The Upper Jurassic Ulaan Malgait Beds in the Shar Teg locality of southwestern Mongolia have yielded remains of a new tritylodontid therapsid (Synapsida), Bienotheroides shartegensis sp. nov. The specimen consists of a fragmentary skull associated with lower jaws. It is assigned to Bienotheroides based on its short snout, a premaxilla-palatine contact, very reduced maxilla, relatively rounded corner of upper postcanine teeth (PC), and PC cusp formula of 2-3-3. It differs from the other species of Bienotheroides in having a much more reduced middle mesial cusp of PC. It further differs from B. zigongensis and B. ultimus in having shorter and wider PC, from B. ultimus in lacking a projection at the middle mesial margin of PC, and from B. wansienensis in lacking the vestigialmost mesiobuccal cusp of PC and in lacking a diastema between upper I1 and I2. This is the first discovery of the Tritylodontidae in Mongolia. This discovery extends the taxonomic (morphological) diversity and geographic range of Bienotheroides and underlies the success of the genus in the Middle to Late Jurassic biota of eastern Eurasia.

Key words: Synapsida, Tritylodontidae, Bienotheroides, Jurassic, Gobi Desert, Mongolia.

Mahito Watabe moldavicum@pa2.so-net.ne.jp, and Takehisa Tsubamoto sorlestes@msc.biglobe.ne.jp, Center for Paleobiological Research, Hayashibara Biochemical Laboratories, Inc., Okayama 700-0907, Japan ; Khishigjav Tsogtbaatar paleolab@magicnet.mn, Mongolian Paleontological Center, Mongolian Academy of Sciences, Ulaanbaatar 46, Mongolia.