Seventy-six million years ago, a group of small mammals huddled in a burrow in what’s now Montana. 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). That’s a big deal because it’s commonly thought that social behavior didn’t arise in mammals until after the death of the dinosaurs, 10 million years after these small critters hung out together. And so the fact we’re finding these multituberculate mammals—a totally unrelated group of mammals—exhibiting social behavior means this was probably not uncommon among these early Mesozoic mammals. And it changes the narrative that sociality is somehow unique to placental mammals.”Even today, social behavior is relatively rare among mammals.