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TAPHONOMIC IMPLICATIONS OF THE TURTLE ASSEMBLAGE AT ASH COULEE QUARRY, SENTINEL BUTTE FORMATION (PALEOCENE), BILLINGS COUNTY, NORTH DAKOTA

KAYS, Glenn B., Dept. of Geology and Geol. Engineering, University of North Dakota, Grand Forks, ND 58201
ERICKSON, J. Mark, Geology Department, St. Lawrence University, Canton, NY 13617
HOGANSON, John W., North Dakota Geological Survey, 600 East Boulevard Ave., Bismarck, ND 58505
   

Approximately 31 soft-shelled turtle specimens belonging to the genus Plastomenus have been excavated from the Ash Coulee Quarry in Billings County, North Dakota. The quarry is in the lower part of the Sentinel Butte Formation of Paleocene age. The varied orientation of the specimens indicates this is a death assemblage resulting from a mass mortality event. The cause of death is unclear, but may have been due to a rapid meteorological shift or sudden limnological change.

Bone preservation is excellent and includes complete carapaces, plastrons, skulls, and other skeletal elements. Very few juveniles are preserved at the site. Carapace lengths range from 21.3 cm to 27.4 cm based on material thus far reconstructed. Studies by others suggest that turtle carcasses in lacustrine environments begin to disarticulate after 12 days and may continue to do so for a number of months. Shell articulation exhibited in the Ash Coulee turtle assemblage indicates that transport as floating carcasses probably occurred for no longer than a month post mortem.

Carbonized logs occur with the soft-shelled turtle remains in the lower portion of the approximately 20 cm-thick layer of carbonaceous claystone. The presence of these logs may indicate a freshwater shoreline environment as the place of burial. Underlying the carbonaceous claystone is a 5-10 cm-thick organic layer, indicative of deposition in a paludal environment.

   
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Updated: 08.11.06 jal