A new Stanford University-led study provides the strongest evidence yet for why some marine animals survived Earth’s largest mass extinction, while many others disappeared forever. The discovery not only explains how modern marine ecosystems arose, but also offers a warning glimpse of how today’s ocean warming could affect marine life.
About 252 million years ago, about 96% of marine species and 70% of land animals became extinct during the Permian-Triassic extinction event, often referred to as the “Great Extinction.” But the devastation was not evenly distributed across the tree of life.
Before their extinction, the ancient ocean floor was dominated for about 280 million years by shell-like brachiopods, crinoids (Crininidae), and other benthic animals. After the disaster, the once dominant group was nearly eliminated. In contrast, only about half of molluscs such as clams and snails disappeared. The survivors continued to dominate the Earth’s oceans, along with fish and echinoderms such as starfish and sea urchins, and this pattern continues today.
Published in the July 6th issue Proceedings of the National Academy of SciencesThis study is the first to combine biological data from both groups devastated by extinction and those that survived. This result shows one major difference. That means species whose metabolisms were less able to cope with warm, oxygen-poor waters suffered the highest extinction rates.
These harsh ocean conditions occurred after massive volcanic eruptions pumped large amounts of carbon dioxide and methane into the atmosphere, causing the Earth to warm dramatically.
“With this study, we essentially wanted to solve the mystery of why people collect clams and snail shells when they go to the beach rather than brachiopod shells,” said study lead author José Andrés Marquez, a former doctoral student in the lab of Eric Anders Sperling at Stanford University. “Our findings show that across a wide range of biological groups, extinctions occurred at much higher rates in groups that were more vulnerable to increasing water temperatures and decreasing oxygen availability.”
Ancient extinctions warn us about modern climate
According to the researchers, the study has important implications today. Environmental conditions before the apocalypse were similar to the relatively cool, oxygen-rich oceans that existed for millions of years before human activity began rapidly changing the Earth’s climate through fossil fuel emissions.
“This study is really the final nail in the coffin of understanding the causes of the Permian-Triassic mass extinction,” said Sperling, lead author of the study and associate professor of Earth and Planetary Sciences at the Stanford Doerr School of Sustainability. “The largest mass extinction in history began in a world similar to today’s in that it had relatively cool, oxygen-rich oceans, followed by a huge injection of carbon dioxide into the Earth system. Understanding how Earth and its biota responded then can help us understand what will happen in the future.”
Why does metabolism determine survival?
Metabolism includes all chemical processes that allow living organisms to generate energy and stay alive. During the Paleozoic Era, which ended with the Catastrophe, many marine animals were slow-moving, bottom-dwelling filter feeders, including brachiopods, lilies (relatives of crinoids and starfish), and some corals and sea anemones.
Marine animals that subsequently flourished were generally much more active. Bivalves such as fish, mobile snails, sea urchins, clams, oysters, and mussels all require faster metabolisms to support movement and often predatory lifestyles.
Compared to brachiopods, bivalves have larger bodies and muscular “legs” that allow them to burrow and crawl, so they require more energy.
“This is why we eat clam chowder and not brachiopod chowder,” Sperling said. “Brachiopods have very little meat.”
Before their extinction, brachiopods greatly outnumbered bivalves. Today, only about 400 species of brachiopods remain, while there are an estimated 10,000 to 15,000 species of bivalves.
Sperling likened this dramatic ecological change to the extinction of non-avian dinosaurs 65 million years ago, “where mammals essentially took over and never handed that niche over to reptiles again.”
Recreating an ancient maritime crisis
The study expands on a 2018 study by Princeton University and Stanford University that concluded ocean warming and oxygen loss were likely to blame for the die-off. However, that early research relied primarily on physiological data collected from modern marine species, particularly economically important fish and crustaceans, leaving large gaps in our knowledge of which animals were actually most affected.
“In our new study, we filled this gap in the physiology of Paleozoic faunas to see if we could explain not only the biogeography of extinctions but also the taxonomic selectivity of extinctions,” Sperling said.
To fill this gap, the research team conducted years of fieldwork that included collecting living brachiopods on the San Juan Islands in Washington state, where brachiopods remain relatively common. Researchers assembled a wide range of marine animals representing both ancient and modern marine ecosystems.
Scientists at the field station and Stanford Research Institute measured how much oxygen each organism consumed at different water temperatures. As the water warms, metabolic activity accelerates and the animal’s oxygen demand increases.
The experiment revealed that Paleozoic animals were better able to survive in low-oxygen conditions than many modern species. However, as temperatures rise, our slow metabolism can’t keep up. Their oxygen requirements increased much faster than those of modern marine animals.
Researchers say differences in body structure may help explain the results. More active modern species require more oxygen under normal conditions, but also have the muscles and gills necessary to cope with increased oxygen demand during warming.
“Warming and oxygen loss are the main factors,” Sperling said.
Other studies have also identified ocean acidification, caused by carbon dioxide making seawater more acidic, as another stressor, as it makes shell formation more difficult. Sperling said the new findings suggest that while acidification likely contributed to the extinction, it was far less significant than warming or oxygen deprivation.
Lessons for today’s oceans
The Stanford team plans to expand their research to other groups of marine animals, especially to better understand how warming, oxygen loss, and acidification interact in today’s oceans.
Researchers warn that history could repeat itself as modern marine life faces increasingly warm and oxygen-depleted waters.
“The bad news is that worst-case scenario projections are moving toward Permian to Triassic levels of warming,” Sperling said. Temperatures rose 8 to 12 degrees Celsius over thousands of years, causing the mass extinction. And today, in just 100 to 200 years, by 2100, temperatures are predicted to rise 1.5 to 4 degrees Celsius above pre-industrial levels. “But the good news is we are still at a stage where we can change things and do something about it.”
Funding was provided by the National Science Foundation, NASA, the Paleontological Society, and the Stanford Woods Institute for the Environment.
