Rocky Mountain Section - 72nd Annual Meeting - 2020

Paper No. 3-1
Presentation Time: 8:30 AM-4:30 PM


SCHMEISSER MCKEAN, 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

MNA V.11350 is a partial skeleton of a short-neck plesiosaur that was discovered in the Tropic Shale in Glen Canyon National Recreation Area in 2012 and excavated in 2013. The specimen was located 1.5 m above bentonite C, just below a prominent limonite bed, in fine-grained black shale. The upper portion of the body is preserved, including the complete skull and lower jaw, pectoral girdle elements from the right side of the body, the right humerus, and isolated vertebrae and ribs. The anterior portion of the skull was located beneath a large sandstone talus block from the overlying Straight Cliffs Formation. This likely aided in the protection of the skull from modern weathering processes, resulting in its outstanding preservation.

The skeleton was mostly articulated, oriented with the left side of the body towards the surface. The skull was preserved with the jaws closed tightly. Most teeth were still in place in the alveoli, with a few teeth scattered near the tip of the snout. The occipital condyle was in articulation with the atlas, with a string of cervical vertebrae extending laterally. The pectoral girdle was mostly articulated, although fragments from the distal ends of the humerus, scapula, and coracoid were scattered from exposure at the surface. A string of dorsal vertebrae in articulation extended along the length of the hillslope, just beneath the surface, with neural spines fused dorsally and ribs extending ventrally. These vertebrae were badly weathered and appeared as a brown powder that could not be restored. Elements from the posterior portion of the body were likely weathered prior to discovery and were not recovered.

With exception of the skeletal elements near or in contact with the modern surface, which were badly weathered from exposure, preservation of the specimen is excellent. Skeletal elements display a fracturing stage of 3 (common for vertebrate fossils from the Tropic Shale), an abrasion stage of 0, and a compression stage of 2 (normal compression). Additionally, an orange mineralized coating is present on the skeletal elements, which may be from bacterial activity during or after decomposition of the soft tissue of the carcass. The preservation of MNA V.11350 confirms prior reconstructions of the Tropic Shale as a low-energy marine environment with weak bottom currents and low levels of benthic oxygen.