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, scientists have revealed.
The precursors of mammals are thought to have split from what eventually became reptiles around 320 million years ago, but it wasn’t until between 70 and 80 million years ago that placental mammals emerged.
Their diversity eventually evolved, with the creatures evolving from primarily small insectivores to a vast array of creatures on land, sea, and wing.
Now, researchers have analyzed the skulls of more than 300 species of extinct and living placental mammals – a subgroup that makes up 94% of all mammals alive today – to tease out trends in their evolution and reveal what their most recent common ancestor may have looked like.
The results suggest that placental mammals got their break around the time of the mass extinction 66 million years ago, when an asteroid plowed into Earth, wiping out the non-avian dinosaurs and a host of other life.
Before this time, the team notes, the ancestors of the large groups that comprise today’s placental mammals all had similarly shaped skulls. But from then onwards diversification, contrary to some theories, happened rapidly.
“We’re seeing 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, says Professor Anjali Goswami of the Natural History Museum and lead author of the research.
Regardless of when exactly the boom began, the team found that the rate of mammal evolution then went through a decline, as some studies have previously suggested.
Crucially, however, the study suggests that this was marked by smaller and smaller peaks of diversity – just as the height of ripples decreases with distance when a stone is thrown into a pond.
“It’s a whole new model of evolution,” Goswami said.
She noted that these repeated peaks are likely to have been linked to climate events that open up new opportunities for mammals. Their declining nature over time is probably due to such niches becoming increasingly occupied by extant species, she said.
Writing in the journal Science, the team says, among other findings, herbivores evolved faster than carnivores and social animals evolved faster than solitary ones.
The former, Goswami said, is likely because herbivores must adapt to changes in plants, which closely follow changes in the environment. “You don’t really see much change in the carnivores because they just eat whatever animal is around regardless of what the 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 snap type of animal,” Goswami said.
In an accompanying article, experts from the University of Washington note that the focus on skulls is powerful because they have so many functions and properties. In addition, they reflect many different adaptations. Goswani said the study provided insights for a world where the climate is changing rapidly, helping to indicate which animals may be most vulnerable.
But when large swaths of biodiversity are lost, she added, all species can be at risk — as was the case when the dinosaur-killing asteroid struck.
“It was a squeak for mammals, too,” she said. “I mean, it’s really just luck that our group made it through.”
Prof Steve Brusatte, 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 that the key finding was that skull evolution in placental mammals increased around the time the dinosaurs disappeared.
“Before that, mammals were background characters in a dinosaur drama, slowly but surely evolving in the shadows and chugging along.
“Then the asteroid hit and the mammals almost 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 dominated by T. rex and Triceratops, and they responded by rapidly developing many new types of skulls, which enabled them to eat new foods, behave in new ways and settling in. new environments.”
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