Mammals appear to have been social creatures much earlier than previously thought, according to US palaeontologists.
The relative absence of sociality in egg-laying and marsupial mammals has led researchers to believe their ancestors led solitary lives until after dinosaurs became extinct around 66 million years ago – and that even then social behaviour occurred mostly in the Placentalia, the group of mammals to which humans belong.
That all changed, however, with the recent discovery of a new genus of multituberculate – a small, rodent-like mammal that lived during the Late Cretaceous of the dinosaur era – called Filikomys primaevus, or “youthful, friendly mouse”.
Fossils extracted from a well-known dinosaur nesting site called Egg Mountain in Montana, by a team led by the University of Washington, date back about 75.5 million years old.
The researchers found the skulls and skeletons of at least 22 individuals clustered together in groups of two to five, with at least 13 individuals found within a 30-square-metre area in the same rock layer.
Based on how well preserved the fossils are, the type of rock they’re preserved in, and the powerful shoulders of F. primaevus, the team believes these animals lived in burrows and were nesting together.
The fossils found were of a mixture of multiple mature adults and young adults, suggesting these were truly social groups, they say, as opposed to just parents raising their young.
“It is really powerful, I think, to see just how deeply rooted social interactions are in mammals,” says Luke Weaver, lead author of the team’s paper in Nature Ecology & Evolution.
“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 behaviour 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.”
Co-author Wilson Mantilla describes the fossils as “game changers”.
“As palaeontologists 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.”