Extreme climate shifts long ago may have helped drive reptile evolution

There’s nothing like a big mass extinction to open up ecological niches and clear out the competition, accelerating evolution for some lucky survivors. Or is there? A new study suggests that the rate of climate change may play just as large a role in speeding up evolution.

The study focuses on reptile evolution across 57 million years — before, during and after the mass extinction at the end of the Permian Period (SN: 12/6/18). That extinction event, triggered by carbon dioxide pumped into the atmosphere and oceans through increased volcanic activity about 252 million years ago, knocked out a whopping 86 percent of Earth’s species. Yet reptiles recovered from the chaos relatively well. Their exploding diversity of species around that time has been widely regarded as a result of their slithering into newly available niches.

But rapid climate fluctuations were already taking place much earlier in the Permian, and so were surges of reptile diversification, researchers say. Analyzing fossils from 125 reptile species shows that bursts of evolutionary diversity in reptiles were tightly correlated with relatively rapid fluctuations in climate throughout the Permian and millions of years into the next geologic period, the Triassic, researchers report August 19 in Science Advances.

Scientists’ understanding of evolution is expanding as they become more tuned into the connection between it and environmental change, says Jessica Whiteside, a geologist at the University of Southampton in England who works on mass extinctions but was not involved in the new work. “This study is bound to become an important part of that conversation.”

To investigate reptile evolution, evolutionary paleobiologist Tiago Simões of Harvard University and colleagues precisely measured and scanned reptile fossils ranging from 294 million to 237 million years old. In all, the researchers examined 1,000 specimens at 50 research institutions in 20 countries.  For climate data, the team used an existing large database of sea surface temperatures based on oxygen isotope data, extending back 450 million years, published in 2021.

By closely tracking changes in body and head size and shape in so many species, paired with that climate data, the researchers found that the faster the rate of climate change, the faster reptiles evolved. The fastest rate of reptile diversification did not occur at the end-Permian extinction, the team found, but several million years later in the Triassic, when climate change was at its most rapid and global temperatures witheringly hot. Ocean surface temperatures during this time soared to 40° Celsius, or 104⁰ Fahrenheit — about the temperature of a hot tub, says Simões.

A few species did evolve less rapidly than their kin, Simões says. The difference? Size. For instance, reptiles with smaller body sizes are already preadapted to live in rapidly warming climates, he says. Due to their greater surface area to body ratio, “small-bodied reptiles can better exchange heat with their surrounding environment,” so stay relatively cooler than larger animals.

“The smaller reptiles were basically being forced by natural selection to stay the same, while during that same period of time, the large reptiles were being told by natural selection ‘You need to change right away or you’re going to go extinct,’” Simões says.

This phenomenon, called the Lilliput effect, is not a new proposal, Simões says, adding that it’s been well established in marine organisms. “But it’s the first time it’s been quantified in limbed vertebrates across this critical period in Earth’s history.”

Simões and colleagues’ detailed work has refined the complex evolutionary tree for reptiles and their ancestors. But, for now, it’s unclear which played a bigger role in reptile evolution long ago — all those open ecological niches after the end-Permian mass extinction, or the dramatic climate fluctuations outside of the extinction event.

“We cannot say which one was more important,” Simões says. “Without either one, the course of evolution in the Triassic and the rise of reptiles to global dominance in terrestrial ecosystems would have been quite different.”