Clidastes Cope, 1868

Lindgren, Johan & Siverson, Mikael, 2004, The first record of the mosasaur Clidastes from Europe and its palaeogeographical implications, Acta Palaeontologica Polonica 49 (2), pp. 219-234 : 221

Clidastes Cope, 1868

Type species: Clidastes propython Cope, 1869 , from the early Campanian part of the Mooreville Chalk Formation of the Selma Group in west−central Alabama, USA (see Kiernan 1992; ICZN Opinion 1750). Additional species: Clidastes liodontus Merriam, 1894 . The (recently) frequently cited C. “ moorevillensis ” is unfortunately still a nomen nudum ( Kiernan 2002). We have examined specimens of the latter from the Mooreville Chalk in Alabama, and they differ dentally from both C. liodontus and C. propython (see below).

Discussion.—The dental and vertebral morphology of Clidastes are closer to those of Mosasaurus Conybeare, 1822 than to any other mosasaurine. However, besides a marked difference in the maximum size of the teeth, marginal tooth−crowns of Campanian Mosasaurus , e.g., M. missouriensis ( Harlan, 1834) and M. conodon ( Cope, 1881) (the two oldest unquestionable Mosasaurus ), differ from those of Clidastes in having a larger number of facets, which are also generally far more distinct than they are in Clidastes .

Cervical vertebrae of Clidastes differ from those of Mosasaurus by their more elongated shape. The total vertebral length/condyle height relationship is about 2: 1 in the juvenile Clidastes cervical centra from the Kristianstad Basin. This agrees very well with the data from North American Clidastes and sets them apart from the relatively shorter cervical centra of Mosasaurus (see Caldwell and Bell 1995).

The bases of the chevrons on caudal vertebrae are laterally compressed in Mosasaurus and extend anteriorly further towards the edge of the centrum than they do in Clidastes (see Kuypers et al. 1998: pl. 9: 11, and personal observation of Mosasaurus ; RMM 3037, from the Demopolis Chalk of Alabama). In the latter, the bases of the chevrons are circular in cross−section, rather than antero−posteriorly elongated as they are in Mosasaurus .

The teeth from an undescribed mosasaurine in the B. mammillatus zone (referred to as a “ Liodon −like” species by Lindgren and Siverson 2002: 79, and to Dollosaurus Yakovlev, 1901 by Lindgren and Siverson 2003) are similar to those of the coexisting Clidastes in being more or less symmetrically bicarinate (does not apply to the anteriorly situated teeth in either of the two species) and lacking basal striations. They differ mainly in being much larger (up to about 40 mm high compared to 20 mm for Clidastes ), strongly recurved, having weakly serrated cutting edges (these are smooth in Clidastes ) and very poorly developed facets or, more commonly, lacking facets altogether. A fragmentary dentary (RM PZ R 1772) with associated teeth of the undescribed mosasaurine from the Maltesholm locality demonstrates that this skeletal element is very robust in this species. In contrast, the dentary is slender in Clidastes .