A New Chasmosaurine Ceratopsid from the Judith River Formation, Montana MICHAEL J. RYAN, ANTHONY P. RUSSELL, AND SCOTT HARTMAN A NEW CHASMOSAURINE CERATOPSID, Medusaceratops lokii, is described based on material collected from a bonebed in the Judith River Formation (Campanian) near Havre, Montana. Originally, all ceratopsid material from the bonebed was referred to the basal centrosaurine Albertaceratops Ryan 2007, the holotype of which was collected from the Oldman Formation of Alberta, Can- ada. Reassessment of several key features of parietals from the Montanan bonebed, including the number and shape of the preserved epiparietals, necessitates referral of at least some material from this site to the new taxon. Although the bonebed does include centrosaurine ele- ments (including a lateral parietal ramus and a squam- osal) that may eventually be referable to Albertaceratops, it appears to be dominated by elements of chas- mosaurine affinity. In addition to being the first un- equivocal occurrence of a Campanian-aged chasmosaurine ceratopsid in Montana, Medusaceratops lokii is also the oldest known Chasmosaurine ceratopsid (approximately 77.5 Ma). Introduction Until the early 2000s, Ceratopsidae was limited to 13 named genera in two clades ("subfamilies"), Centrosaurinae and Chasmosaurinae (Ryan and Russell 2001). The specimens were known exclusively from Upper Cretaceous sediments of the Western Interior Seaway of North America, with the ex- ception of the fragmentary taxon Turanoceratops (discussed by Chinnery-Allgeier and Kirkland this volume), which is from the Turonian of Uzbekistan (Nessov et al. 1989). Recent work (e.g., Ryan and Russell 2005; Ryan 2007; Lucas et al. 2006; and contributions to this volume) has significantly in- creased the number of ceratopsid taxa, and, taken together with new descriptions of basal neoceratopsians (see You et al. and Chinnery-AlIgeier and Kirkland this volume, for reviews), the number of described ceratopsians has more than doubled in the past decade. Although the Upper Cretaceous rocks of Montana have yielded a wealth of dinosaur taxa (more than two dozen: Trex- ler 2001; Weishampel et al. 2004), they have been surprisingly sparing in producing ceratopsid material, especially consid- ering the abundance of horned-dinosaur taxa known from the adjacent, and at least partially contemporaneous. Belly River Group in Alberta. Basal neoceratopsian material from Mon- tana includes Cerasinops, Prenoceratops, and Montanocera- tops from the Two Medicine Formation. Campanian-aged Montanan ceratopsids are all centrosaurines and include Ache- lousaurus (Sampson 1995), Einiosaurus (Sampson 1995) and Styracosaurus ovatus (Gilmore 1930; although this taxon is transferred to a new genus by McDonald and Horner this vol- ume) from the upper portion of the Two Medicine Formation, and Avaceratops from the Judith River Formation. Additional problematic, putative centrosaurine elements have been col- 181