Shell density as proxy for reconstructing prehistoric aquatic resource exploitation, perspectives from southern Africa

Aquatic resource use has gained enormous attention in recent years, particularly in terms of its role in human evolution. Significant strides are being made regarding the potential nutritional significance of aquatic foods for hominin diets, and explicit conceptual frameworks for understanding the evolutionary context of coastal adaptations are also being developed. Finding out when and where systematic use of aquatic resources took place and what constitutes a well-developed coastal adaptation requires a corpus of data that can offer insights into the organization of marine resource procurement. The earliest evidence for the exploitation of marine coastal habitats in the world is found in southern-most Africa (≤ 164 ka), where the most evident and abundant material expression of such foraging adaptation is revealed by marine mollusc shells found in mid to late Pleistocene archaeological sites. Hence, it becomes imperative to understand what can be meaningfully inferred from the variable quantities of such a significant component of early marine aquatic exploitation. This paper approaches the issue of aquatic resource use by investigating one of the most frequently employed quantitative measure for inferring such behaviour, namely shell density. Holocene assemblages from the South African west coast are used as a case study. It is found that shell densities can be misleading for inferring intensity of coastal resource use in the absence of palaeoshoreline reconstruction and when deposition rates are assumed to remain constant and when geomorphological and taphonomic contexts are unknown. Overall, deposition rates of marine shells is a better proxy for comparison between sites and through time, and thus for gaining insight into past marine coastal adaptations.