The impact of sediment abrasion on tooth microwear analysis: an experimental study

Abstract

Dental microwear analysis is a proxy for analysing the diet in extinct and extant vertebrates, especially mammals. The limits of these approaches are still rather poorly known, especially in terms of taphonomic impacts. Indeed, several physical or chemical phenomena may have altered the microscopic features linked to the diet and compromised their study. In this article, we evaluate the effect of sediment abrasion on teeth on low-magnification tooth wear studies. We used a tumbling machine in order to reproduce abrasion marks on 57 molars and premolars of Equus sp., Capra hircus and Sus scrofa employing two types of sediments: a mixture of clay and sand sediment with small (150–200 μm) and rounded particles and a sandy one with larger (350–500 μm) and sub-angular particles. The teeth underwent up to 2 h of tumbling simulation, and casts were made at regular intervals in order to evaluate the evolution of the taphonomic impact over time. Our experiment shows that (1) both sediments strongly alter the teeth after a certain time; (2) the fine particles contained in the mix of sand and clay sediment have a much stronger impact on the enamel than the sand; (3) the mix of clay and sand sediment tends to increase the number of pits and reduce the number of scratches, vice versa for the sand; and (4) sedimentary and dietary marks do not have the same morphology and can be distinguished. The abrasion marks (compared to dietary scratches) tend to be wider, shorter, with an isotropic distribution, more frequent on the most exposed parts of the teeth (such as the cusps or the edges). The pits resulting from sediment tumbling present an irregular morphology in comparison with dietary pits, which are rounder. Both sediments have an impact on the enamel surfaces. Thus, when signs of taphonomic alteration (e.g. presence of abrasion marks, taphonomic pits, notches in the edges of enamel) are documented, we recommend avoiding studying the tips of the cups of the Suidae (and probably other bunodont teeth) and the portions of enamel at the edge of equid teeth which are more affected by taphonomic processes, especially in the mix of sand and clay sediment. This work has important implications for microwear studies applied to fossil samples. It makes it possible to recognize some taphonomic features linked to mechanical abrasion of the enamel, to consider with more caution the teeth that have been preserved in fine sediment and to choose, in order to characterise the diet, the areas least impacted by taphonomic alterations.