Many mammals work together in packs to stay alive. Since reptiles don't do this, it seems likely the first mammals weren't social creatures, raising the question of when this important trait arose. New evidence suggests it dates back to at least the late Cretaceous.

A newly discovered mammal from 75.5 million years ago in western Montana is described in Nature Ecology and Evolution. Named Filikomys primaevus, or “youthful, friendly mouse”, the new find represents the most complete pre-asteroid mammal fossils in North America. More significantly, the clustering of specimens indicate F. primaevus nested in multi-generational burrows.

"It was crazy finishing up this paper right as the stay-at-home orders were going into effect — here we all are trying our best to socially distance and isolate, and I'm writing about how mammals were socially interacting way back when dinosaurs were still roaming the Earth!" said University of Washington graduate student Luke Weaver in a statement. 

Birds and reptiles may nest close together for safety or easy mate-finding, but seldom help raise each other's young. Some mammals are similarly solitary but many prefer to work collectively, a vital feature of human success.

Yet prior to Weaver's discovery, it appeared this was a relatively late development in mammalian evolution. Beneficial as social grouping can be, it takes a lot of brain power to distinguish friend from foe and treat each appropriately.

F. primaevus's strong shoulder muscles suggest it was a burrower. Some of the co-located fossils could be parents and young, others suggest multiple adults raising their offspring communally, for example three adults buried with two juveniles whose teeth suggest they were weaned. Most remarkably, the colony long precedes the first appearance of placental mammals, thought to have originated vertebrate social behavior.

"These fossils are game changers," said senior author Professor Wilson Mantilla, a UW biology professor and curator of vertebrate paleontology at the Burke Museum. "As paleontologists working to reconstruct the biology of mammals from this time period, we're usually stuck staring at individual teeth and maybe a jaw that rolled down a river, but here we have multiple, near complete skulls and skeletons preserved in the exact place where the animals lived.”

"It is really powerful, I think, to see just how deeply rooted social interactions are in mammals. Because humans are such social animals, we tend to think that sociality is somehow unique to us, or at least to our close evolutionary relatives, but now we can see that social behavior goes way further back in the mammalian family tree,” Weaver added. 

Thirteen of the 22 F. primaevus fossils found so far were in a 30 square meter (320 square feet) area in a layer of rock just 9 centimeters (3.5 inches) thick. It is thought likely, but not certain, all died together in some calamitous event. The Egg Mountain site where they were found has produced abundant dinosaur fossils, some of which no doubt hunted the friendly mice.