Earth’s ‘Great Dying’ killed 80-90% of life. How some amphibians survived.

When we talk about mass extinction events, the first case that usually comes to mind is when an asteroid struck Earth about 66 million years ago and triggered the extinction of the dinosaurs. However, the Cretaceous–Paleogene extinction event was not the worst loss of life in our planet’s history. That distinction belongs to the Permian-Triassic extinction or the Great Dying. During this dramatic period of climate change about 252 million years ago, about 80 to 90 percent of all species on Earth were wiped out and the biosphere of the planet was completely altered.

Yet still, in the face of this devastation, some species managed to survive. Namely, a group of primitive amphibians called the temnospondyls. They may have survived the Great Dying by feeding on some freshwater prey that larger land-based predators couldn’t get to and by not being picky eaters. These new findings are detailed in a study published March 4 in the journal Royal Society Open Science.

About 250 million years ago during the Early Triassic, nearly constant volcanic activity led to long phases of global warming, aridification, reductions in oxygen in the atmosphere, mega El Niños, acid rain, and wildfire. The landscape eventually became so hostile that the tropics became completely devoid of animal life. The eventual tropical dead zone impacted the distributions of both marine and terrestrial organisms throughout Earth. Some organisms, including sharks, horseshoe crabs, and temnospondyls, managed to tough it out. 

“One of the great mysteries has been the survival and flourishing of a major group of amphibians called the temnospondyls,” Aamir Mehmood, a study co-author and evolutionary biologist at the University of Bristol in the United Kingdom, said in a statement. “These were predatory animals that fed on fishes and other prey, but were primarily linked to the water, just like modern amphibians such as frogs and salamanders. We know that climates then were hot, and especially so after the extinction event. How could these water-loving animals have been so successful?”

To figure out why, Mehmood and the team from this new study collected fossil data from 100 temnospondyls that lived throughout the Triassic. They studied how their ecologies changed, measuring parts of their skulls, teeth, and body sizes to see what specific functions they may have been used for. 

Surprisingly, they found that the temnospondyls did not change much throughout the crisis. Instead, they showed the same range of body sizes that they did during the earlier days of the Permian period. Some of the temnospondyls were small and fed on insects while others were larger. These bigger temnospondyls hold a critical survival clue. 

“These larger forms included long-snouted animals that trapped fishes and broad-snouted generalist feeders,” study co-author and paleontologist Armin Elsler said in a statement. “What was unusual though was how their diversity of body sizes and functional variety expanded about 5 million years after the crisis and then dropped back.”

Due to the intense global warming in the first five million years of the Triassic period, there is evidence both terrestrial and marine organisms moved away from the tropics to get away from the extreme heat. According to the team, the temnospondyls were surprisingly able to cross that tropical dead zone.

[ Related: These pleasantly plump salamanders dominated the Cretaceous period. ]

“Fossils are known from South Africa and Australia in the south, as well as North America, Europe and Russia in the north,” study co-author and paleontologist Mike Benton said in a statement. “The temnospondyls must have been able to criss-cross the tropical zone during cooler episodes.”

The study suggests that their generalist feeding ecology was key to this success. Temnospondyls were able to feed on a variety of prey, despite the environmental changes happening around them. It wasn’t that they could survive by eating less, but also their ability to hide in sparse water bodies and consume different types of prey.

However, this success did not last. The temnospondyls began their decline by the Middle Triassic, as the ancestors of mammals and dinosaurs began to diversify. 

“Their burst of success in the Early Triassic was not followed up,” said Mehmood. 

Temnospondyls ultimately went extinct about 120 million years ago. While they do not have any living relatives, some evolutionary biologists do consider them an important step towards today’s diverse amphibians. Studying these past periods can help scientists understand how frogs, salamanders, and toads may fare during today’s environmental challenges. Amphibians remain one of the most threatened groups of animals due to widespread diseases and climate change. 

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