Lagomorphs from the late Eocene and Eocene-Oligocene transition of North America, apart from the abundant and fairly speciose Palaeolagus, are represented by some rare and enigmatic genera, including monospecific Chadrolagus and Litolagus, of uncertain infraordinal relationship. In this paper new specimens of lagomorphs from the Chadronian and Orellan of Montana and Wyoming are presented. They include Chadrolagus emryi from the early Chadronian of the Renova Formation, Beaverhead Basin (Montana), and the late Chadronian of the Dunbar Creek and Cook Ranch Formations (Montana), extending the stratigraphic range of this species to the entire Chadronian interval. Further, previously unreported material, originally collected by M.F. Skinner during his fieldwork in Wyoming (near the Chadronian-Orellan boundary in age), is described and figured. This includes an exquisitely preserved skull of Litolagus molidens, and Limitolagus roosevelti, gen. et sp. nov., represented by mandibular and dental material from the Chadronian-Orellan boundary. L. roosevelti shares many similarities in dental structure with Chadrolagus, but its size is close to Litolagus molidens. The information provided by newly described material and a revision of the type specimens and topotypic series of Chadrolagus emryi and Litolagus molidens allow for an extended diagnosis of these taxa and a preliminary phylogenetic analysis of Paleogene North American lagomorphs with Desmatolagus gobiensis as an outgroup. The skull of Litolagus molidens shows many derived characters: shortening of the palatal bridge and the reduction of its palatine portion, greater skull height, and an increase in the size of the auditory bullae. The greater basicranial angle and relatively wider choanae indicate increased cursorial adaptations. A cladistic analysis shows that there is no immediate ancestral relationship between Chadrolagus and Litolagus, and it supports an advanced phylogenetic position for Litolagus closer to Archaeolaginae, suggested by the cranial and dental morphology. Limitolagus is related closely to Chadrolagus and paraphyly of Palaeolagus is strongly suggested, due to a placement of P. burkei between Limitolagus and the clade composed of Archaeolagus Litolagus.
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Vol. 2013 • No. 3773