WASHED ASHORE – A NEW ELASMOSAURID SPECIMEN (PLESIOSAURIA: SAUROPTERYGIA) FROM SHOREFACE BIOCLASTIC DEPOSITS OF THE JUANA LOPEZ MEMBER (MIDDLE TURONIAN), CARLILE SHALE; DISCOVERED, EXCAVATED, AND PREPARED BY CITIZEN STEWARDS OF U.S. FOREST SERVICE AND DENVER MUSEUM OF NATURE AND SCIENCE

A unique elasmosaurid plesiosaur partial skeleton was discovered by Carol McClure (resident of La Junta, CO) on the Comanche National Grassland. Carol and other citizen stewards excavated the skeleton along with the Forest Service Minerals and Geology staff in recent years. The specimen (Denver Museum V.90000) is from the Juana Lopez Member (Carlile Shale), a regressive lag deposit along the western shoreline of the Western Interior Seaway. Winnowing by wave energy produced a bioclastic deposit composed of particulate inoceramid shell, oysters, and including small vertebrate bone fragments, teeth, and phosphatic coprolites. No large associated vertebrate skeleton was known from the Juana Lopez prior to this discovery. Unique fossil occurrences often yield unique kinds of fossils, and very little elasmosaur material of Turonian age is known globally. Accordingly, the specimen exhibits derived and plesiomorphic morphologies indicating an ‘intermediate’ grade of derivation, best diagnosed by cervical centra dimensional data. Diagnosis beyond Elasmosauridae may not be possible.

The bones occur atop and partly contained within a 20 cm thick calcarenite bed, widely strewn over an area of 20 m². Only a small percentage of the skeleton is present, but fortunately vertebrae throughout the axial skeleton are present, including a broad sample of the cervical series, and several trunk, sacral, and caudal vertebrae. Also present are articulated coracoids, numerous rib/gastralia portions, phalanges, a single rooted tooth, and numerous polished silicic pebbles (likely gastroliths).

The bedding plane contains numerous ammonite molds (Prionocyclus) and an abundant network of Thalassinoides feeding traces. Elasmosaur bones projecting above the bedding plane are heavily eroded, while those portions encased within the calcarenite bed are well preserved. This suggests the scattered bones lay half submerged in the bioclastic sand, and were subject to considerable weathering (perhaps subaerial exposure) prior to burial. The result is an intriguing taphonomic scene, that of a large elasmosaurid carcass washed ashore by wave action. Wave energy dispersed the skeletal elements after decay of soft tissues, but large size of individual bones prevented them from moving so far as to be completely disassociated.