Fracture mechanics, enamel thickness and the evolution of molar form in hominins

Abstract

As the tissue most directly responsible for breaking down food in the oral cavity, the form and function of enamel is obviously of evolutionary significance in humans, non-human primates and other vertebrates. Accordingly, a standard metric, relative enamel thickness (RET), has been used for many decades to provide insights into vertebrate and human palaeobiology. Relatively thick enamel has evolved many times in vertebrates including hominoids (the group to which living humans and fossil hominins belong), and this pattern is thought to provide information about taxonomy, phylogeny, functional anatomy and diet. In particular, relatively thick enamel is thought to make tooth crowns strong so that they resist fractures associated with eating mechanically resistant foods. Here, we use current models of tooth biomechanics to show that RET is at best only moderately informative of function and diet in living hominoids and fossil hominins, and at worst provides misleading information. We propose a new metric, absolute crown strength, to assess the resistance of teeth to fracture, and identify what may be a novel characteristic of tooth strength in fossil hominins.

Quantification of enamel decussation in gracile and robust capuchins (Cebus, Sapajus, Cebidae, Platyrrhini)

“…However, enamel complexity is not the only factor determining fracture resistance in teeth; other factors such as enamel thickness also play a role. For example, Schwartz et al (2020) recently advanced a variable that they refer to as "absolute crown strength" (ACS), which accounts for both enamel thickness and crown shape to describe tooth durability. While the authors did not account for enamel complexity in their study, their point remains that tooth crown mechanics are reliant on multiple variables.…”

Similarities and differences in the dental tissue proportions of the deciduous and permanent canines of Early and Middle Pleistocene human populations

“…The dental tissue proportions of each group were evaluated using the computerized microtomographic techniques at both two‐dimensional (areas and lengths) and three‐dimensional (volumes and surfaces) level. Various indices on enamel thickness were also assessed, including the traditional indices proposed by Martin in 1983 and adapted by Olejnickzak et al in 2008a, as well as the recently described ACS (Schwartz et al, 2020) and 3DRED (Yi et al, 2021) indices. The aim was to describe the possible similarities and differences present in the histological pattern of the deciduous canines of each of the samples analysed, as well as to evaluate the degree of covariance between deciduous and permanent dentitions.…”

“…Subsequently, these variables were employed to compute the 2D average enamel thickness index (2DAET, mm); the 2D relative enamel thickness index (2DRET, scale free) and the relative coronal dentine and pulp complex area (Acdp/Ac, percentage scale). We also assessed the absolute crown strength (ACS, scale free) proposed by Schwartz et al (2020).…”

A robust alternative to assessing three‐dimensional relative enamel thickness for the use in taxonomic assessment

“…Other than the classic enamel thickness indices like the average enamel thickness (AET) and relative enamel thickness (RET) (Kono, 2004; Martin, 1985), attempts were made to develop derivatives like the lateral RET to study worn teeth (e.g., Benazzi et al, 2011; Kono & Suwa, 2008; Toussaint et al, 2010; Zanolli et al, 2018). Most recently, a novel metric was proposed, the absolute crown strength estimated on 2D sections, to estimate tooth crown resistance to fracture (Schwartz, McGrowsky, & Strait, 2020).…”