EXCEPTIONALLY PRESERVED SKELETONS FROM THE LATE CAMPANIAN OF MONTANA PROVIDE A UNIQUE GLIMPSE INTO THE PALEOBIOLOGY OF MULTITUBERCULATES

Multituberculates were among the most taxonomically diverse and numerically abundant mammals in Late Cretaceous terrestrial faunas of the northern hemisphere. In North America, they are known almost exclusively from isolated teeth and a handful of jaws, whereas the few associated skeletons of Late Cretaceous multituberculates are known from Mongolia, China, and Romania. Here we report on exceptional new multituberculate specimens, which we provisionally refer to Cimexomys judithae, from the late Campanian-age (ca. 75.5 Ma) Egg Mountain (EM) locality of the Two Medicine Formation in western Montana, USA. The multituberculate remains at EM consist of monospecific, multi-individual aggregates of semi-articulated craniodental and postcranial elements. In one instance, five individuals are preserved within <1 m², represented by five partial skulls with associated lower jaws, two partial pectoral girdles with one articulated forelimb, two articulated pelvic girdles and partially articulated hind limbs, and numerous semi-articulated to disarticulated postcranial remains in close association. Both adult and subadult individuals occur in these aggregates; adult specimens exhibit fused distal epiphyses, fused cranial sutures, and heavily worn cheek teeth, whereas subadults exhibit unerupted incisors, erupting premolars, unfused distal epiphyses, unfused cranial sutures, and unworn cheek teeth.

The presence of multiple well-preserved individuals of varying ontogenetic stages, to the exclusion of any other vertebrate taxa, suggests that these animals aggregated in life and died in close proximity to one another. The sedimentology and diverse invertebrate ichnology of EM indicate that the locality was strictly terrestrial; thus, these animals may have come together in either an above-ground nest or a burrow. The latter scenario is supported by: (1) evidence for low sedimentation rates with extensive bioturbation at EM, (2) postcranial features of C. judithae indicative of scratch-digging capabilities, and (3) multivariate analyses of linear measurements taken on the appendicular skeleton, which suggest a semifossorial locomotor mode. Taken together, these new specimens provide an unprecedented look at the paleobiology of multituberculates during the Late Cretaceous in North America.