Prognathodon stadtmani was discovered in the Campanian portion of the Upper Cretaceous Mancos Shale in western Colorado in 1975. Following its original description almost two decades ago, more material of the holotype was prepared. Those new elements include a quadrate, often considered one of the most critical bones for understanding mosasaur systematics, portions of the skull roof, posterior mandible, and braincase. Prognathodon stadtmani exhibits a suite of features that include both derived character states observed in later mosasaurines and primitive features present in earlier members of the clade, such as Clidastes. Although the quadrate resembles those of Prognathodon and Globidens in some respects, it lacks fusion between the suprastapedial process and ascending tympanic rim observed in those taxa. I ran a series of phylogenetic analyses using parsimony and Bayesian inference to understand the relationships of this taxon. Regardless of character selection or methodology, ‘Prognathodon’ stadtmani was not found to be monophyletic with the rest of the genus Prognathodon. Depending on method, it was recovered as the sister taxon to the rest of Globidensini, the sister taxon to Mosasaurini + Globidens, or an early diverging mosasaurine outside of both Globidensini and Mosasaurini. Owing to these results, I assign the type to a new genus, Gnathomortis, gen. nov., yielding the species Gnathomortis stadtmani, comb. nov. Furthermore, the rest of the genus Prognathodon and the clade Globidensini more broadly were not consistently recovered as monophyletic. Many characters of the quadrate and the dentition that typically united those clades are optimized as homoplastic based on my resulting topologies.