PLESIOSAUR GASTROLITHS FROM THE TROPIC SHALE (CENOMANIAN-TURONIAN) OF SOUTHERN UTAH

Recent fieldwork conducted by the Museum of Northern Arizona has resulted in the recovery of more than a dozen short-neck plesiosaur skeletons from the marine Tropic Shale (Cenomanian-Turonian) in Glen Canyon National Recreation Area and Grand Staircase-Escalante National Monument of southern Utah. To date, four species of plesiosaur have been recognized in the Tropic Shale fauna, providing insight into the biodiversity of large marine reptiles for this portion of the Late Cretaceous Greenhorn Cyclothem and for the western margin of the Cretaceous Western Interior Seaway. A closely associated short-neck plesiosaur skeleton excavated in 2005 from a site in the Lower Turonian of Glen Canyon National Recreation Area is the first nearly complete individual recovered during the past six years of intensive fieldwork in this area. The skeleton includes skull and jaws, teeth, and most of the axial and appendicular elements. Although the bones are heavily fractured from compression of the shale, they are well preserved. Identification of the taxon must await laboratory preparation, but preliminary assessment suggests Trinacromerum sp. The skeleton contains numerous gastroliths, making it the first plesiosaur we have collected in the region to contain stomach stones. Ninety-seven gastroliths were collected and recorded in the field; additional gastroliths are likely to be discovered during laboratory preparation. The stones are relatively small (0.5 to 3.5 cm maximum length) and most are well-rounded. A few of the stones are somewhat angular, indicating ingestion by the plesiosaur shortly before its death and burial. The gastroliths were in clusters and in some cases were situated on top of skeletal elements. Based on field mapping of the quarry and these preliminary observations, we conclude that the carcass was buried ventral side up. Only one other accumulation of small, rounded stones has been recorded in this fauna. These stones are proposed to be plesiosaur gastroliths, but were not associated with skeletal remains.