Abstract
Ivanantonia efremovi Shevyreva 1989, known from the Tsagan Khushu locality (Early Eocene of Mongolia), erroneously has been described asOrogomys obscurus Dashzeveg 1990. This form shows an interesting association of primitive and derived characters: the enamel of the incisors is two-layered and has pauciserial Hunter-Schreger bands in portio interna; the lower tooth row has only three jugal teeth; the tooth pattern of lower molars is primitive and looks very similar to that ofAlagomys, but the trigonid is less reduced with regards to the talonid. It also differs from other primitive rodents in the distribution of wear facets, suggesting that mandibular propalineal movement was pre-eminent during chewing. Comparisons ofIvanantonia with several Eocene-Oligocene rodents indicate possible relationships to the Early Oligocene North American genusNonomys.
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