The largest documented cimolestid, Alveugena carbonensis new genus and species, is both morphologically and temporally intermediate between small, early cimolestids (such as Procerberus and Cimolestes) and the earliest documented conoryctid taeniodont Onychodectes tisonensis; this represents a transition between the suborders Didelphodonta and Taeniodonta. Diagnosis and description of A. carbonensis is based upon a partial skull and two isolated upper molars recovered from fluvial sandstones at UW locality V-91005, in upper parts of the Ferris Formation, western Hanna Basin, Wyoming. An earliest middle Puercan age for UW locality V-91005 is based upon: presence of taxa that are morphologically intermediate between characteristic early and middle Puercan species; presence of a species of Ectoconus morphologically more primitive than Ectoconus ditrigonus; and co-occurrence of taxa characteristic of Pu1 and Pu2 stratigraphically below and above it. Morphologic trends in evolution of the upper dentition of cimolestids reflect a relative broadening of the crown. A trend toward increasing body size amongst Puercan cimolestids appears to have coincided with a change from a carnivorous (or perhaps insectivorous) to an omnivorous diet, suggested by increased grinding function of the cheek teeth. Cladistic analyses support A. carbonensis as the sister group to O. tisonensis. No known autapomorphies preclude A. carbonensis from having been a plausible ancestor to O. tisonensis.