Evidence that metallic proxies are unsuitable for assessing the mechanics of microwear formation and a new theory of the meaning of microwear

Abstract

Mammalian tooth wear research reveals contrasting patterns seemingly linked to diet: irregularly pitted enamel surfaces, possibly from consuming hard seeds, versus roughly aligned linearly grooved surfaces, associated with eating tough leaves. These patterns are important for assigning diet to fossils, including hominins. However, experiments establishing conditions necessary for such damage challenge this paradigm. Lucas et al. (Lucas et al. 2013 J. R. Soc. Interface 10, 20120923. (doi:10.1098/rsif.2012.092310.1098/rsif.2012.0923)) slid natural objects against enamel, concluding anything less hard than enamel would rub, not abrade, its surface (producing no immediate wear). This category includes all organic plant matter. Particles harder than enamel, with sufficiently angular surfaces, could abrade it immediately, prerequisites that silica/silicate particles alone possess. Xia et al. (Xia, Zheng, Huang, Tian, Chen, Zhou, Ungar, Qian. 2015 Proc. Natl Acad. Sci. USA 112, 10 669–10 672. (doi:10.1073/pnas.150949111210.1073/pnas.1509491112)) countered with experiments using brass and aluminium balls. Their bulk hardness was lower than enamel, but the latter was abraded. We examined the ball exteriors to address this discrepancy. The aluminium was surfaced by a thin rough oxide layer harder than enamel. Brass surfaces were smoother, but work hardening during manufacture gave them comparable or higher hardness than enamel. We conclude that Xia et al.'s results are actually predicted by the mechanical model of Lucas et al. To explain wear patterns, we present a new model of textural formation, based on particle properties and presence/absence of silica(tes).

Ontogenetic Dietary Shifts and Microscopic Tooth Wear in Western Chimpanzees

“…Although dust originating from the same trade-wind has been contested as the major cause for microwear in neighboring Ghana (Merceron et al, 2016), seasonal variation in dust accumulation plays a fundamental role in microwear formation of the Taï chimpanzees (Schulz-Kornas et al, 2019). The findings of Schulz-Kornas et al (2019) support previous suggestions, that dust abrades teeth at even relatively low loads (Lucas et al, 2013;van Casteren et al, 2018). In the present analysis we can exclude the possibility that our results are biased by dust accumulation because the 3DST parameters suggesting age group differences (Str, Sp, FLTp) were previously shown to be unaffected by seasonality in the same population (Schulz-Kornas et al, 2019).…”

The dental microwear of hard‐object feeding in laboratory Sapajus apella and its implications for dental microwear formation

“…Our findings do not speak directly to interpretations of dental microwear texture (e.g., Merceron et al, ; Schmidt et al, ; Ungar, Scott, & Steininger, ). However, recognition of the complexity of dental microwear formation from studies such as ours is gradually leading to a convergence between theoretical/in vitro (e.g., Lucas et al, ; van Casteren et al, ) and empirical/in vivo (e.g., Percher et al, ; Teaford et al, ) perspectives on the key variables that impact formation of dental microwear textures, and to a much needed holistic approach to understanding the processes underlying dental microwear formation and their implications for paleobiological interpretations. In short, the characterization of dental microwear textures as being either representative of, for example, the consumption of hard foods (e.g., Scott et al, ) or extraneous abrasives (e.g., Strait et al, ) is an oversimplification.…”

“…It is possible that some items consumed by the UChicago animals (e.g., raisins, grapes, and apple slices) were simply incapable of causing microwear. But, if that were the case, then were nuts and pieces of their shells—or perhaps abrasives embedded in their shells (e.g., van Casteren et al, )—the most likely cause of the new microwear in these animals? If so, as UC‐M and UC‐C each consumed more nuts than did UC‐S (Table ), why are their rates of microwear not more different?…”

“…As a result, most foods have traditionally been thought to be unable to scratch it (e.g., Lucas, ). While this perspective makes intuitive sense, recent work has indicated that the actual situation is far more complicated as it most likely depends on dynamic interactions, at a range of magnifications, between abrasive/food particles (which are changing in size and shape during chewing), and the complex anisotropic enamel surface they are striking (Lucas et al, ; Teaford, Ungar, Taylor, Ross, & Vinyard, ; van Casteren et al, ). In essence, measures of the mechanical and geometrical properties of whole food items, and of pristine enamel surfaces, may be of limited usefulness in studies of the formation of dental microwear patterns (see Sections 4.2 and 4.7 for further discussion).…”

“…However, more recent in vitro studies have suggested that relatively soft materials and light loads can still mark enamel, in part due to the composite nature of enamel structure at a microscopic level (Borrero‐Lopez, Pajares, Constantino, & Lawn, ; Daegling, Hua, & Ungar, ; Karme, Rannikko, Kallonen, Clauss, & Fortelius, ; Xia et al, ). Some of these results have not been without their critiques (e.g., van Casteren et al, ; Xia et al, ; Xia, Zhou, Qian, & Ungar, ), but the point remains that, even if the most rapid wear of enamel is still caused by exogenous grit (e.g., Karme et al, ), food material properties still impact the abrasive effects of grit (Hua, Brandt, Meullenet, Zhou, & Ungar, ). Thus, better data are needed on the in vivo mechanics of microwear formation by foods of known geometric/material properties and particle composition.…”