How reptiles took over the world

Climate change is shaping the way animals evolve, be it the way they lookor behave. Similarly, climate warming that occurred millions of years ago helped reptiles evolve faster, diversify, and conquer the world, says a new study. 

Researchers have long believed the extinction of some of our mammalian ancestors at the end of the Permian Period (some 252 million years ago) slashed the competition for food and habitats, ultimately allowing reptiles to flourish in the succeeding Triassic Period. But the rise of the reptiles was not just about less competition, the recent study found. Reptiles started to flourish much earlier, at least 270 million years ago, thanks to intense bouts of global warming.

Harvard researchers published their findings in the journal Science Advances.

The period between 300 and 200 million years ago is a very interesting time to see how different species evolved, because this era experienced several successive climate change events, explained Tiago Simões, the study’s lead author and a postdoctoral fellow at Harvard University.

"It was the hardest time period in the history of the planet. It’s related to not just one, but two major mass extinctions," Simões told Mashable. 

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Out of the five mass extinction events that Earth has experienced so far, the Permian-Triassic extinction that happened roughly 250 million years ago is undoubtedly the largest. It wiped away nearly 90 percent of life on the planet. How reptiles survived this mass extinction event was the looming question that Simões and his team at Harvard chose to research. 

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"It was the hardest time period in the history of the planet."

The research team used reptile fossil records from 20 countries for the study. While working to answer how reptiles survived the mass extinctions, the researchers found that reptiles took a foothold in diverse places, in both water and on land, long before the extinction events started. These animals adopted unique adaptations that allowed them to sustain hot temperatures, ultimately making them evolutionary winners. 

Hotter climes, changing bodies

Reptiles were already developing new body plans that probably helped them cope well with relatively sudden and drastic conditions that emerged during the extinction events, Simões told Mashable. The speed at which reptiles evolved was faster when compared to the evolutionary forerunner of mammals, called the "synapsids," that were around at that time, he said.

Not all species of reptiles responded to global warming in the same way.  Reptiles are cold-blooded animals that largely depend on their surroundings for regulating body heat. Natural selection favors small-bodied animals in hot climates, as small creatures more easily release heat from their bodies, compared to larger ones. This adaptation helped small-bodied reptiles succeed, evolve, and diversify faster.

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The larger reptiles, the study says, could have used two strategies to grapple with the extreme temperatures. They might have either ventured into the water, or migrated to colder regions. In either case, cooler environs would have helped the large-bodied reptiles better tolerate warming climes.

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One lineage of small-bodied reptiles call the lepidosaurs, however, evolved slowly. The animals (which included snakes and lizards) were already very small, and they didn’t have as much environmental pressure to change their bodies to adapt to the heat. They kept their original body plan. "Basically, that worked out for them," explained Simões. 

"It's been a question that has been fascinating researchers for a long time."

This study addresses the interesting point of explaining how different groups of reptiles responded differently to climate change, said evolutionary ecologist Davide Foffa, who was not part of this newly published study. "It's been a question that has been fascinating researchers for a long time," he said. 

Climate change threatens over 20 percent of reptile species with extinction, according to a global reptile assessment. Credit: Brett Monroe Garner / Getty Images

But the findings of this study cannot be readily applied to how reptiles adapt to climate change today, said Simões. The impact of climate change on reptile evolution is different now. The rate of heating trapping carbon dioxide entering the atmosphere today is higher than it's been in tens of millions of years, and the increase is likely higher than rates observed before the Permian extinction, Simões added.

Many animals today won't have time to adapt to such significant warming. Climate change threatens over 20 percentof reptile species with extinction, according to a global reptile assessment released this year. 

"This is quite alarming," Simões said.