North American archaic ochotonids Hesperolagomys and Russellagus (Mammalia: Lagomorpha) and geometric constraints on the evolution of hypsodonty in lagomorphs

New specimens of Russellagus and Hesperolagomys (Mammalia, Lagomorpha, Ochotonidae), reexamination of type material, and new information from topotypic specimens provide a context for reassessment of these taxa. Hesperolagomys niobrarensis (new species) is described from a large sample from the late Barstovian (Miocene) of Nebraska. Cranial and dental characters of Hesperolagomys and Russellagus, including presence of a premolar foramen in the maxilla, place these genera within Family Ochotonidae. These taxa share characters including: rooted cheek teeth, talonids narrower than trigonids in lower molariform teeth and having an anterior projection, persistent crescentic valley in P4-M2, and p3 with deep buccal fold and slender trigonid. Hesperolagomys P3 is unique among North American lagomorphs. Hesperolagomys is smaller than Russellagus and has a deep, persistent lingual hypostria in P4-M2; Russellagus additionally differs from Hesperolagomys in having wider anterior projections of the talonid in lower molariform teeth and ventral border of posterior portion of horizontal ramus curves more strongly ventrad. The direction and magnitude of size changes in Russellagus vonhofi occur in apparent coordination with size changes in Hesperolagomys species in the Barstovian. Morphologic and taxonomic turnover in lagomorphs suggests dentally archaic forms (such as Russellagus and Hesperolagomys) may have been unable to develop hypselodont cheek teeth. I use a theoretical morphology and geometric model approach to investigate how the interaction of tooth shape and wear can cause occlusal size and shape changes in different types of hypsodont teeth, and potentially impose structural constraints on crown height. Modeled results of wear surface topography and dimensions closely correspond to observed patterns in P4s of Russellagus, Hesperolagomys, and lower incisors of the muskrat Ondatra. Model results on wear in theoretical tooth morphologies identify two major shape factors influencing wear: orientation of the wear surface ( incisor-like or cheek tooth-like ), and tooth curvature ( concentric or nonconcentric ). Modeled wear also suggests two geometric constraints on crown height. Potential tooth area limits crown height in some teeth with nonconcentric curvatures. Incomplete wear in any tooth can present severe constraints on increasing crown height, causing structurally untenable morphologies in very tall-crowned to hypselodont teeth