A NEW LONG-SNOUTED AND LONG-NECKED POLYCOTYLID PLESIOSAUR FROM THE PIERRE SHALE OF WYOMING

GPM5001 is a new plesiosaur skeleton excavated from the Pierre Shale, along the Old Woman Anticline, in the Redbird Area of Wyoming. Anatomically most similar to Dolichorhynchops spp., GPM5001 is a derived polycotylid. Most derived plesiosaurs conform to one of two basic body types: those with proportionately small heads, short snouts, and elongated necks and those with proportionately large heads, elongated snouts, and short necks. However, GPM5001 presents the unusual trait combination of both an elongate snout and an elongate neck, with a fully articulated cervical series composed of 32 vertebrae. Additional diagnostic traits include the overall form of the ilium, a prominent and hooked projection along the posterolateral edge of the pubis, and the relative contributions of various elements to the mandibular symphysis. As in other polycotylids, the elongate and narrow jaws of GPM5001 appear consistent with rapid lateral strikes in pursuit of small- to medium-sized prey. The conical, posteriorly curved, and narrow teeth of GPM5001 are also consistent with this mode of predation. The cervical neural spines are tall and anteroposteriorly broad, while the cervical zygapophyseal articulations are flat and horizontally oriented. These traits suggest that the neck supported large epaxial muscles and was capable of substantial lateral flexibility, adaptations consistent with the neck contributing to the lateral acceleration and reach of the elongate jaws. The GPM5001 quarry is located approximately 12 meters below contact with the overlaying Fox Hills Formation, putting the new taxon in the uppermost member of the Pierre Shale (~69-71 MYA). Contemporaneous with other short- and long-necked plesiosaurs, GPM5001 increases the taxonomic and ecological diversity of the Western Interior Seaway during this interval.