pattern of extinction.
The fit is strikingly close. Hit hardest were sluggish-metabolism animals anchored to the seafloor: brachiopods and sea lilies (crinoids), which had dominated ocean bottoms for nearly 280 million years, disappeared almost entirely. In warmer water, their need for oxygen rose faster than their bodies could meet it. More mobile, higher-metabolism creatures – clams, snails, fish and echinoderms such as starfish and sea urchins – fared far better. They have shaped the seas ever since.
Why it matters today
The study is more than a look backward. Warming and falling oxygen levels are precisely the stresses now bearing down on modern oceans under climate change. "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 Erik Sperling's lab at Stanford. The answer also hints at which communities may be most vulnerable to future warming.
That a 252-million-year-old extinction pattern can be reproduced from the animals' physiology alone counts as a methodological advance – one that lets scientists not merely describe past crises, but understand their causes, and learn from them.