New materials of Desmatolagus gobiensis from Ulantatal, Nei Mongol provide morphological evidence for the origin of Ochotonidae (Lagomorpha, Mammalia).

We report the first skull of Desmatolagus gobiensis, along with well-preserved maxillae and mandibles, discovered from Early Oligocene strata in Ulantatal, Nei Mongol, China, with detailed morphological comparison with extant Ochotona and other fossil taxa with skulls reported. Our updated phylogenetic analysis supports the monophyly of the family Ochotonidae, with Desmatolagus placed at the most basal position of the family, confirming Desmatolagus as the earliest member of Ochotonidae, as also evidenced by the present fossil record. Exploration of the morphological transitions in Ochotonidae suggests that major characters of extant Ochotonidae occurred at different evolutionary stages. Additionally, new juvenile specimens of Desmatolagus gobiensis from Ulantatal suggest unusual replacement of milk teeth, from the last to the first, as in extant Ochotona and Lepus.