Abstract

Mosasaurs were large, carnivorous aquatic lizards with a global distribution that lived during the Late Cretaceous. After two hundred years of scientific study, new mosasaur species are yet discovered as new localities are explored and specimens collected long ago are re–evaluated using modern standards of species delimitation. Even so, the phylogenetic positions of many key taxa are unresolved and therefore our understanding of mosasaur macroevolution is muddled. Here, we describe a new genus and species of mosasaurine mosasaur comprising a partial skull and skeleton from the Pembina Member of the Pierre Formation in Cavalier County, North Dakota. The lower bound on the age of the specimen is 80.04 ±0.11 Ma, provided by the underlying Q3 bentonite bed. Its skull and jaws are nearly complete, and the postcranial skeleton includes seven cervical vertebrae, three hypapophyses, eleven ribs, and five anterior dorsal vertebrae. The new specimen was scored into a modified version of an existing phylogenetic matrix of Mosasauroidea and was recovered in a polytomy with Clidastes; however, given that its morphology is significantly different from that of Clidastes, we refer it to a new genus and species, Jormungandr walhallaensis. Notably, this new taxon shares a mosaic of features seen in both basal (e.g, Clidastes; high dental counts) and derived (e.g., Mosasaurus; subrectangular quadrate) mosasaurines, in addition to possessing its own unique suite of autapomorphies. Given that it possesses morphology intermediate between Clidastes and Plotosaurini, we suspect that future analyses of mosasaur phylogeny, following the addition of new characters and taxa, will recover Jormungandr as transitional between them. Its occurrence increases the known diversity of mosasaurs known from the Pembina Member, extends the northern range of Pierre Formation mosasaurines, and is the earliest mosasaur to possess autapomorphies of Plotosaurini. Finally, we also analyzed the matrix using different outgroups, and found that including multiple non–mosasauroid anguimorphs increased resolution, but not support, of mosasaurid ingroup relationships.