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This is Scientific American's 60-second science. I'm Christopher in Taguata.

76 million years ago, a group of small mammals huddled in a burrow in what's now Montana,

they were good diggers, most likely furry and petite.

They could sit comfortably in the palm of your hand. I mean, if you saw them running around today,

you probably think it looks like a small rodent of some sort of like a, you know, a chipmunk or a mouse or something like that.

Lucas Weaver is a mammal paleobiologist at the University of Washington.

These little creatures didn't belong to any of the three main mammal groups on the planet today, which are the placental mammals like us,

monotremes like the platypus, and marsupials like koalas and kangaroos. Instead, they belong to another

now extinct group called the multi-tuberculates. Their teeth is what really distinguishes them from

any other group of mammals. So they have these really bizarre molars with these multiple bumps on

the teeth, which is where they get their name multi-tuberculate just means many bumps.

Weaver and his colleagues have studied the fossilized skulls and skeletons of these animals,

dug up in Montana, and they've given them a name, Felicamis Primavis, friendly or neighborly mouse.

The details are in the journal Nature Ecology and Evolution.

Weaver says drought or climate change may have killed the animals, though it's hard to be sure,

but the critters were fossilized together in ways that suggest they sought out each other's company.

That's a big deal, because it's commonly thought that social behavior didn't arise in mammals

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