Mechanisms and causes of wear in tooth enamel: implications for hominin diets

Abstract

The wear of teeth is a major factor limiting mammalian lifespans in the wild. One method of describing worn surfaces, dental microwear texture analysis, has proved powerful for reconstructing the diets of extinct vertebrates, but has yielded unexpected results in early hominins. In particular, although australopiths exhibit derived craniodental features interpreted as adaptations for eating hard foods, most do not exhibit microwear signals indicative of this diet. However, no experiments have yet demonstrated the fundamental mechanisms and causes of this wear. Here, we report nanowear experiments where individual dust particles, phytoliths and enamel chips were slid across a flat enamel surface. Microwear features produced were influenced strongly by interacting mechanical properties and particle geometry. Quartz dust was a rigid abrasive, capable of fracturing and removing enamel pieces. By contrast, phytoliths and enamel chips deformed during sliding, forming U-shaped grooves or flat troughs in enamel, without tissue loss. Other plant tissues seem too soft to mark enamel, acting as particle transporters. We conclude that dust has overwhelming importance as a wear agent and that dietary signals preserved in dental microwear are indirect. Nanowear studies should resolve controversies over adaptive trends in mammals like enamel thickening or hypsodonty that delay functional dental loss.

Figure 1.

(a) Scanning electron microscopy (SEM) image of a squash phytolith (arrowed) mounted on the flat-ended titanium nanoindenter tip. (b–d) Pre-test screening of chemical identity of each mounted particle by energy dispersive spectroscopy (EDS). No sputter-coating was necessary. Dust and phytoliths have distinct elemental compositions, while Ca and P peaks for enamel reflect hydroxyapatite. (e) A squash phytolith has rubbed an enamel surface, a broken fragment of which remains embedded in the enamel. Note the U-shaped trough lying to the left of the fragment. (f) Part of the surface of a quartz dust particle, post-test, littered with small enamel chips, one of which is arrowed. (g) The joint identity of quartz particle and an enamel chip was confirmed by spot EDS analysis, with the Ca and P peaks reflecting enamel, as in (c), with the other peaks mirroring those in (d).

Figure 2.

A rigid particle sliding on a surface rubs or abrades depending on the attack angle β. (a) Abrasion removes material by chipping, while rubbing produces plastic displacement with a prow in front of the particle displacing enamel into lateral ridges. Assuming a triangular pyramid moving facet-first on enamel (angles as shown), a sliding force F, normalized to enamel shear yield stress τ_y and the square of indentation depth t (= 0.5 µm to conform with experiment), abrades when β > 40°, but rubs when β < 40°. (b–e) Features of a quartz dust particle that removed an enamel chip at a fixed vertical force of 1800 µN. (b) Stereo light microscopic views show quartz attached to the titanium tip (cyanoacrylate glue produces the halo effect) prior to the scratching test. (c) The same quartz dust particle post-test, showing a clump of enamel chips (arrowed) retained on the particle after having been fractured away from the enamel surface. Inset for (c) shows a clump of these enamel chips (SEM). (d,e) Top and side views of same particle post-test, but the enamel chips having been removed (SEM). Particle has peaks, arrowed in (e), with β > 40° that removed the enamel in (c).

Figure 3.

Rubbing action of a phytolith. Topography of a nanogroove produced by squash phytolith on enamel (vertical force of 600 µN, atomic force microscope (AFM) image). The groove is oriented along an enamel prism. The depth profile shows the prow at the end of the groove.

Figure 4.

Flat channel-like troughs produced by an enamel chip. (a) Chip on the nanoindenter tip prior to test. (b) Topography of multiple troughs produced by this chip on parental enamel surface (vertical force 1600 µN, AFM image). Depth profiles (graphs) show these scratches that are shallow troughs approximately 10 nm deep.