AbstractNewly discovered mandibles and lower dentition of the middle Eocene rodentThisbemys brevicristafrom the Green River Basin, Wyoming, are the basis for a species re-diagnosis. Previously, only the upper dentition and a partial maxilla ofT. brevicristawere known from Br3. New specimens from Br2 now include the lower molars, additional upper molars, and maxillary fragments including a partial zygomatic arch that preserves the relationship of the arch to the first upper molar. In addition, the presence ofT. brevicristaat Br2 documents the co-existence ofT. brevicristawithT. perditus, T. nini, T. plicatus,andT. corrugatus. Formerly, the latter two species were differentiated primarily using stratigraphy. Now, morphology and size can also be used.Thisbemys brevicristais intermediate in size betweenT. plicatusandT. corrugatus, and can readily be distinguished from these species based on unique features of the upper and lower molars, which include an additional loph on the two anterior upper molars and a complete metalophid on m1-3. The original type specimen ofT. brevicristaappears to be lost, thus a neotype is designated as part of the re-diagnosis. Discovery and description of the lower dentition and mandibles ofT. brevicristaclarifies the alpha taxonomy of this species, and increases its utility for studies of phylogenetic relationships and for documenting Eocene mammalian diversity patterns.
