Reptile boom 250 million years ago may have been due to global warming

A boom in reptile abundance and diversity around 250 million years ago may have resulted from soaring temperatures beginning millions of years earlier, rather than filling the gap left by a mass extinction of mammals as was previously thought.

Towards the end of the Permian Period around 250 million years ago, two massive volcanic eruptions caused global temperatures to increase by roughly 30°C (54°F). “[The volcanoes] released huge amounts of greenhouse gases into the atmosphere, which resulted in a huge global warming effect,” says Tiago R. Simões at Harvard University. In the tropics, “the surface of the ocean was basically as hot as your hot tub.”

The explosions may not be as famous as the more recent asteroid strike thought to have killed off the dinosaurs, but the eruptions were among the most destructive mass extinction events in our planet’s history, with the second, more powerful blast wiping out 86 per cent of species.

The planet was already on a warming trend, but the eruptions spurred a roughly 20-million-year-long hot streak. While early mammal ancestors began dying en masse, reptiles appeared to evolve at breakneck speed, ranging from small gecko-like creatures on land to domineering ichthyosaurs at sea.

Simões and his colleagues spent eight years measuring and comparing museum fossils of extinct amniotes – the four-legged ancestors of mammals, reptiles and birds – that lived in a period spanning 70 million years before and 70 million years after the major extinction event. He compiled 348 morphological characteristics, like skull dimensions and tail length, for 1000 fossil specimens from 125 species. Then, he and his team compared that information with global temperatures during the same period.

Their statistical analysis revealed that reptiles were increasing in number and diversity around 40 million years before the dramatic explosions, indicating the reptiles’ success was tied to a warming climate, not the sudden loss of mammalian competition.

“As you reach the peak of those climatic changes, reptiles were already evolving quite fast,” says Simões. “This takeover of reptiles was already under course.”

The findings shake up how palaeontologists think about reptile evolution, says Christopher J. Raxworthy at the American Museum of Natural History in New York. “New climates themselves could be stimulating evolution to ultimately produce very diverse new forms,” he says.

Raxworthy notes that, compared with the rapid pace of human-caused climate change, this stretch of warming happened relatively slowly. “We won’t actually see the evolutionary implications of the climate change we’re inducing now for millions of years,” he says. “The consequences could be huge.”