The skull of Hapalodectes (Hapalodectidae, Mesonychia) from the Paleocene of Mongolia

The incomplete skull of Hapalodectes (Hapalodectidae, Mesonychia) is described from the Upper Paleocene Naran Member of the Naran Bulak Formation of the Khaychin-Ula 2 locality in the Bugin Tsav Basin in the south of Mongolia. This is the first discovery of the skull of a Paleocene member of the genus. The skull corresponds in size and dental occlusion to the species H. dux Lopatin, 2001, previously known only from a lower jaw from the Upper Paleocene Zhigden Member of the Naran Bulak Formation of the Tsagan-Khushu locality in the Nemegt Basin. In the structure of the facial part of the skull, H. dux exhibits features shared with Mesonychidae, namely the significant participation of the lacrimal and jugal in the preorbital region. The upper molars of H. dux were found to have distinct conules (a first for Hapalodectidae). The evolutionary changes in the structure of the upper molars of Hapalodectes (disappearance of the conules, decrease of the metacone, and narrowing of the lingual lobe) are determined. They were correlated with the transformations of the lower molars (reduction of the metaconid, protocristid, and entoconid, transverse compression of the trigonid and talonid) and together with them were aimed at enhancing the shearing action of the longitudinal trenchant blades of the posterior part of the dentitions.