A systematic review of natural processes in creating pierced shells: implications for the archaeological record

Background. The shells of molluscs survive well in most sedimentary contexts and yield information about the diet of prehistoric humans. They also yield evidence of symbolic behaviours, through their use as beads for body adornments. Researchers often analyse the location of perforations in shells to make judgements about their use as symbolic objects (i.e., beads), the assumption being, that holes attributable to deliberate human behaviour are more likely to exhibit low variability in their anatomical locations, while holes attributable to natural processes yield more random perforations. However, there are non-anthropogenic factors that can cause perforations in shells and these may not be random. The aim of the study is to look at the association between variation of holes in shell beads from archaeological sites and shells pierced by natural (non-human) processes. Methods. Two hundred and sixty scientific papers retrieved from online databases by using keywords, (e.g., ‘shell beads’; ‘pierced shells’); 77 of these publications enabled us to conduct a systematic review and assess the location of the hole in the shell beads in the published articles. Results. Almost all archaeological sites described shells beads with holes in a variety of anatomical locations. High variation of hole-placement was also found within the same species from the same site, as well as among sites. In contrast, predators were more specific in where they attacked molluscs; birds often select the thinnest part of the shell, while molluscs and cephalopods target thicker parts. Discussion. These results indicate that variation in hole-location on shells pierced by humans is greater than variation in the placement of holes created by natural processes. Consequently, these patterns are opposite to those expected. We also found that Gastropod and Bivalve predators choose similar hole locations to humans. Research into human shell-beads recovered from archaeological contexts should take into account non-anthropogenic factors, which can lead to more realistic scenarios of the cultural behaviours of prehistoric people.