INTRODUCTION

The family Ochotonidae (Lagomorpha) includes 16 extinct genera and one extant genus (Ochotona). Studies indicate that the ochotonids show general evolutionary trends in the cheek teeth. These trends include a shift toward rootlessness and hypsodonty, increasing complexity in the enamel structure of the P³ and P₃, increasing depth of the hypostria of P⁴-M², and widening of the talonids of P₄-M₂ from narrow to wide (Erbajeva 1994).

Morphological examination of cranial and postcranial skeletons of both extinct and living taxa indicates that the main external features of ochotonids remained invariable for a long time (Gureev 1964; Polyakova and Erbajeva 1974; Erbajeva 1994). Current data suggest that the principal ochotonid adaptations developed in the Oligocene and have mostly been retained. Based on similar dental structures in fossil and modern pikas, it is likely that fossil forms had similar ecologic requirements (e.g., open landscapes, grass diet) (Gureev 1964; Erbajeva 1988; Mead and Spaulding 1995).