A plesiosaur skeleton from the base of the Greenhorn Limestone (lower upper Cenomanian), western South Dakota, represents a new taxon tentatively referred to the Polycotylidae. The specimen possesses a number of features in common with the Polycotylidae, including an exceptionally elongate muzzle and mandibular symphysis, lack of a parietal foramen, homodont dentition, and faint striae only on the medial side of teeth. However, the palate and limb morphologies differ with respect to Polycotylidae as currently known. The pterygoids are united along their midline anterior to the parasphenoid, lacking an anterior interpterygoid vacuity. The preserved parasphenoid is relatively robust, possesses a...