A mosasaur from the Maastrichtian Fox Hills Formation of the northern Western Interior Seaway of the United States and the synonymy of <i>Mosasaurus maximus</i> with <i>Mosasaurus hoffmanni</i> (Reptilia: Mosasauridae)
Abstract We report here a large mosasaur skull, preserved three-dimensionally in a concretion recovered from Ziebach County, South Dakota, USA. This fossil represents the first articulated mosasaur skull from the Trail City Member of the Fox Hills Formation and the first definitive occurrence of Mosasaurus hoffmanni Mantell, 1829 from that area and the northernmost occurrence in the Western Interior Seaway, greatly extending the paleobiogeographic range of this taxon. The age of this specimen is determined to be between 68.3 and 67.6 Ma based on the associated invertebrate fauna. Although previous authors have suggested synonomy of the North American Mosasaurus maximus Cope, 1869 and the European M. hoffmanni, this is the most comprehensive analysis to date and is based on comparisons with Mosasaurus specimens recovered across the Northern Hemisphere, allowing an emended diagnosis of the species M . hoffmanni. Minor differences are considered individual variation or to reflect ontogenetic stage, including slender dentaries in some individuals, range of development of the C-shaped notch of the coronoid and differences in the shape of the supratemporal fenestra.