Medusaceratops

Extinct genus of dinosaurs

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.

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. 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.

Two partial parietals (frills) that are housed at the Wyoming Dinosaur Center were chosen as the type material of Medusaceratops, including the holotype WDC DJR 001 and the paratype WDC DJR 002. Although all the chasmosaurine material from the Mansfield Bonebed was thought to be referable to Medusaceratops, which totals at several hundred individual elements, upon its original description only these two partial frills have been scientifically described while the rest of the material was being reexamined. Many of the other elements are not diagnosable to a genus level and can only be confidently referable to Ceratopsidae.

Among valid ceratopsids from the Judith River Formation, Medusaceratops can be directly distinguished from centrosaurine Avaceratops, based on its unique frill ornamentation. It differs from chasmosaurine Mercuriceratops based on its less unique squamosal bone, as evident from newly described squamosal bones of Medusaceratops. All material previously assigned to the centrosaurine Albertaceratops from the formation is now assigned to Medusaceratops or considered too fragmentary. The cladogram presented below follows a phylogenetic analysis by Chiba et al. (2017) who redescribed Medusaceratops as a centrosaurine: