Palaeobiological and sedimentological implications of fossil concentrations

Abstract

Concentrations of fossil hardparts are common features of the stratigraphical record and are preferred collecting sites for most palaeontological data. Nonetheless, most investigations into the nature of the fossil record have analysed the biasing effects of selective hardpart transport and destruction1,2 rather than the consequences of the concentration process itself. Genetic classification is discouraged by the diverse origins of skeletal accumulations, which range from predator gastric residues to shelly shoals and biostratigraphically condensed deposits; concentrations can thus form over time intervals of a few minutes to hundreds of thousands of years. I show here that the close association of shell beds with stratigraphical discontinuities in Miocene shallow marine deposits of Maryland3,4 provides the basis of a model of skeletal accumulation cast entirely in terms of changes in net sedimentation. This simple sedimentological model is a surprisingly powerful predictor of post-mortem bias and ecological composition of fossil assemblages, suggesting that fossil-rich and fossil-poor strata are qualitatively different, both as repositories of palaeontological information and as settings for biotic interactions. Moreover, the apparent primary importance of rates of sedimentation in skeletal accumulation—despite emphasis usually placed on rates of hardpart input—suggests a new approach to inferring the detailed dynamics of sediment deposition and erosion in the formation of stratigraphical sequences.