Landmark- and semilandmark-based geometric morphometrics were used to explore morphological variation in the occlusal surface and linea sinuosa (enamel-dentine junction) of first lower molars (m1s) of Ogmodontomys sawrockensis and O. poaphagus from the Meade Basin in southwestern Kansas. Morphological differences between the two species were determined in an effort to explore interspecific variation and to test the power of landmark geometry of dental variation for species discrimination. Significant differences were found between O. sawrockensis and O. poaphagus in both occlusal surface and the linea sinuosa patterns. Multivariate tests demonstrate that the most significant morphological changes occurred at the presumed O. sawrockensis-O. poaphagus speciation event. An intraspecific multivariate analysis of O. poaphagus m1s, however, also identified directional evolutionary tendencies in shape of both the occlusal surface and linea sinuosa.