Filikomys primaevus: Rodent-Like Mammal from Cretaceous Period Lived in Colonies

Wednesday, November 4, 2020

Filikomys primaevus’ powerful shoulders and elbows suggest it was a burrowing mammal. Art by Misaki Ouchida.

Filikomys primaevus, a new genus and species of multituberculate mammal that lived during the Late Cretaceous period, has been identified from multi-individual aggregates of skulls and skeletons found at a dinosaur nesting site in Montana, the United States. The well-preserved fossils indicate that Filikomys primaevus engaged in multi-generational, group-nesting and burrowing behavior, representing the first example of social behavior in a Mesozoic mammal.

Multituberculates were mostly small-bodied, omnivorous or herbivorous mammals that lived between 166 and 35 million years ago, from the Middle Jurassic to the late Eocene.

Most species of multituberculates are generally recovered as loose teeth; preserved skulls and jaws are rare except in Mongolia.

They are known from North America and Europe — which were a single continent until the early Eocene — in the uppermost Triassic to the lower Oligocene, and from northern Asia in the upper Cretaceous and Paleocene.

“Multituberculata is 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,” said lead author Luke Weaver, a graduate student in the Department of Biology at the University of Washington.

An artistic reconstruction of a social group of Filikomys primaevus in a burrow while dinosaurs roam above. Image credit: Misaki Ouchida.

Filikomys primaevus roamed Earth during the Cretaceous period, about 75.5 million years ago.

The fossilized skulls and skeletons of at least 22 individuals were recovered from a well-known dinosaur nesting site called Egg Mountain in western Montana.

They were typically clustered together in groups of two to five, with at least 13 individuals found within a 30 m2 area in the same rock layer.

Based on how well preserved the fossils are, the type of rock they’re preserved in, and Filikomys primaevus’ powerful shoulders and elbows, Weaver and colleagues hypothesize these animals lived in burrows and were nesting together.

Furthermore, the ancient animals were a mixture of multiple mature adults and young adults, suggesting these were truly social groups as opposed to just parents raising their young.

“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 said.

Skeletal elements of the five Filikomys primaevus individuals from Egg Mountain, Montana, the United States. Scale bars – 1 cm. Image credit: Misaki Ouchida / Weaver et al., doi: 10.1038/s41559-020-01325-8.

Previously, paleontologists thought social behavior in mammals first emerged after the mass extinction that killed off the dinosaurs, and mostly in the Placentalia — the group of mammals humans belong to, which all carry the fetus in the mother’s uterus until a late stage of development.

But Filikomys primaevus shows mammals were socializing during the Age of Dinosaurs, and in an entirely different and more ancient group of mammals — the multituberculates.

“These fossils are game changers,” said senior author Dr. Gregory Wilson Mantilla, a researcher in the Department of Biology at the University of Washington and the Burke Museum of Natural History & Culture.

“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 discovery of Filikomys primaevus is reported in a paper in the journal Nature Ecology & Evolution.


L.N. Weaver et al. Early mammalian social behaviour revealed by multituberculates from a dinosaur nesting site. Nat Ecol Evol, published online November 2, 2020; doi: 10.1038/s41559-020-01325-8