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 (~9 cm thick) and geographic (<32 m²) 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.

社会性何时进化以及哪些群体在哺乳动物进化中仍然是悬而未决的问题，这主要是由于零碎的中生代哺乳动物化石记录。尽管如此，在严格约束的地质和空间框架中收集的保存完好的化石可以让我们一瞥早期哺乳动物行为的这些更短暂的方面。在这里，我们报告了多结核 Filikomys primaevus gen 的特殊标本。11 月，来自蒙大拿州晚白垩世，主要作为多个体、单种的半铰接头骨和骨骼聚集体出现在狭窄的地层（约 9 厘米厚）和地理（<32 米²）间隔内。埋藏地学和地质学证据表明F. primaevus从事多代、群体筑巢和穴居行为，代表了中生代哺乳动物社会行为的第一个例子。F. primaevus是一个挖掘者，这得到了其后颅骨的功能形态学和形态测量分析的进一步支持。F. primaevus的社会行为表明，哺乳动物形成社会群体的能力可以追溯到中生代，并不仅限于 Therians。社会性可能是在哺乳动物进化过程中多次出现的进化不稳定特征。