FIRST OCCURRENCE OF A LONG-NECK PLESIOSAUR FROM THE TROPIC SHALE (CENOMANIAN-TURONIAN), SOUTHERN UTAH

The Tropic Shale was deposited along the western margin of the Western Interior Seaway during the Late Cretaceous (Cenomanian to Turonian). The formation crops out in southern Utah and is dominated by fine-grained black shale separated by prominent layers of bentonite that can be traced laterally. The presence of invertebrates in the formation has been well-known for several decades, but the vertebrate fauna is only recently being described in detail. As a result of recent work, an incredible diversity of vertebrates has been identified from the Tropic Shale including: marine turtles, several species of fish, several species of shark occupying various ecological niches, several types of short-neck plesiosaurs (including three new species), a coastal dinosaur, and very recently, a mosasaur. Prior to this study, no long-neck plesiosaurs had been described from the Tropic Shale.

Seven vertebrae (MNA V10090) from the collections of the Museum of Northern Arizona (Flagstaff, Arizona) have been identified as long-neck plesiosaur vertebrae. The vertebrae were found in Glen Canyon National Recreation Area in southern Utah. Six vertebrae were collected in 2003 and one was collected in 2012 from the same locality. All vertebrae were found in float, just below the Mammites nodosoides ammonite biostratigraphic zone, which places them in the Lower Turonian. The vertebrae are weakly amphicoelous, have concave lateral surfaces, and a slight ventral keel. No facets for ribs appear on the centra. All have facets for neural arches that span the length of the centra, with no evidence of fusion between neural arches and centra. The vertebrae are longer than they are wide and wider than they are tall. The morphology and proportions of the vertebrae indicate that they are dorsal vertebrae from a long-neck plesiosaur. The lack of fused neural arches and the relatively small size of the vertebrae compared to dorsal vertebrae from other long-neck plesiosaur specimens indicates that these are from a small juvenile. Long-neck plesiosaurs are found more commonly in formations along the eastern margin of the seaway. It has been suggested that long-neck plesiosaurs preferred shallower water due to their hunting strategies. This may explain the rarity of long-neck plesiosaurs from the deeper, western side of the seaway.