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 strong ventral keel, and is sutured to the dorsal surface of the pterygoids. The paddles possess many plesiomorphic features, including only relatively minor posterodistal expansion of the propodials, epipodials that are longer than wide, a distinct antebrachial foramen, and relatively elongate phalanges. Derived features make it a plausible representative sister taxon to Polycotylidae, a prospect that is complemented by its stratigraphic position. This hypothesis is hampered by the palate morphology, which suggests a separate lineage of short-necked plesiosaurs far removed from genera classically assigned to Polycotylidae (Dolichorhynchops, Polycotylus, Trinacromerum). Nevertheless, the specimen represents the persistence of plesiomorphic limb traits and possession of a closed palate within an early polycotylid lineage until at least the early late Cenomanian in the Midcontinent of North America.