- Copyright © Cambridge University Press 2016
The large Late Cretaceous marine reptile Mosasaurus has remained poorly defined, in part owing to the unorthodox (by today's nomenclatural standards) manner in which the name was erected. The lack of a diagnosis accompanying the first use of either the genus or species names allowed the genus to become a catchall taxon, and subsequent diagnoses did little to refine the concept of Mosasaurus. We herein present emended diagnoses for both Mosasaurus and the type species M. hoffmannii, based solely on personal examination of the holotype, and a description of the type species based on personal examination of many specimens. Mosasaurus exhibits a premaxilla with a short, conical edentulous rostrum, a maxilla with little to no dorsal excavation for the external naris, posteromedial processes of the frontal that deeply invade the parietal, a quadrate taller than long with a short suprastapedial process and the stapedial pit dorsal to the mid-height of the shaft, an angular that is laterally visible for only a short length of the post-dentary unit, a very tall surangular, a humerus with the postglenoid process robust and offset and a distal width greater than the length, and a pubis with an anteriorly projecting tubercle. M. hoffmannii is distinguished from other species assignable to the genus by the anteroventral corner formed on the tympanic rim of the quadrate, the asymmetric carinae of the anterior marginal teeth dividing the tooth circumference into short labial and long lingual segments, and the proximal and distal expansion of the femur.