How survivors spanned the globe after Earth’s biggest mass extinction

Scientists don’t call it the “Great Dying” for nothing. About 252 million years ago, upward of 80% of all marine species vanished during the end-Permian mass extinction – the most extreme event of its kind in Earth’s history. 

What followed was a mysterious, multimillion-year span that could be called the “Great Dulling,” when marine animal communities looked remarkably alike all over the planet, from the equator to the poles. Researchers have long sought an explanation for this so-called taxonomic homogenization – a scene that played out after other mass extinctions over the past half-billion years.

Now, Stanford researchers have shown that profound environmental change likely provided the means for select survivors of the end-Permian extinction to dramatically expand their ranges. The researchers focused on the marine animal fossil record – the most complete evidence of the extinction’s aftermath – and created a model that predicted how creatures like clams, oysters, snails, and slugs flourished in suddenly warmer, less-oxygenated waters. The findings – published in Science Advances on March 26 – offer insight into life’s recovery not only in bygone eras but also for the present, ongoing mass extinction wrought by human activity.

“For us in the paleobiology field, this model is the equivalent to climate scientists getting computerized climate models to make quantitative predictions of how the world should change based on some simple mathematical representations,” said senior study author Jonathan Payne, the Dorrell William Kirby Professor of Earth and Planetary Sciences in the Stanford Doerr School of Sustainability. “We are now able to study big biogeographic changes of mass extinctions in a new way and get a better sense of why some animal groups made it through while others perished.”

Reconstructing the past

In addition to the fossil record, scientists understand ancient oceans based on naturally occurring chemical markers that reveal past temperatures and environmental conditions. Toward the end of the Permian period, the planet was reeling from cataclysmic volcanic activity in modern-day Siberia, which ushered in intense global warming, oxygen depletion, and ocean acidification that killed most marine organisms 252 million years ago. 

But the extinction alone doesn’t explain the bizarre presence of its surviving species – previously constrained to certain specific locations – in every ocean across the globe in the millions of years that followed, known as the earliest Triassic geological period. To convey the surreal concept of taxonomic homogenization on a planetary scale, lead study author Jood Al Aswad, a PhD candidate in Earth and planetary sciences, offered a modern analogy with land animals: 

“If someone asked you today where you’d find kangaroos, you’d say Australia,” she says. “But now imagine some major disaster happened, like a giant volcano erupted, and afterward you’re finding kangaroos in great numbers all over the globe – they’re all the way out in Antarctica, they’re hopping by the pyramids in Egypt, and they’re even in Stanford, California.”

Fossils before and after the end-Permian extinction “go from richly diverse communities to almost boringly alike communities, wherever you look,” Payne said. According to the research, the variety of species across different parts of the world was reduced by more than half after the extinction event.

Setting up shop all over

Researchers have debated the cause of these stark fossil record differences for nearly 200 years and, in recent decades, proposed multiple mechanisms for why different locations had remarkably similar inhabitants following the end-Permian extinction. 

One hypothesis is “ecological release,” where the die-offs of certain predator and competitor creatures allow one surviving group of organisms to go gangbusters. Another common theory is that the climate changes in ways that produce a favorable environment for the same few organism groups just about everywhere.

The study authors put these hypotheses to the test, using geochemical data that provides information about ancient ocean oxygen levels and temperature conditions to build a climate model for end-Permian environmental change in the oceans. 

They then applied data from physiological experiments on living marine invertebrate animals such as clams and snails that are related to the survivors and victims of the Great Dying to populate a climate model with simulated species. These virtual species were able to respond to environmental changes of the end-Permian era based on their ability to survive alterations in temperature and oxygen availability. In this way, the model provided a “physiology-only” evaluation of how species’ geographical distribution would be expected to change if oxygen and temperature were the main drivers of where species could go. 

The results show that the hardy clique of mollusks monopolizing the marine fossil record in the Great Dying’s aftermath were indeed well suited for the conditions of the changed world. As a result, the model did not even have to consider ecosystem-level factors such as loss of predators and competitors, which might have also played a secondary role.

“Our study has provided a simple environmental explanation, rather than an ecological one, for why certain survivors of the end-Permian extinction prospered and why homogenization happened on a global scale,” Payne said. 

Views into the future

In addition to illuminating the deep past, the new model can also help scientists and policymakers predict and better understand the presently unfolding biodiversity crisis, an impending mass extinction caused by the planet-altering activities of billions of humans.

“The current biodiversity crisis is anticipated to herald changes in ecosystem composition that surpass even those seen in the earliest Triassic, which has been the greatest homogenization event to date,” the study authors wrote.

Al Aswad, Payne, and colleagues are now extending their model to examine other past mass extinctions, such as the end-Cretaceous event that famously wiped out the non-avian dinosaurs. 

“Our model offers a great way of studying how animals respond to extreme changes in the environment,” Al Aswad said. “With anthropogenically spurred climate change, there has been some warning that if we continue, then in the future we’re going to see taxonomic homogenization of organisms in modern oceans as well.”

Other Stanford co-authors of the study are Pedro Monarrez (previously a postdoctoral fellow at Stanford and now an assistant professor at Virginia Tech) and Mohamad Bazzi, a current postdoctoral scholar in Payne’s lab. Justin Penn and Curtis Deutsch from Princeton University are also co-authors. The research was supported by funding from the National Science Foundation.