Shrew-like creature was placental mammals’ last common ancestor

The last common ancestor of today’s placental mammals – a group that includes humans, whales and armadillos – was probably a shrew-like creature with a long snout, researchers have revealed.

The forerunners of mammals are believed to have split from what eventually became reptiles around 320m years ago, but it was not until some time between 70 and 80m years ago that placental mammals arose.

Their diversity eventually mushroomed, with the creatures evolving from primarily small insectivores to a huge array of creatures on land, sea and wing.

Now researchers have analysed the skulls of more than 300 species of extinct and living placental mammals – a subgroup that makes up 94% of mammals alive today – to unpick the trends in their evolution and reveal what their last common ancestor might have looked like.

The results suggest placental mammals got their break around the time of the mass extinction 66m years ago, when an asteroid ploughed into Earth and wiped out the non-avian dinosaurs and a host of other life.

Before this time, the team note, the ancestors of the major groups encompassing today’s placental mammals all had similar shaped skulls. But from then on, contrary to some theories, diversification happened rapidly.

“We see that there is a huge boom, in terms of mammal diversification, right after the boundary, or right around that boundary – depending on when you think [placental] mammals actually originated,” said Prof Anjali Goswami of the Natural History Museum and lead author of the research.

Regardless of exactly when the boom began, the team found the pace of mammal evolution subsequently underwent a decline, as some studies have previously suggested.

Crucially though, the study suggests this was punctuated by smaller and smaller peaks of diversity – just as the height of ripples decrease with distance when a pebble is thrown in a pond.

“That’s a completely new model of evolution,” said Goswami.

She noted these repeated peaks were likely to have been linked to climate events opening up new opportunities for mammals. Their diminishing nature over time is probably because such niches became increasingly occupied by existing species, she said.

Writing in the journal Science, the team say that, among other discoveries, herbivores evolved more rapidly than carnivores and social animals more quickly than solitary ones.

The former, said Goswami, is likely because herbivores must adapt to changes in plants, which closely track changes in the environment. “You don’t really see much change in the carnivores because they’re just eating whatever animal is around regardless of what that animal is eating itself,” she said.

The team also used the data to explore what the skull of the last common ancestor of placental mammals looked like, revealing it was likely to have been a diminutive creature.

“I think realistically we’re looking at a shrew-kind of animal,” said Goswami.

In an accompanying article, experts from the University of Washington note the focus on skulls is powerful because they have so many functions and features. What’s more they reflect many different adaptations.Goswani said the study offered insights for a world where the climate is rapidly changing, helping to indicate which animals might be most at risk.

But when huge swathes biodiversity are being lost, she added, all species might be in danger – as was the case when the dinosaur-killing asteroid struck.

“It was a squeaker for mammals too,” she said. “I mean, it’s really just luck that our group made it through.”

Prof Steve Brusatte, a palaeontologist at the University of Edinburgh and author of The Rise and Reign of the Mammals, described the study as hugely ambitious and impressive, noting the key finding was that rates of skull evolution in placental mammals spiked around the time the dinosaurs disappeared.

“Before then, mammals were background characters in a dinosaur drama, slowly but surely evolving in the shadows, chugging along.

“Then the asteroid hit and mammals nearly went the way of the dinosaurs, but a handful of species were able to survive, including distant but direct ancestors of ours,” he said.

“Now they suddenly found themselves in a world free of dinosaurs, no longer lorded over by T. rex and Triceratops, and they responded by rapidly evolving many new types of skulls, which enabled them to eat new foods, behave in new ways and settle new environments.”

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