Osteology and taxonomy of Mosasaurus conodon Cope 1881 from the Late Cretaceous of North America

Two well-preserved skeletons of Mosasaurus conodon Cope 1881 (Squamata, Mosasaurinae) from the Pierre Shale (late Campanian) of Colorado and the Bearpaw Shale (Late Campanian to Early Maastrichtian) of Montana are described. The two specimens are important because they provide new osteological information, especially on the skull (including jaws with teeth) and forelimbs, whereas those elements are largely missing in the holotype (AMNH 1380) of M. conodon. Morphological comparisons of the holotype with the two new specimens allow us to emend the diagnosis of the species in the genus Mosasaurus, primarily using tooth and forelimb morphologies. Teeth of M. conodon are unique in their combination of having a slender, gently recurved overall shape (similar to Clidastes) with no serration on the developed carinae (less developed in Clidastes). The tooth count of M. conodon tends to be low (14–15 in the maxilla, 16 in the dentary and eight in the pterygoid, respectively) when compared to other species, such as Mosasaurus lemonnieri, Mosasaurus missouriensis and Mosasaurus hoffmanni–Mosasaurus maximus. The forelimb is short in the species, characterised by a much lower number of the manual digital formula, 4(+1?)–4(+2?)–4(+1?)–4(+1)–2 than other species of Mosasaurus. The forelimb bones are generally robust, especially the box-shaped humerus (width-to-length ratio 3/2). A variety of new morphological data support the conclusions that (1) M. conodon is a nominal species, (2) the European species M. lemonnieri is not a junior synonym and (3) one of the most complete skeletons of Mosasaurus from South Dakota (SDSM 452) is not assigned to M. conodon (but is likely to be Mosasaurus sp.). To date, M. conodon occurs only in North America during the late Campanian to early Maastrichtian.