We can now verify that Megalneusaurus rex was collected from the upper Redwater Shale member of the Sundance Formation, about 10 m below the Windy Hill sandstone of the Morrison Formation. It appears that M. rex frequented the shallower portions of the Sundance Sea during the last regressive phase. The discovery of the site is significant in establishing the stratigraphic context of this largest member of the Sundance marine reptile fauna.
The large pliosaurid, Megalneusaurus rex, is the only known pliosaurid from the Sundance Formation (O’Keefe and Wahl 2003a). Wilbur Knight excavated the partially articulated, but incomplete, skeleton near the town of Ervay, Wyoming in 1895 (Knight, 1895, 1898). Knight (1898) described a specimen that included a complete forelimb, portion of the pectoral girdle, ribs, and “cervical, dorsal and caudal vertebra”. Part of a second limb was found but not explicitly described by Knight (1898). Since Knight’s initial description, only a cursory mention of M. rex has been made in the literature (Brown, 1981; Weems and Blodgett, 1996; O’Keefe and Wahl, 2003a; Wahl, 2006). With the exception of an isolated neural arch (UW24238) from the Upper Redwater Shale of the Sundance Formation, and pieces of a humerus from the Naknek Formation of Alaska (Weems and Blodgett, 1996), no other North American Jurassic pliosaurid material has been referred to this taxon. As M. rex represents the largest pliosaurid from North America and the only one from the Sundance Formation, new information on this giant predator would be useful.