The mass extinction event known as the Great Dying, which occurred 252 million years ago, resulted in the loss of 96% of marine species, and for many years, scientists could not fully explain why certain animals disappeared while others survived.
That longstanding mystery may be closer to a resolution, thanks to a recent study in the Proceedings of the National Academy of Sciences, which suggests that differences in physiology, metabolism, and oxygen use helped determine which marine animals survived the catastrophe.
Stanford University researchers found that physiological differences largely determined which marine animals survived the end-Permian mass extinction after warming oceans lost much of their oxygen. Animals whose bodies could not meet rising oxygen demands, including brachiopods and crinoids that had long ruled the seafloor, had a hard time surviving as ocean temperatures rose and oxygen in the water dropped.
By contrast, mollusks and fish possessed body plans better suited to supplying oxygen as temperatures climbed, allowing them to survive in greater numbers.
“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, a former PhD student in the lab of Stanford’s Erik Anders Sperling.
Marquez and his team developed a model based on species’ metabolic oxygen balance and tested it using direct measurements of oxygen use in living relatives of both ancient groups. This included brachiopods collected from the San Juan Islands in Washington. Their results revealed that the risk of extinction closely aligned with how much oxygen a species required and how effectively it could transport oxygen within its body.
A Body Built for Survival
Modern sea animals, such as bivalves, gastropods, and fish, have become more active and muscular over time. Their hunting habits demand more oxygen, even under normal conditions. Although this could seem like a disadvantage during harsh times, it actually worked in their favor. When ocean temperatures rose and oxygen levels dropped during the Permian-Triassic period, these animals already relied on features such as gills, strong blood flow, and larger muscles to get more oxygen when needed.
On the other hand, Paleozoic animals like brachiopods and crinoids were slow-moving filter feeders that thrived in low-oxygen environments when things were stable. However, as temperatures rose, they required more oxygen, and their bodies could not keep up with the demand in the now-oxygen-poor waters.
“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,” Marquez said.
The findings support decades of evidence from the fossil record. Brachiopods, which once accounted for thousands of species, now number about 400. In contrast, bivalves have diversified and now account for between 10,000 and 15,000 species.
Settling the Debate
Sperling previously co-authored a 2018 study that identified oxygen loss and warming as the main causes of the Great Dying. However, that earlier research used only physiological data from modern species, so the results were influenced by data from commercially important fish and crustaceans rather than from the ancient groups that have largely disappeared. The new study addresses this limitation by directly measuring the physiology of living representatives of ancient animal groups, including brachiopods.
“This study is really the final nail in the coffin for what caused the Permian-Triassic mass extinction,” Sperling said.
A Familiar Climate
The environmental changes that led to the Great Dying began in oceans that were relatively cool and well-oxygenated, similar to present-day conditions. A massive release of volcanic carbon dioxide altered the atmosphere, driving global temperatures up by 8 to 12 degrees Celsius over several thousand years. By comparison, current climate projections estimate a rise of 1.5 to 4 degrees Celsius above preindustrial levels by 2100, but over a much shorter timescale. “The bad news is, we are on track for Permian-Triassic levels of warming in worst-case scenario projections,” Sperling said.
By comparing more groups of marine animals, the researchers hope to determine whether the physiological patterns that shaped Earth’s greatest mass extinction could help predict which species are most vulnerable as today’s oceans continue to warm. “We’re still at the point where we can change things and do something about it,” Sperling emphasized.
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.