Amphibians bounce-back from Earth’s greatest mass extinction

Ancient frog relatives survived the aftermath of the largest mass extinction of species by feeding on freshwater prey that evaded terrestrial predators, University of Bristol academics have found.

In the study, published today in the journal Royal Society Open Science, their findings suggest the amphibians’ success lay in their generalist feeding ecology, enabling them to feed on a wide variety of prey despite the array of environmental changes happening all around them through the Triassic. Broader examination of Triassic ecosystems also indicates that the freshwater habitats they preferred provided them with a relatively stable variety of food resources, allowing them to thrive while strictly terrestrial predators made do with meagre, unstable resource availability on land.

The end-Permian mass extinction event, 252 million years ago, was the largest ever, marked by the loss of as many as 90% of species. A key line of research has been to focus on the survivors and their fate in the Triassic, the timespan that followed the Permian.

“One of the great mysteries has been the survival and flourishing of a major group of amphibians called the temnospondyls,” explained lead author Aamir Mehmood from Bristol’s School of Biological Sciences. “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?”

The Early Triassic was a time of repeated volcanic activity leading to long phases of global warming, aridification, reductions in atmospheric oxygen, acid rain and widespread wildfires, creating conditions so hostile that the tropics became devoid of animal life. This ‘tropical dead zone’ drastically impacted the distributions of both marine and terrestrial organisms.

Co-author Dr Suresh Singh said: “We collected data on 100 temnospondyls that lived throughout the Triassic and wanted to look at how their ecologies changed.

“We measured their body sizes and features of the skulls and teeth that tell us about function.”

“Much to our surprise, we found that they did not change much through the crisis,” said co-author Dr Armin Elsler. “The temnospondyls showed the same range of body sizes as in the Permian, some of them small and feeding on insects, and others larger. These larger forms included long-snouted animals that trapped fishes and broad-snouted generalist feeders.

“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 of the first five million years of the Triassic, there is evidence that life on land and in the sea moved away from the tropics to avoid the heat.

Professor Mike Benton explained: “Our work shows that the temnospondyls, unexpectedly, were able to cross the tropical dead zone.

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

Aamir concluded: “Their burst of success in the Early Triassic was not followed up.

“They coped with the hot conditions probably by having a low requirement for food, by being able to eat most prey animals, and by hiding in sparse water bodies. But when the ancestors of dinosaurs and of mammals began to diversify in the Middle Triassic, the temnospondyls began their long decline.”

Paper:

‘The ecology and geography of temnospondyl recovery after the Permian – Triassic mass extinction’ by Aamir Mehmood, Dr Suresh Singh, Dr Armin Elsler and Professor Michael Benton in Royal Society Open Science.