A sort of rodent called Filikomys primaevus. Eurekalert offers an artist’s concept:
Misaki Ouchida
The evidence lies in the fossil record of a new genus of multituberculate—a small, rodent-like mammal that lived during the Late Cretaceous of the dinosaur era—called Filikomys primaevus (translates to “youthful, friendly mouse”). The fossils, which are the most complete mammal fossils ever found from the Mesozoic in North America, indicate that F. primaevus engaged in multi-generational, group-nesting and burrowing behavior, and possibly lived in colonies. University of Washington Biology and Burke Museum Graduate Student Luke Weaver, UW Professor of Biology and Burke Museum Curator of Vertebrate Paleontology Gregory Wilson Mantilla, and co-authors analyzed several fossils, extracted from a well-known dinosaur nesting site called Egg Mountain in western Montana, that are about 75.5 million years old.
Fossil skulls and skeletons of at least 22 individuals of F. primaevus were discovered at Egg Mountain, typically clustered together in groups of two to five, with at least 13 individuals being 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 F. primaevus’ powerful shoulders and elbows that are similar to today’s living burrowing animals, Weaver, Wilson Mantilla, and co-authors hypothesize these animals lived in burrows and were nesting together. Furthermore, the animals found were a mixture of multiple mature adults and young adults, suggesting these were truly social groups as opposed to just parents raising their young.
News, “New Study Finds Earliest Evidence for Mammal Social Behavior” at Burke Museum
We learn more from bones all the time, especially as more of them are found. Some dinosaurs appear to have lived in social groups (herds) too. It prompts one to wonder, at what point did truly social behavior arise?