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SOFT-SHELLED TURTLE (PLASTOMENUS) MASS MORTALITY ASSEMBLAGE, SENTINEL BUTTE FM. (PALEOCENE), BILLINGS COUNTY, NORTH DAKOTA

KAYS, Glenn B., Dept. of Geology and Geol. Engineering., University of North Dakota, Grand Forks, ND 58202
HOGANSON, John W., North Dakota Geological Survey, 600 East Boulevard Ave.,Bismarck, ND 58505
   

The Ash Coulee Quarry, Billings County, North Dakota, where the Paleocene Sentinel Butte Formation is exposed in badland terrain, has produced a large number of specimens identified as the soft-shelled turtle, Plastomenus. The turtles occur in a 10 cm-thick layer of carbonaceous claystone indicative of a quiet water setting. Bone preservation is excellent including complete carapaces, plastrons, skulls, and other skeletal elements. On average one Plastomenus individual per m2 was recovered in the 30 m2 excavation area. Plastomenus is by far the dominant taxon in this assemblage. Few remains of Amia, Champsosaurus, Protochelydra, Piceoerpeton, crocodile, molluscs (Campeloma, Plesielliptio, pill clams) and carbonized logs are also present. The sedimentology, stratigraphy, and paleontology of the site suggest that these turtles inhabited an ephemeral, calm, shallow water environment. Because of the abundance of Plastomenus fossils within a restricted stratigraphic interval, it is suggested that a mass mortality event exterminated this turtle population. The cause of death is unclear, but may have been due to a rapid meteorological shift or a sudden limnological change. The Ash Coulee Quarry, which contains one of the largest assemblages of Plastomenus fossils in North America, is one of the most important soft-shelled turtle sites yet discovered. Fossils from this site will help clarify the taxonomy of plastomenid turtles.

   
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