Taphonomy of an Upper Cretaceous Edmontosaurus Bone Bed
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 paucispecific (in this case, one predominant species with a few other kinds of dinosaurs mixed in, in contrast to monospecific – only one species present) 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 (about 100 acres). 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 (a sedimentary layer with large particles on the bottom, smaller ones on top) 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 are seeking to understand the taphonomy (the study of the processes of death, decay and burial of organisms) of the remains of this large population (greater than probable herd size) of ornithopods, and to understand how they were catastrophically decimated. At the present level of our investigation, the animals appear to have initially accumulated in a nearshore freshwater environment that was subsequently remobilized and transported basinward to a deeper water setting as a graded bone bed.
We are continuing to expose the remains of the animals over a large area with major quarries in the central region and test quarries in remote areas. We are also studying the sedimentology (the study of the processes of deposition of sedimentary rock layers – rocks composed of particles that have settled out of a fluid) over a wider area and seeking stratigraphic (the study of the layers of rock) information in this notoriously challenging deposit. We are carrying out palynological (study of fossil pollen and spores) analysis and paleobotanical (study of fossil plants) work in parallel with the stratigraphic and taphonomic studies.