EXCEPTIONALLY WELL-PRESERVED MAMMAL FOSSILS FROM THE UPPER CRETACEOUS (CAMPANIAN) EGG MOUNTAIN LOCALITY (TWO MEDICINE FORMATION)

The Late Cretaceous represents a critical but poorly understood period in the evolution of mammals. Spectacular but rare specimens and dental analyses suggest ecological diversification among non-therian mammals, and molecular clock studies assert that many modern lineages originated during this time. However, analyses of fossil data do not support this claim for crown therians, and stem therians, although taxonomically diverse, appear to remain morphologically and ecologically uniform. A largely untested assumption is that these Mesozoic mammals were constrained to be generalized, small-bodied, nocturnal insectivores because of selective pressures (predation, competition) imposed by dinosaurs at this time. The Cretaceous record of mammals consists predominantly of isolated elements (e.g., jaws and teeth), occurring within allochthonous and time-averaged microsite assemblages. These records permit gross analysis of diversity and disparity through time, but they are inadequate to fully address the ecological factors potentially underlying these large-scale patterns.

Recent field work at the Egg Mountain locality, a dinosaur-nesting site in the Upper Cretaceous Two Medicine Formation of Montana, has yielded exceptionally well-preserved and largely autochthonous mammalian fossils. Here, we describe two new specimen blocks. One contains semi-articulated dental, cranial, and postcranial (distal humerus, radius, ulna, femur, tibia, fibula) remains, likely of a single individual of the multituberculate Cimexomys judithae. The other specimen block contains associated, disarticulated dental, cranial, and postcranial (ulna, hand) remains of at least three individuals of the marsupialiform Alphadon halleyi. Study of these specimens has just begun, but they hold potential to improve our assessment of the ecologies of these taxa, clarify our understanding of their phylogenetic relationships, and establish associations among their dental, cranial, and postcranial elements. Moreover, because Egg Mountain is situated in the Western Interior of North America among several intensely studied, often well-dated Late Cretaceous localities with large fossil sample sizes of isolated remains, these specimens will inform studies of these other sites.