Earth's Greatest Mass Extinction Paved the Way for the Rise of Mammals

Mass extinction events may not be the end of the world for some species. It turns out that the Earth's greatest mass extinction of all time around 252 million years ago may have helped pave the way for the rise of mammals when their ancient ancestors survived the incident.

The first mammals arose in the Triassic period, more than 225 million years ago. These included shrew-like animals, such as the Morganucodon from England and the Megazostrodon from Africa and the Bienotherium from China. Yet these creatures with their mammalian features, which included warm blood, fur and large brains, probably arose gradually over a long period of time.

So who were the ancestors of these mammals? Their closest relatives were the cynodont therapsids. These creatures managed to survive the end-Permian mass extinction, which wiped out about 90 percent of marine organisms and about 70 percent of terrestrial species. In this latest study, the researchers found that cynodont diversity rose steadily during the recovery of life following the mass extinction, with their range of form rising rapidly before hitting a plateau.

"Mass extinctions are seen as entirely negative," said Marcello Ruta, the lead author of the new paper, in a news release. "However, in this case, cynodont therapsids, which included a very small number of species before the extinction, really took off afterwards and were able to adapt to fill many different niches in the Triassic--from carnivores to herbivores."

The fact that the extinction event allowed these creatures to diversify gave them the chance to survive far into the future. During the Triassic, the cynodonts split into two groups--cynognathians and probainognathians. The first were mainly plant-eaters while the second were mainly flesh-eaters. These two groups seemed to rise and fall at random, but also gave rise to the first mammals about 25 million years after the mass extinction.

"We saw that when a major group, such as cynodonts, diversifies, it is the body shape or range of adaptations that expands first," said Michael Benton, co-author of the new paper, in a news release. "The diversity, or number of species, rises after all the morphologies available to the group have been tried out."

The findings reveal that extinction events aren't always bad for species during the long term. They can open up new niches for animals to take advantage of. In this case, it allowed mammals to gain a foothold and persist into the present day.

The findings are published in the journal Proceedings of the Royal Society B: Biological Sciences.