INTRODUCTION

Little is known regarding the fossil record of North American ochotonid lagomorphs. Seven species of ochotonids with rooted cheek teeth have been described from late Oligocene to late Miocene faunas: Hesperolagomys galbreathi (Clark et al. 1964), ?Desmatolagus schizopetrus (Dawson 1965), Russellagus vonhofi (Storer 1970, 1975), Hesperolagomys fluviatilis (Storer 1970, 1975), Gripholagomys lavocati (Green 1972), Cuyamalagus dawsoni (Hutchison and Lindsay 1974), and Oklahomalagus whisenhunti (Dalquest et al. 1996). These species are based largely on small samples of isolated teeth and are difficult to confidently assign to species, genus, or in some cases, to family. Dentally archaic ochotonids pose a particular challenge to workers because of dramatic changes in the size and shape of the occlusal surface of upper cheek teeth with wear (Bair 2007), and with only a few isolated teeth, distinguishing ontogenetic from taxonomic differences in morphology has proven problematic.

Hesperolagomys galbreathi, the type species of the genus, was first described from deposits of the Clarendonian Esmeralda Formation in Fish Lake Valley, Nevada, by Clark et al (1964), who noted its dental similarities to both Amphilagus fontannesi, a late Miocene European ochotonid, and Desmatolagus gobiensis from the Oligocene of Asia. At the time of its initial description, Hesperolagomys was arguably the most 'primitive' late Miocene ochotonid; Oreolagus (possessing many 'advanced' dental characters including rootless cheek teeth) was the only other North American Miocene ochotonid known. Clark et al (1964) described plesiomorphic characters of Hesperolagomys galbreathi including: (1) rooted cheek teeth, (2) buccal folds persisting on the occlusal surface of P4 and M1, and (3) talonids transversely narrower than trigonids in p4-m2. Apomorphic characters of Hesperolagomys noted by Clark et al (1964) are the unique position and emphasis of mental foramina, and "marked anterior projections of the talonids on p4-m2."

A slightly older species of Hesperolagomys, H. fluviatilis, was described by Storer (1970, 1975) from the Barstovian Wood Mountain Formation of south-central Saskatchewan, along with a new genus of dentally archaic North American ochotonid, Russellagus. Since then, Hesperolagomys galbreathi has been reported in Clarendonian faunas in Nebraska (Korth 1998) and Utah (Tedrow and Robison 1999), and Hesperolagomys fluviatilis has been reported from Barstovian localities in Nebraska (Voorhies 1990a, 1990b).

Part of a larger-scale project to document dental variation and revise the systematics and biochronology of North American Oligo-Miocene ochotonids, the purpose of this study is to describe a new species of Hesperolagomys from a large sample from the late Barstovian of Nebraska, and to review and revise Hesperolagomys. Previously undescribed topotypic specimens of Hesperolagomys galbreathi and H. fluviatilis and previously undescribed specimens from Nebraska allow taxonomic reassessment and more definite characterization of these taxa. Having many more specimens at my disposal than were available to previous authors, I can now document pronounced wear-related changes in the occlusal patterns of Hesperolagomys upper cheek teeth and can confidently distinguish Hesperolagomys from the commonly co-occurring ochotonid Russellagus. Both dental and skull morphology, along with size, distinguish the three species of Hesperolagomys, and advances in biochronology allow refinement of their chronologic ranges.