TAPHONOMIC MODES IN THE SCENIC MEMBER OF THE BRULE FORMATION: CONTINUOUS OR DISCRETE?

A taphonomic mode is a “recurring pattern of preservation of organic remains in a particular sedimentary context, accompanied by characteristic taphonomic features”, produced by the sum total of the taphonomic processes acting on a particular fossil assemblage. Given the variability in the physical characteristics of individuals preserved in a typical vertebrate fossil assemblage, individual elements experience different taphonomic microenvironments, and so, even with a constant set of biasing processes, a range of taphonomic characteristics is displayed within an assemblage. For example, if each preserved element is subject to the same amount of physical damage, less robust elements will suffer more damage than sturdier elements. It is possible to quantitatively characterise the taphonomic mode of a fossil assemblage using the unique combination of taphonomic features shared by all of the members of that modal group.

By using ordination and multiple regression to study two fossil assemblages from the Scenic Member of the Brule Formation, Badlands National Park, it is possible to determine that virtually all specimens from these assemblages have a single taphonomic mode, although this is expressed as a small, but continuous range of taphonomic characteristics. No discrete groups of elements, subject to different sets of taphonomic processes, can be identified, implying that there was little mixing of elements from different environments prior to final burial. The influences of different taphonomic processes on the fossil assemblages can also be quantified, showing that taphonomic processes related to three main factors (density, shape and surface area to volume ratio) have produced this taphonomic mode. These results are used in combination with a contrasting fossil assemblage from the Cretaceous Wayan Formation to illustrate quantitative comparison of taphonomic modes, and the interpretations that are possible with such quantitative data.