Paper No. 14
Presentation Time: 11:15 AM
TAPHONOMY OF A NEARLY COMPLETE POLYCOTYLID PLESIOSAUR SKELETON FROM THE TROPIC SHALE OF SOUTHERN UTAH
SCHMEISSER, Rebecca L., Department of Geology, St. Norbert College, 100 Grant Street, De Pere, WI 54115 and GILLETTE, David D., Department of Geology, Museum of Northern Arizona, 3101 N. Fort Valley Road, Flagstaff, AZ 86001, rebecca.schmeisser@snc.edu
More than a dozen short-neck plesiosaur skeletons and many additional fragments of plesiosaur bone have been discovered in the Tropic Shale of southern Utah, in both Glen Canyon National Recreation Area and Grand Staircase-Escalante National Monument. The skeletons have been found in varying stages of preservation, articulation, and completeness. The most well preserved and complete skeleton was excavated during the summer of 2005 and has been identified as a new species of polycotylid plesiosaur, within the genus
Dolichorhynchops. This specimen was found within thinly laminated, black shale and has been placed stratigraphically within the Early Turonian. The specimen consists of the skull, an incomplete set of vertebrae and ribs, scapulae, coracoids, nearly complete forelimbs, ilia, ischia, pubes, and nearly complete hindlimbs. Missing from the skeleton are about three-quarters of the phalanges, about half of the podial elements, about two-thirds of the right coracoid, the majority of the left ischium, and a few vertebrae and ribs.
The skeleton is remarkably well preserved, though heavily fractured, probably from contraction of the surrounding shale over time. Although the skeleton was disarticulated, some elements were associated (the pectoral and pelvic girdles were near their respective limb elements in the quarry, vertebrae were grouped in series). The jaws of the skull were closed with some teeth in the alveoli and others scattered nearby. Additionally, 298 gastroliths were found associated with the skeleton, clustered in groups near the back of the skull and in the surrounding area. Some of the stones were located directly on top of bones and the majority of the skeletal elements were ventral side up, indicating that the carcass came to rest belly up.
The high level of preservation and completeness of the skeleton suggest that the carcass was likely deposited shortly after settling to the sea floor, escaping destruction or corrosion of the bones. Some missing skeletal elements (such as the phalanges) may have been carried away by weak bottom currents prior to sedimentation, while others were lost by erosion slightly prior to discovery. Some scavenging on the carcass took place, though scavenging marks on the bones were minimal.
