A groundbreaking study led by Stanford University offers the strongest evidence yet explaining why certain marine animals survived Earth’s largest mass extinction, while many others vanished forever. Not only does this research shed light on the origins of modern marine ecosystems, but it also serves as a critical warning about the possible impacts of current ocean warming on marine life.
Approximately 252 million years ago, the Permian-Triassic extinction event, often termed the “Great Extinction,” resulted in the extinction of about 96% of marine species and 70% of terrestrial animals. However, the devastation was not uniformly distributed across the tree of life.
Before this extinction, the ancient ocean floor was dominated for roughly 280 million years by shell-like brachiopods, crinoids (Crinidae), and other benthic organisms. Post-extinction, this once-dominant group was nearly eradicated, while about half of mollusks, including clams and snails, managed to survive. The surviving species, alongside fish and echinoderms like starfish and sea urchins, continued to dominate the oceans, a trend that persists today.
Published in the July 6th edition of Proceedings of the National Academy of Sciences, this pioneering study merges biological data from both extinct and surviving groups. The findings reveal a significant discrepancy: species with metabolisms less equipped to handle elevated temperatures and low oxygen levels experienced the highest extinction rates.
These inhospitable ocean conditions emerged following massive volcanic eruptions, which released large quantities of carbon dioxide and methane into the atmosphere, leading to dramatic global warming.
“With this study, we aimed to solve the mystery of why beachgoers collect clams and snail shells instead of brachiopod shells,” explained the study’s lead author, José Andrés Marquez, a former doctoral candidate in Eric Anders Sperling’s lab at Stanford University. “Our results indicate that across diverse biological groups, extinctions were significantly higher in those more susceptible to rising water temperatures and diminishing oxygen availability.”
Ancient Extinctions: A Warning for Modern Climate
The researchers believe that their findings hold crucial implications for today’s world. The environmental conditions prior to the extinction mirror the relatively cool, oxygen-rich oceans that existed for millions of years before human activities began to dramatically alter the Earth’s climate through fossil fuel emissions.
“This study is essentially the final piece in understanding the causes behind the Permian-Triassic mass extinction,” stated Sperling, lead author and an associate professor of Earth and Planetary Sciences at the Stanford Doerr School of Sustainability. “The largest mass extinction in history began in a world akin to ours, characterized by relatively cool, oxygen-rich oceans, followed by a significant surge of carbon dioxide into the Earth system. Grasping how Earth and its biota responded then can illuminate what might unfold in the future.”
How Metabolism Influences Survival
Metabolism encompasses all chemical processes enabling living organisms to produce energy and survive. During the Paleozoic Era, which culminated with the mass extinction, many marine creatures functioned as slow-moving, bottom-dwelling filter feeders, such as brachiopods, lilies (related to crinoids and starfish), and some corals and sea anemones.
The marine species that thrived afterward were generally more active. Bivalves like fish, mobile snails, sea urchins, clams, oysters, and mussels require faster metabolisms to sustain their movement and often predatory lifestyles.
Compared to brachiopods, bivalves possess larger bodies and muscular “legs” for burrowing and crawling, necessitating more energy.
“This explains why we enjoy clam chowder instead of brachiopod chowder,” remarked Sperling. “Brachiopods contain very little edible flesh.”
Historically, brachiopods outnumbered bivalves. Today, roughly 400 species of brachiopods persist, while an estimated 10,000 to 15,000 bivalve species remain.
Sperling likened this significant ecological shift to the extinction of non-avian dinosaurs 65 million years ago, “where mammals effectively took over and never relinquished that niche to reptiles again.”
Reconstructing an Ancient Marine Crisis
This study builds upon previous research from Princeton University and Stanford University conducted in 2018, which posited that ocean warming and oxygen depletion were likely core contributors to the extinction event. However, earlier research primarily utilized physiological data from modern marine organisms, particularly economically significant fish and crustaceans, leaving significant knowledge gaps regarding the animals most adversely affected.
“In our new study, we addressed these gaps in the physiology of Paleozoic faunas to elucidate not only the biogeographical patterns of extinction but also the taxonomic selectivity,” said Sperling.
To bridge this knowledge gap, the research team dedicated years to fieldwork, collecting living brachiopods from the San Juan Islands in Washington state, where these organisms remain relatively abundant. The researchers compiled an extensive variety of marine animals from both ancient and contemporary ecosystems.
At the field station and Stanford Research Institute, scientists measured the oxygen consumption of each organism at various water temperatures. As the water heats up, metabolic activity escalates, leading to heightened oxygen demands.
The experiments revealed that Paleozoic organisms were better adapted to endure low-oxygen environments compared to many modern species. However, rising temperatures hinder their metabolism, making it increasingly challenging to meet oxygen needs. Their oxygen requirements surged at a much quicker rate than those of modern marine organisms.
Researchers posit that differences in body structure explain these outcomes. More active modern species require greater oxygen under typical conditions but are also equipped with muscles and gills to manage increased oxygen demand during warming.
“Warming and oxygen deprivation are the primary factors at play,” noted Sperling.
Other studies have highlighted ocean acidification—caused by carbon dioxide increasing seawater acidity—as an additional stressor, complicating shell formation. However, Sperling stated that while acidification likely contributed to the extinction, it was significantly less impactful than warming or oxygen deficiency.
Lessons for Today’s Oceans
The Stanford team aims to broaden their research into other marine animal groups to enhance understanding of how warming, oxygen depletion, and acidification intersect in contemporary oceans.
The researchers caution that history may repeat itself as today’s marine life contends with increasingly warmer and oxygen-poor waters.
“The troubling news is that worst-case scenario predictions are approaching Permian to Triassic levels of warming,” Sperling warned. Historical temperatures surged 8 to 12 degrees Celsius over thousands of years, ushering in the mass extinction. Today, projections estimate a temperature rise of 1.5 to 4 degrees Celsius above pre-industrial levels within just 100 to 200 years, by 2100. “Yet, the encouraging news is that we are still at a pivotal moment where we can effect change.”
This research was funded by the National Science Foundation, NASA, the Paleontological Society, and the Stanford Woods Institute for the Environment.
Source: www.sciencedaily.com