Abstract

A new, articulated skeleton of an ichthyosaur from the Lower Cretaceous (lower Albian) Wabiskaw Member of the Clearwater Formation near Fort McMurray, Alberta, is the most complete and stratigraphically oldest known ichthyosaur from the Cretaceous of North America and represents a new genus and species of ophthalmosaurian, Athabascasaurus bitumineus. The specimen consists of a nearly complete, dorsoventrally compressed skull, a complete and articulated presacral and partial caudal vertebral series, portions of the right pectoral girdle, and the right pelvic girdle and femur. The new taxon is characterized by the lack of a robust supranarial process of the premaxilla, an elongate maxilla that has its tallest point (in lateral view) posterior to the external naris, a wide postorbital region, the presence of a rectangular squamosal, an angular with greater lateral exposure on the posterior jaw ramus than the surangular, a dentition with extremely light enameled ridges, and a reduced presacral count of 42 vertebrae. The first species-level phylogenetic analysis of Ophthalmosauria reveals that Athabascasaurus is neither the sister taxon of, nor nests within Platypterygius, a geographically widespread, geologically long-lived, and taxonomically problematic genus. Athabascasaurus adds important new data on the morphology of Cretaceous ichthyosaurs and expands our knowledge of the palaeoecology and marine tetrapod diversity of the early Albian Boreal Sea.