Mosasauridae Gervais 1853

Mosasauridae Gervais, 1853 (figs. 54F, 55F)

DEFINITION: Halisaurus platyspondylus, Tylosaurus proriger, Mosasaurus hoffmanni, and all descendants of their last common ancestor.

DIAGNOSIS: Mosasaurids are diagnosed by two unambiguous synapomorphies: 247(1) absence of a functional sacrum and 275(1) presence of a flattened ‘‘hourglass-shaped’’ humerus.

COMMENTS: The definition of Mosasauridae implemented here follows that of Bell (1997) who divided mosasaurids into Halisaurus and Natantia. The sister-group relationship of Halisaurus and all other mosasaurids has been recovered in several analyses (DeBraga and Carroll, 1993; Caldwell, 1996; Bardet and Suberbiola, 2001; Bardet et al., 2003, 2005), but has been questioned recently (Bell and Polcyn, 2005; Polcyn and Bell, 2005). This problem will require further examination given the recent description of new taxa such as Russellosaurus coheni (Polcyn and Bell, 2005) and Dallasaurus turneri (Bell and Polcyn, 2005). Note, however, that although some existing phylogenetic studies of mosasauroids have sampled densely for species than the current analysis (Bell, 1997; Bell and Polcyn, 2005; Polcyn and Bell, 2005), they have limited their taxonomic sampling to mainly North American and European taxa. The current analysis benefits from more complete taxonomic sampling from all over the world (e.g., Lakumasaurus antarcticus from Antarctica and Goronyosaurus nigeriensis from Africa), but does not include many of the American species represented in earlier analyses. A more definitive phylogenetic and taxonomic analysis should include all of these data.