A new Stanford led study has provided the strongest evidence yet for why some marine animals survived Earth’s largest mass extinction while many others disappeared forever. The findings not only explain how modern ocean ecosystems came to be, but also offer a cautionary glimpse of how today’s warming oceans could affect marine life.
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 across the tree of life.
Before the extinction, ancient seafloors were dominated for about 280 million years by brachiopods, which resemble 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, along with fish and echinoderms such as starfish and sea urchins, went on to dominate Earth’s oceans, a pattern that continues today.
Published July 6 in the Proceedings of the National Academy of Sciences, the study is the first to combine biological data from both the groups devastated by the extinction and those that survived. The results point to one major difference: species whose metabolisms were less able to cope with warmer, oxygen poor water suffered the highest extinction rates.
Those harsh ocean conditions developed after massive volcanic eruptions pumped enormous amounts of carbon dioxide and methane into the atmosphere, dramatically warming the planet.
“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 study author Jose Andres Marquez, a former PhD student in the lab of Erik Anders Sperling at Stanford. “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.”
Ancient Extinction Offers a Modern Climate Warning
According to the researchers, the work also has important implications for the present. The environmental conditions before the Great Dying resembled the relatively cool, oxygen rich oceans that existed for millions of years before human activities began rapidly altering Earth’s climate through fossil fuel emissions.
“This study is really the final nail in the coffin for what caused the Permian-Triassic mass extinction,” said Sperling, the study’s senior author and an associate professor of Earth and planetary sciences in the Stanford Doerr School of Sustainability. “The biggest mass extinction of all time started from a world that is very similar to today in having a relatively cool, relatively well-oxygenated ocean, and then there was a giant injection of carbon dioxide into the Earth system. Understanding how Earth and Earth’s biota responded back then could inform us of what’s to come.”
Why Metabolism Determined Survival
Metabolism includes all of the chemical processes that allow living organisms to produce energy and stay alive. During the Paleozoic era, which ended with the Great Dying, many marine animals were slow moving, bottom dwelling filter feeders, including brachiopods, crinoids (sea lilies, related to starfish), and some corals and sea anemones.
The marine animals that flourished afterward were generally much more active. Fish, mobile snails, sea urchins, and bivalves such as clams, oysters, and mussels all require faster metabolisms to support movement and, in many cases, predatory lifestyles.
Compared with brachiopods, bivalves have greater energy demands because of their larger bodies and muscular “foot” that allows them to burrow and crawl.
“This is why we eat clam chowder and we don’t eat brachiopod chowder,” Sperling said. “Brachiopods have almost no meat.”
Before the extinction, brachiopods greatly outnumbered bivalves. Today, only about 400 brachiopod species remain, while an estimated 10,000 to 15,000 species of bivalves exist.
Sperling compared this dramatic ecological shift to the extinction of the non-avian dinosaurs 65 million years ago, “where mammals essentially took over and never gave up that niche to reptiles again.”
Recreating an Ancient Ocean Crisis
The research expands on a 2018 Princeton and Stanford study, which concluded that warming oceans and oxygen loss were likely responsible for the Great Dying. However, that earlier work relied largely on physiological data collected from modern marine species, particularly economically important fish and crustaceans, leaving major gaps in knowledge about the animals that were actually hardest hit.
“In our new study, we filled in this gap about the physiology of the Paleozoic fauna to see if we could explain not only the biogeography of the extinction but the taxonomic selectivity of the extinction,” said Sperling.
To close that gap, the team conducted years of fieldwork, including collecting living brachiopods in Washington state’s San Juan Islands, where they remain relatively common. Researchers assembled a wide range of marine animals representing both ancient and modern ocean ecosystems.
At field stations and in Stanford laboratories, the scientists measured how much oxygen each organism consumed under different water temperatures. As water warms, metabolic activity speeds up, increasing an animal’s demand for oxygen.
The experiments revealed that Paleozoic animals could survive in lower oxygen conditions than many modern species. However, once temperatures rose, their slow metabolisms could no longer keep up. Their oxygen demands increased much faster than those of modern marine animals.
According to the researchers, differences in body structure help explain the result. More active modern species require more oxygen under normal conditions, but they also possess the muscles and gills needed to handle rising oxygen demands during warming.
“Warming and oxygen loss are the key drivers,” said Sperling.
Other studies have also identified ocean acidification, caused by carbon dioxide making seawater more acidic, as another stressor because it makes shell formation more difficult. Sperling said the new findings suggest acidification likely contributed to the extinction but was far less significant than warming and oxygen depletion.
Lessons for Today’s Oceans
The Stanford team plans to expand its research to additional groups of marine animals to better understand how warming, oxygen loss, and acidification interact, particularly as all three are becoming more severe in today’s oceans.
The researchers warn that history could repeat itself if modern marine species face increasingly warm, oxygen depleted waters.
“The bad news is, we are on track for Permian-Triassic levels of warming in worst-case scenario projections,” said Sperling. Temperatures increased 8-12° Celsius over thousands of years to cause the Great Dying, and today, over just 100-200 years, temperatures are projected to be 1.5-4° Celsius warmer than pre-industrial times 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.
