QUANTIFYING WINNOWING OF SKELETONS IN VERTEBRATE ASSEMBLAGES

Preferential preservation of vertebrate skeletal elements in a bone bed (i.e., localized concentration of skeletal elements) can be indicative of different modes of accumulation or of winnowing. The effects of hydrodynamic sorting observed in bone beds are often discussed in terms of “Voorhies Groups”, based on flume experiments by Voorhies in the 1960s on the dispersal potential of mammal bones. However, subsequent flume experiments on turtle and crocodilian skeletons have demonstrated a continuum of dispersal potential influenced by bone density and complexity of morphology. In addition, homologous elements between turtle, crocodilian, and mammal skeletons did not follow the same hydrodynamic dispersal patterns; this has been largely attributed to differences in gross morphology and density (i.e., variance in hydrodynamic equivalence). As such, Voorhies group assignments may not fully capture hydrodynamic dispersal potentials if homologous elements differ enough in gross morphology, density, or both across disparate taxa. These issues are compounded with fossil taxa because diagenetic alteration of bone density and taphonomic deformation of gross morphology mask original conditions present at the time of deposition. In an attempt to mitigate uncertainties surrounding original bone density and element geometries of specific fossil taxa, we employ a simple count of non-articulated skeletal elements normalized to the minimum number of individuals to infer hydrodynamic conditions. We hypothesize that a normalized distribution of elements can offer a more robust comparison between bone beds of similar taxa better supporting paleoenvironmental and paleoecological interpretations. We suggest that this method will provide a framework that is more versatile than discrete bins defined by limited taxon sampling. This framework can be further tested through modeling and actualistic studies of more disparate taxa. Although this method relies on proper anatomical and taxonomic identification of all elements within the bone beds being compared, it avoids assumptions of the hydrodynamic equivalence of homologous bones.