We provide a new morphological description of the holotype and a previously undescribed large sample of the type species of the genus CopemysWood, 1936, Copemys loxodon (Cope, 1874a), a Miocene North American cricetid rodent occasionally treated as the ancestor of the non-arvicoline cricetids (i.e., neotomines, sigmodontines, and tylomyines). Dental, mandibular, and skull morphology is described from material recovered in a quarry in the vicinity of the type locality in the Española Valley of New Mexico, U.S.A. Copemys loxodon is a small, brachydont cricetid characterized by a mandible with a large mental foramen lateral in position and an enlarged retromolar fossa; the skull having wide infraorbital foraminae and robust zygomatic arches; and the upper molars being pentalophodont, the lower molars tetralophodont. The procingulum of the M1 has two conules, an enamel atoll is present on the M2, and the procingulum of m1 is composed exclusively of the lingual conulid. Preliminary comparisons among representatives of the 11 species of Copemys suggest that some forms currently recognized as Copemys probably do not belong to that genus, as indicated both by our results and those of other recent studies. Understanding the limits of variation in the type species of Copemys provides a starting point for the development of a more detailed phylogenetic analysis of early North American cricetids and their possible descendants.
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