We describe Dissacus zanabazari new species from a partial skeleton collected from the early Eocene Bumban Member of the Naran Bulak Formation at Tsagaan Khushuu (Omnogov Province, Mongolia). The holotype includes most of the skull with basicranium, mandibles, well preserved upper and lower dentitions, partially articulated left manus and right tarsus, and most of the long bones in the limbs. The presence of a rudimentary 1st metatarsal is confirmed in mesonychids, and the relatively unworn lower incisors display an unexpected trilobed morphology. Autapomorphies of this new species are short face, absence of diastemata between the lower premolars (except between p1 and p2), m3 metaconid subequal to protoconid, and foramen for superior ramus of stapedial artery entirely within the petrosal. A phylogenetic analysis of 89 characters scored for 14 mesonychians and 5 outgroups resulted in 8 most parsimonious trees. Dissacus zanabazari is in a clade with D. navajovius, but this genus is otherwise paraphyletic. The strict consensus of the eight trees has a monophyletic Mesonychia, Hapalodectidae, and Mesonychidae; Dissacus and Ankalagon as the most basal mesonychid genera; and paraphyly of Pachyaena.

Key words: Mammalia, Mesonychia, Mesonychidae, Dissacus, Eocene, Bumbanian, Tsagan Khushuu, Mongolia.

Jonathan H. Geisler geislerj@georgiasouthern.edu, Department of Geology and Geography and Georgia Southern Museum, Georgia Southern University, Statesboro, GA 30460, and Research Associate, Division of Paleontology, American Museum of Natural History; Malcolm C. McKenna m4pmck@indra.com, Curator Emeritus, Division of Paleontology, American Museum of Natural History, and Adjunct Professor, Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071.