Re-characterization of Tylosaurus nepaeolicus (Cope, 1874) and Tylosaurus kansasensis Everhart, 2005: Ontogeny or sympatry?

Tylosaurus nepaeolicus (Cope, 1874), from the lower Smokey Hill Chalk upper Coniacian of Kansas is reassessed and compared to T. kansasensis Everhart, 2005, its sympatric contemporary from the same formation; both are compared to a later species from the upper Smoky Hill Chalk, T. proriger (Cope, 1869). Tylosaurus nepaeolicus (Cope, 1874) is virtually indistinguishable from T. kansasensis Everhart, 2005, and both show important similarities with T. proriger, particularly the smaller individuals of T. kansasensis. Many of the anatomical features of T. kansasensis are indicative of a juvenile stage based on comparisons to T. proriger. In addition to the aforementioned spatial and temporal sympatry between T. nepaeolicus and T. kansasensis, it is anatomically difficult to distinguish the two species from each other, with the few notable differences being ontogenetically variable, and possibly indicating allometric changes during post-embryonic development; in addition, T. nepaeolicus is known from fragmentary remains of very large individuals, while T. kansasensis is known from a small number of complete and recently collected skulls, though of much smaller size than the type materials of T. nepaeolicus. We suggest that T. kansasensis is a juvenile of Tylosaurus nepaeolicus, and is thus the junior synonym of the latter. Furthermore, we find ontogenetic evidence to suggest that T. proriger is likely a paedomorph of T. nepaeolicus, albeit, a gigantic one.