Medusaceratops is an extinct genus of centrosaurine ceratopsian dinosaur known from the Late Cretaceous Judith River Formation (middle Campanian stage) of Montana, northern United States. It contains a single species, Medusaceratops lokii.
Discovery The known material of Medusaceratops came from a bonebed in the badlands on the west side of Kennedy Coulee adjacent to the Milk River, in the Milk River Natural Area, near Havre, Hill County of Montana. The material was first reported by Sweeney and Boyden (1993), who considered it to represent the southernmost occurrence of Styracosaurus albertensis, based on misidentified frill spikes. Trexler and Sweeney (1995) reinterpreted the spikes as eye-socket horncores and noted their similarity to those of the nomen dubium Ceratops montanus from a nearby area, however, could not refer the bonebed material to any valid existing taxon. The bonebed, known as the Mansfield Bonebed honoring its landowner, is located on private land and historically has been excavated by several commercial companies. The type material of Medusaceratops and other specimens were excavated more recently and have been purchased by the Wyoming Dinosaur Center from Canada Fossils, Ltd., of Calgary, Alberta. Additional material from the same excavation was purchased and accessioned by the Royal Tyrrell Museum of Palaeontology. Canada Fossils, Ltd., also assembled two composite skeletons using the Mansfield Bonebed material which are in the collections of the Wyoming Dinosaur Center and the Fukui Prefectural Dinosaur Museum, however neither of the casts has an exact reconstruction of Medusaceratops as it was later described. The name Medusaceratops was coined by Canadian paleontologist Michael J. Ryan of the Cleveland Museum of Natural History in 2003 in a dissertation. Its fossils were subsequently confused with those of Albertaceratops, another centrosaurine ceratopsian from Alberta which had been described by Ryan in 2007. Later, Ryan realized that the Mansfield Bonebed fossils did not belong to Albertaceratops. Medusaceratops was formally described and name by Michael J. Ryan, Anthony P. Russell and Scott Hartman in 2010 and the type species is Medusaceratops lokii. The generic name refers to Medusa, a monster from Greek mythology whose "hair" consists of snakes and its gaze could turn men to stone, alluding to a unique trait of this genus - the large, thick snake-like spikes that extend to the sides of the frill, in combination with Latinized Greek ceratops, meaning "horned-face", which is a common suffix for ceratopsian genera names. The specific name lokii honors Loki, a troublemaking god in the Norse mythology, in reference to the years confusion that surrounded the taxonomic designations of the Mansfield Bonebed material before it was given its own name. The Mansfield Bonebed material was collected from the upper part of the Judith River Formation, in a region where it is lithologically equivalent to the Oldman Formation of Canada. The bonebed is located at approximately the same level as the holotype of Albertaceratops, dating to 77.5 million years ago, to the middle Campanian stage of the Late Cretaceous. Thus, Medusaceratops was considered to represent the oldest known chasmosaurine, until the naming of Judiceratops by Longrich (2013), also from the Judith River Formation, but from an area equivalent to the lower Oldman Formation or upper Foremost Formation. However, Ryan had already indicated that part of the Mansfield material represented not a chasmosaurine, but a centrosaurine. Chiba et al. in 2017 described new material of Medusaceratops from the Mansfield Bonebed, found in 2011 and 2012 by David Trexler, indicating the presence of traits that were characteristic of Centrosaurinae in the skeleton of M. lokii. They concluded that all the material could be referred to a single species. The phylogenetic analysis conducted by the authors indicated that Medusaceratops was not a member of Chasmosaurinae after all, but rather an early centrosaurine ceratopsid that was more closely related to Centrosaurini and Pachyrhinosaurini than Nasutoceratopsini.
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