A new large zalambdalestid mammal from the Gobi Desert and formal stratigraphic description of the Upper Cretaceous Zos Formation

We describe Zoslestes gongori , a new species of non-placental eutherian mammal from Upper Cretaceous rocks of Zos Canyon in the Nemegt Basin of the western Gobi Desert, Mongolia. Zoslestes gongori is the largest known zalambdalestid. The Zos Canyon locality, Red Rum, that yielded the specimen is adjacent to beds that have produced several extinct crocodyliforms and the ornithischian dinosaur Haya griva . We provide the first formal stratigraphic description of these fossiliferous Zos Canyon rocks by naming and describing the Zos Formation for the unit at Red Rum. The Zos Formation has a distinct set of sedimentologic associations that differentiate it from the overlying Djadokhta Formation and also has the only fauna in the Nemegt Basin older than that of the Djadokhta Formation. We consider the Zos Formation roughly comparable in age to the Javkhlant Formation of the Eastern Gobi Desert, and likely older than the Alagteeg Formation of the Ulan Nur Basin. Zoslestes gongori is known from the holotype, a well-preserved posterior cranium, worn dentition, and fragmentary postcranial elements, and one referred specimen. The referred specimen consists of a damaged anterior skull and dentition, and a fragmentary left mandible with roots and alveoli, including an enlarged tooth resembling the elongate first lower incisor of zalambdalestids. Zoslestes shares several derived features with zalambdalestids, including the presence of a prominent entoglenoid process that, in ventral view, partly conceals the anteriorly-positioned postglenoid foramen, a large maxillary tuberosity, multiple hypoglossal foramina, a large, elongate i1 with a horizontally directed root extending deep within the mandible, lower molars with mesiodistally compressed trigonids and relatively wide talonids, a relatively large P4, and upper molars that are distinctly wider labiolingually than long mesiodistally and that are not interlocked mesiodistally. The significantly large size of Zoslestes and the large contribution of its jugal to the zygomatic process are autapomorphies distinguishing this taxon from other zalambdalestids.