Postcranial anatomy of Platypterygius americanus (Reptilia: Ichthyosauria) from the Cretaceous of Wyoming

ABSTRACT

Ichthyosaurs remains from the Albian and Cenomanian of Wyoming have been assigned to Platypterygius americanus, but apart from structures of the distal humerus, few other unequivocal character states have been proposed to differentiate this species from its congeners. The purpose of this study was to reevaluate the postcranial morphology of Platypterygius americanus, in order to assess the validity of this species and distinguish postcranial traits relevant for specific diagnoses. Several applicable features were identified, including the location of extrazeugopodial facets on the distal humerus and femur, shape of the intermedium, relative degree of fusion within the atlas-axis complex, number of presacral centra, and varying lengths of the anterior caudal centra. Comparative assessment suggests that Platypterygius americanus is a valid taxon, and that it is possible to differentiate the various Platypterygius spp. independent of stratigraphical and/or geographical occurrence data. Nevertheless, a future review of cranial morphology in all Platypterygius spp. is necessary to bolster these conclusions.

ACKNOWLEDGMENTS

Thanks to M. Feuerstack and K. Shepherd (CMN), T. Fiorillo (DMNH), D. Henderson (RTM), S. Hocknull and A. Cook (QM), T. Rowe (TMM), and M. Cassiliano (UW) for hospitality and collections access. FQRNT and I. W. Killam postdoctoral fellowships, and M. Caldwell provided funding for this work (E.M.). Thanks to M. Caldwell and lab members for critical discussion and help with the manuscript, and to R. O'Keefe and two anonymous reviewers for comments that improved the manuscript.