PRELIMINARY DEPOSITIONAL MODEL FOR AN UPPER CRETACEOUS EDMONTOSAURUS BONEBED

CHADWICK, Arthur, Southwestern Adventist Univ., Keene, TX; SPENCER, Lee, Southern Adventist Univ., Collegedale, TN; TURNER, Larry, Southwestern Adventist Univ., Keene, TX

GPS surveying and GIS mapping have enabled us to obtain accurate three dimensional data on the position and distribution of Edmontosaurus bones from an extensive monotypic bone bed in the Upper Cretaceous Lance Formation of eastern Wyoming.  The bone bed occurs over an area in excess of one square kilometer, but bones apperar to be concentrated in an area of about 40 hectares.  While we do not yet have quantitative bone data covering the entire area, the main quarries and six test quarries have yielded a consistent picture. Estimates based upon these quarries, extrapolated over the known extent of the 40 hectares, suggest the bones of 10,000 to 25,000 animals are interred here.

The bones occur as individual disarticulated elements or rarely, as partially disarticulated assemblages.  The mass mortality event is preserved within a normally graded bed in a poorly consolidated claystone or mudstone with large limb bones at the base, grading upward to vertebrae and toe bones at all quarry sites.  The bones universally exhibit little evidence of weathering; abrasion and other transport degradation are also conspicuously absent.  The claystone is conformably overlain by a fine-grained, well-sorted immature sandstone showing evidence of rapid accumulation.  We propose that a large population of ornithopods (greater than probable herd size) was catastrophically decimated and initially accumulated in a nearshore freshwater environment.  Subsequently, the disarticulating remains were remobilized and transported basinward to a deeper water setting as a graded bone bed.