A Stagodontid Marsupial from the Upper Cretaceous Hell Creek Formation of Montana

ATWATER, Amy L; Museum of the Rockies, Montana State University, Bozeman, MT. SCANNELLA, John B; Museum of the Rockies, Montana State University, Bozeman, MT.


The uppermost Cretaceous Hell Creek Formation (HCF) of Montana and surrounding regions preserves the remains of one of the last Mesozoic ecosystems in North America. Mammals from the HCF are primarily known from teeth and less commonly from other cranial and postcranial remains. Metatherians are a clade of mammals that includes marsupials and stem marsupials and those currently recognized from the HCF of Montana include pediomyids, alphadontids, glasbiids, deltatheridiids, herpetotheriids, and stagodontids. Stagodontids are an extinct family of carnivorous metatherian mammals that lived during the Late Cretaceous and reached relatively large body sizes. Here we describe a new stagodontid fossil from the upper half of the HCF in eastern Montana. MOR 10904 is a partial left dentary with a complete m4 and a partial m2. The m4 displays a reduced metaconid and strong development of the paraconid as well as an emphasis on prevallid shearing. The m4 paraconid is lingually positioned and appressed to the metaconid and this distalmost molar is medial to the coronoid process. The coronoid process is near vertical displaying an angle between 95 and 105 degrees. These characters indicate MOR 10904 is referable to the genus Didelphodon. However, MOR 10904 exhibits features which appear to distinguish it from previously described examples of this genus. MOR 10904 exhibits a less pronounced ventral boss for the insertion of the mandibular adductor and unique dental dimensions. This new specimen is further distinctive in having a shallower mandible height as measured from the base of the final molar compared to the known species of Didelphodon. MOR 10904 appears to most closely resemble D. coyi and may illustrate individual variation within the species; alternatively, it may represent a new species of Didelphodon. This specimen provides new insights into the morphology of Late Cretaceous metatherians and furthers our understanding of HCF ecosystems.