Counting Individuals in Mammalian Fossil Assemblages from Fluvial Environments

Abstract

Many paleoecological analyses utilize estimates of the relative abundances of taxa in fossil or subfossil assemblages. Such estimates depend upon determination of the number of individuals present or some other measure of the amount of material collected for each taxon. I assess the validity of methods currently used to estimate numbers of individuals per taxon in mammalian assemblages. For any method of counting selected, the underlying assumptions about the probability of association among skeletal elements should correspond to processes of accumulation for the assemblage, as inferred from its taphonomic characteristics. No method is appropriate for all mammalian assemblages. Equating the number of individuals with the number of specimens per taxon is the appropriate method when formerly articulated material has been widely dispersed and has accumulated as isolated specimens. Determining the minimum number of individuals represented by multiple skeletal elements is appropriate when material originally accumulated in articulation. For mammalian assemblages from Middle Siwalik (late Miocene) sediments of Pakistan, different methods of counting are appropriate for assemblages from different sedimentary environments. For assemblages produced by current transport, I counted each specimen as a single individual. For assemblages produced by the activities of predators and scavengers, I determined the minimum number of individuals. Two taphonomic processes that influence the degree of association among skeletal remains are mortality and transport. Taphonomic histories that combine different causes of mortality and different conditions of transport produce fossil assemblages with expected taphonomic characteristics. These include the amount of articulated material, the spatial distribution of specimens, the presence of hydraulic sorting, the frequency of juvenile remains, and patterns of bone damage. Taphonomic characteristics of fossil assemblages do not permit all combinations of mortality and transport to be distinguished. Processes of transport generally obscure taphonomic characteristics imposed by different agents of mortality. INTRODUCTION