Mammal social behavior dates back to the Age of Dinosaurs

The earliest evidence for mammal social behavior dates back to the Age of Dinosaurs, according to a new study led by paleontologists at the University of Washington Burke Museum of Natural History & Culture. This evidence is tied to the discovery of a small mammal that lived during the Late Cretaceous period.

The previously unknown species was part of an extinct group of rodent-like mammals known as multituberculates. The new mammal has been named Filikomys primaevus, which translates to “youthful, friendly mouse.” 

The fossils examined for the study are the most complete mammal fossils ever found from the Mesozoic era in North America. The analysis suggests that F. primaevus engaged in group nesting and burrowing behavior, and possibly lived in colonies. 

Dating back about 75 million years, the fossils were recovered from a dinosaur nesting site called Egg Mountain in modern-day western Montana.

The excavation produced skulls and skeletons of at least 22 F. primaevus individuals that were typically clustered together in groups of two to five. About 13 individuals were found in the same rock layer within a 30 square-meter area. 

The researchers noted that the ancient mammal had powerful shoulders and elbows that are similar to those of burrowing animals that exist today. The experts theorize that the ancient animals also lived in burrows and nested together. 

The combination of F. primaevus individuals found clustered together included multiple mature adults and young adults, which indicates that these were truly social groups as opposed to just parents raising their young.

“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 study lead author Luke Weaver. 

“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.” “Multituberculates are one of the most ancient mammal groups, and they’ve been extinct for 35 million years, yet in the Late Cretaceous they were apparently interacting in groups similar to what you would see in modern-day ground squirrels.”

It was previously assumed that social behavior first emerged in mammals after the mass extinction that killed off the dinosaurs. 

Social behavior was also thought to have initially appeared in the group Placentalia – mammals that carry the fetus in the mother’s uterus until a late stage of development, including humans. However, the new fossils show that an entirely different group of mammals was socializing during the Age of Dinosaurs.

“These fossils are game changers,” said study senior author Gregory Wilson Mantilla. “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. We can now credibly look at how mammals really interacted with dinosaurs and other animals that lived at this time.”

The study is published in the journal Nature Ecology & Evolution.

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By Chrissy Sexton, Earth.com Staff Writer