TAPHONOMIC WINDOWS INTO AN UPPER CRETACEOUS EDMONTOSAURUS BONEBED

Studies on an extensive Upper Cretaceous monotypic Edmontosaurus bone bed in the Lance Formation in eastern Wyoming conducted using GPS and GIS mapping have enabled us to obtain accurate three dimensional data on the position and distribution of the excavated bones. We have also mapped the extent of the bed and estimated the numbers of individuals represented. We are reconstructing the taphonomy of the site based upon these and other data.

The bone bed is extensive, covering an area in excess of 1 square kilometer, but most concentrated in an area of about ten hectares. While we do not yet have quantitative bone data over much of this area, test quarries suggest the distribution of bones is reasonably consistent throughout. Estimates based upon test quarries and the main quarry area, extrapolated over the known extent of the ten hectares, suggest the bones of 5000 to 10,000 animals are interred here.

The bones occur as individual disarticulated elements or occasionally, partially disarticulated assemblages. The bones were transported to the site in a poorly consolidated mudstone. The mechanism of transport is not known, but occasionally maintained elements with articulation fidelity suggesting that at least some tissue connections remained intact. The bones occur as a graded bed within the mudstone with large limb bones at the base of the layer, grading upward to vertebra and toe bones in all quarry sites. The mudstone thickens to the north and west and pinches out to the south where it is displaced by a fine-grained, well sorted immature sandstone showing evidence of rapid accumulation. The bone bed itself provides one of the rare stratigraphic markers in this section of the Lance Formation, being traceable for more than a kilometer.