The shrew-like creature was the last common ancestor of placental mammals

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

Precursors to mammals are thought to have split off from what eventually became reptiles around 320 million years ago, but it wasn’t until 70 to 80 million years ago that placental mammals appeared.

Their diversity eventually multiplied, with the creatures evolving primarily from small insectivores to a vast array of land, sea, and air creatures.

Today, researchers analyzed the skulls of more than 300 species of extinct and living placental mammals – a subgroup that accounts for 94% of mammals alive today – to uncover evolutionary trends and reveal what might have been resemble their last common ancestor.

Findings suggest placental mammals got their chance at the time of the mass extinction 66 million years ago, when an asteroid slammed into Earth and wiped out non-avian dinosaurs and a host of other forms of life.

Prior to that time, the team notes, the ancestors of the major groups encompassing today’s placental mammals all had similarly shaped skulls. But then, contrary to some theories, diversification happened quickly.

“We see there’s a huge boom, in terms of mammalian diversification, just past the border, or just around that border – depending on when you think [placental] mammals actually originated,” said Professor Anjali Goswami of the Natural History Museum and lead author of the research.

Regardless of when exactly the boom started, the team found that the pace of mammalian evolution subsequently suffered a decline, as some studies had previously suggested.

Crucially, however, the study suggests this has been punctuated by smaller and smaller peaks in diversity – much like the height of ripples decreases with distance when a pebble is thrown into a pond.

“It’s a whole new pattern of evolution,” Goswami said.

She noted that these repeated peaks were likely linked to climatic events opening up new opportunities for mammals. Their dwindling nature over time is likely because these niches are increasingly being filled by existing species, she said.

Writing in the journal Science, the team say that, among other findings, herbivores evolved faster than carnivores and social animals faster than solitaries.

The first, Goswami said, is likely because herbivores have to adapt to changes in plants, which closely follow changes in the environment. “You don’t really see much of a change in carnivores because they just eat any animal, no matter what that animal eats 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 that it was likely a small creature.

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

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

But when huge swaths of biodiversity are lost, she added, all species could be at risk – as was the case when the dinosaur-killing asteroid hit.

“It was also a mammalian squeaker,” she said. “I mean, it’s really lucky that our group made it through.”

Professor Steve Brusatte, a paleontologist at the University of Edinburgh and author of The Rise and Reign of the Mammals, described the study as extremely ambitious and impressive, noting that the key finding was that the rates of skull evolution in placental mammals grew at the time the dinosaurs died out.

“Before that, mammals were background characters in a dinosaur drama, moving slowly but surely in the shadows, huffing.

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

“Now they suddenly found themselves in a world without dinosaurs, more dominated by T. rex and Triceratops, and they responded by rapidly evolving many new types of skulls, which allowed them to eat new foods, behave in new ways and settle into new environments.

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