Fossils Show Mass Extinction Occurred 10 Times Faster in Ocean

Tuesday, April 20, 2021

An illustration shows Lystrosaurus during the end-Permian mass extinction. (Credit: Gina Viglietti)

Studies of ancient mammal remains reveal how the planet’s biggest extinction event played out on land and in the sea.

Some 250 million years ago, Earth’s warming climate and extreme volcanic activity caused most animal species to go extinct, an event now known as the End-Permian Extinction which eventually ushered in the early days of the dinosaurs. 

To study trends in how the mass extinction happened, researchers looked at the difference between terrestrial and marine die-offs. They found that, on land, the process took 10 times as long as it did in the ocean. 

“The focus for studying terrestrial extinction has basically been, ‘Can we match up the pattern in the terrestrial realm with what’s observed in oceans?’ And the answer is, ‘Not really,'” said Ken Angielczyk, the paper’s senior author and curator of vertebrate paleontology at Chicago’s Field Museum, in remarks released along with the research. 

Previously, it had been assumed that since extinction happened quickly in the ocean, the same would have been true for land-dwellers. Part of the reason for the focus on marine extinction is that the fossil record underwater is better-preserved, with sediment blanketing fossilized animals. 

Now, researchers can compare the previously known submarine extinction patterns to what happened on land. 

“This paper is the first really focusing on vertebrates and saying, ‘No, something was going on that was unique to the terrestrial realm,'” Angielczyk said. 

The results were published in the journal Proceedings of the National Academy of Sciences

Angielczyk and colleagues examined fossils from 588 four-legged animals, discovered in the modern-day Karoo Basin in South Africa. Grouping fossils by age, they created a database spanning 4 million years, in 300,000-year intervals.

With the data in-hand, researchers used a statistical analysis to characterize the array of animals present during the transition period between the Permian and Triassic geologic periods. 

That helped the team “quantify how much extinction is happening and how quickly new species are appearing,” said Pia Viglietti, the study’s lead author and a postdoctoral researcher at the Field Museum

“Instead of putting too much focus on any one fossil,” Viglietti said, “you compile hundreds of observations roughly in the same time interval.”

However, one species in particular helped elucidate patterns of extinction and recovery: an ancient mammal called Lystrosaurus, a name meaning “shovel lizard.” 

Lystrosaurus, an herbivore, included species ranging between the size of a small dog to 2.5 meters long. It made up 54% of species by the onset of the mass extinction, and 70% in its aftermath.

“It had a beak and tusks, it wasn’t the most attractive animal, but I have a soft spot for Lystrosaurus because it was like the first animal I studied as a grad student,” Viglietti said. “So coming full circle with Lystrosaurus in this study made me quite happy.” 

Lystrosaurus is an example of a “disaster taxon,” or a species that thrived when most other life was struggling to survive. 

“Lystrosaurus is like a poster child for the end-Permian extinction that’s always been portrayed as this animal that flourishes in the aftermath of all this extinction and just takes over,” Viglietti said. “But we see Lystrosaurus appearing before the extinction even got started, it was already abundant.”

The researchers believe that Lystrosaurus’ expansion was facilitated by adapting to environmental changes, rather than an ability to expand its range after other species went extinct. 

Fossils like Lystrosaurus help to confirm that the Permian extinction was a longer, more drawn-out process on land, compared to the sea. The reason for that difference may come down to the ocean’s ability to absorb chemicals like carbon dioxide, which can break down ocean ecosystems rather quickly. 

Researchers also drew a parallel between the mass extinctions that happened hundreds of millions of years ago, and the current threat to many of Earth’s species, brought on by climate change. 

“The changes to the Earth’s climate were cumulative and added up over time. Ecosystems were slowly disrupted, and then it just got to a point where everything collapsed, like the straw that breaks the camel’s back,” Viglietti said. “Everything’s fine, until it’s not.”