Fossil specimens of rodent-like multituberculate mammals, dated to 75.5 million years ago, including skeletons of multiple individuals burrowed together, suggest that mammals may have been social since the Mesozoic era. These findings are reported in a paper published this week in Nature Ecology & Evolution.
Although a large number of placental mammals are social today, the relative absence of sociality in egg-laying and marsupial mammals has led researchers to believe that the ancestors of mammals led solitary lives until after dinosaurs became extinct approximately 66 million years ago.
Lucas Weaver and colleagues describe deposits of small mammal bones found in Montana, United States, from multiple individuals of different generations buried together, dating to the late Cretaceous. The skeletons represent a new genus of rodent-sized multituberculate mammal the authors named Filikomys primaevus. The genus name comes from the Greek filikós, meaning friendly or neighbourly, because of the behaviour the authors interpreted from the fossils. F. primaevus had powerful legs well-adapted for digging, which allowed them to burrow together in multigenerational groups of up to five individuals. Based on the behaviour of living burrowing social mammals like rabbits, the authors suggest that the fossil individuals were related.
The authors conclude that the fossils provide evidence of social mammal behaviour more than 75 million years ago.
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