Description and revised diagnosis of Asia's first recorded pachycephalosaurid, Sinocephale bexelli gen. nov., from the Upper Cretaceous of Inner Mongolia, China

The first recorded pachycephalosaurid dinosaur from outside of North America, "Troodon" bexelli, was described from the Upper Cretaceous of Nei Mongol (Inner Mongolia) Autonomous Region, China, in 1953 based on a partial parietal dome. The holotype, and only, specimen has not been redescribed or figured since the original description and is currently considered lost. As a result, researchers have generally considered this taxon a nomen dubium. Here, we identify and describe two high- fidelity plaster casts of the holotype and assign them as plastotypes for this taxon. Examining these replicas allows for an updated comparative description and complete systematic revision of this enigmatic taxon and its inclusion within a phylogenetic analysis for the first time. The material is distinct from all other pachycephalosaur material and can be diagnosed by a single autapomorphy (a wide and deeply embayed posterior parietal margin) and a unique combination of characters, including lack of primary nodes on the parietal and a highly arched, fully roofed temporal chamber. A new genus, Sinocephale gen. nov. is established to receive this species as Sinocephale bexelli. Sinocephale bexelli is phylogenetically removed from Stegoceras (formerly "Troodon"), to which it was previously affiliated, and recovered as a pachycephalosaurine pachycephalosaurid.