When sociality evolved and in which groups remain open questions in mammalian evolution, largely due to the fragmentary Mesozoic mammal fossil record. Nevertheless, exceptionally preserved fossils collected in well-constrained geologic and spatial frameworks can provide glimpses into these more fleeting aspects of early mammalian behaviour. Here we report on exceptional specimens of a multituberculate, Filikomys primaevus gen. nov., from the Late Cretaceous of Montana, primarily occurring as multi-individual, monospecific aggregates of semi-articulated skulls and skeletons within a narrow stratigraphic (similar to 9 cm thick) and geographic (<32 m(2)) interval. Taphonomic and geologic evidence indicates that F. primaevus engaged in multigenerational, group-nesting and burrowing behaviour, representing the first example of social behaviour in a Mesozoic mammal. That F. primaevus was a digger is further supported by functional morphological and morphometric analyses of its postcranium. The social behaviour of F. primaevus suggests that the capacity for mammals to form social groups extends back to the Mesozoic and is not restricted to therians. Sociality is probably an evolutionarily labile trait that has arisen numerous times during mammalian evolution.