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

Researchers have revealed that the last common ancestor of today’s placental mammals – a group that includes humans, whales and armadillos – may have been a shrew-like creature with a long nose.

The ancestors of mammals are thought to have separated from what eventually became reptiles about 320 million years ago, but placental mammals did not appear until sometime between 70 and 80 million years ago.

Their diversity eventually spread, with the creatures evolving from primarily small insects to a large group of creatures on land, sea, and wing.

The researchers have now analyzed the skulls of more than 300 species of extinct and vital placental mammals – a subgroup that makes up 94% of mammals alive today – to delineate trends in their evolution and reveal the shape of their last common ancestor.

The findings suggest that the placental mammals got a break around the time of the mass extinction 66 million years ago, when an asteroid smashed into Earth and wiped out non-bird dinosaurs and a host of other living things.

Before this time, the team noted, the ancestors of the major groups that comprise today’s placental mammals all had similarly shaped skulls. But since then, and contrary to some theories, diversification has occurred rapidly.

“We see there’s been a huge boom, in terms of mammalian diversity, right after the border, or right around that border — depending on when you think about it. [placental] “Mammals really did originate,” said 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 pace of mammalian evolution subsequently declined, as some studies previously indicated.

But crucially, the study indicates that this is punctuated by smaller and smaller peaks of diversity – just as the height of ripples decreases with distance when a pebble is thrown into a pond.

“This is a completely new paradigm for evolution,” Goswami said.

She noted that it is possible that these recurring peaks are related to climatic events that open new opportunities for mammals. She said their dwindling nature over time is probably due to such niches becoming increasingly occupied by existing species.

The team wrote in the journal Science that, among other discoveries, herbivores evolved faster than carnivores and social animals more quickly than solitary animals.

The first reason, 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 eat whatever animal is out there regardless of 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 an animal that looks like a shrew,” Goswami said.

In an accompanying article, experts from the University of Washington note that the focus on skulls is strong because they have many functions and features. What’s more, they reflect many different adaptations, and Joswani said the study provided insights for a world in which climate is changing rapidly, helping to identify which animals may be most vulnerable.

And when vast areas of biodiversity are lost, she added, all species may be at risk – as was the case when an asteroid hit that killed the dinosaurs.

“It was a mammal squeak, too,” she said. “I mean, it’s really just luck 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, called the study very ambitious and impressive, noting that the main finding was that rates of skull evolution in placental mammals rose around the time that dinosaurs. disappear.

“Before that time, mammals were background characters in the dinosaur drama, slowly but surely evolving in the shadows, swaying.

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

“Now they have suddenly found themselves in a world without dinosaurs, no longer controlled by T-Rex and Triceratops, and they have responded by rapidly evolving many new types of skulls, enabling them to eat new foods, behave in new ways and settle in new environments.”

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