Nano-indentation of native phytoliths and dental tissues: implications for herbivore-plant combat and dental wear proxies

Abstract

Tooth wear induced by abrasive particles is a key process affecting dental function and life expectancy in mammals. Abrasive particles may be plant endogenous opal phytoliths, exogene wind-blown quartz dust or rain borne mineral particles ingested by mammals. Nano-indentation hardness of abrasive particles and dental tissues is a significant yet not fully established parameter of this tribological system. We provide consistent nano-indentation hardness data for some of the major antagonists in the dental tribosystem (tooth enamel, tooth dentine and opaline phytoliths from silica controlled cultivation). All indentation data were gathered from native tissues under stable and controlled conditions and thus maximize comparability to natural systems. Here we show that native (hydrated) wild boar enamel exceeds any hardness measures known for dry herbivore tooth enamel by at least 3 GPa. The native tooth enamel is not necessarily softer then environmental quartz grit, although there is little overlap. The native hardness of the tooth enamel exceeds that of any silica phytolith hardness recently published. Further, we find that native reed phytoliths equal native suine dentine in hardness, but does not exceed native suine enamel. We also find that native suine enamel is significantly harder than dry enamel and dry phytoliths are harder than native phytoliths. Our data challenge the claim that the culprit of tooth wear may be the food we chew, but suggest instead that wear may relates more to exogenous than endogenous abrasives.

Forage silica and water content control dental surface texture in guinea pigs and provide implications for dietary reconstruction

“…Thus, the inherent structural properties of the grass blades or their abrasiveness was changed by drying, even though they became more brittle and should fracture more easily. A recent study (37) suggests that phytolith characteristics depend on the extraction method, with native phytoliths showing lower indentation hardness than phytoliths extracted by dry ashing. Consequently, the dehydration of plant tissues could have altered the hardness of phytoliths and led to more abrasion in the dry grass group.…”

“…On the one hand, internal abrasives, namely opal phytoliths [amorphous silica bodies with complex ultrastructural patterns (30)], have long been suggested as the causal agents of the scratch-dominated tooth wear characteristic of grazers (31–33). Recent studies, however, produced equivocal results on whether phytoliths can actually scratch enamel (34–37). On the other hand, grazing is associated with feeding close to the ground and often also with seasonally arid and hence dusty grassland environments.…”

“…For grass, it is known that small changes in the volume fraction of the sclerenchyma fibers will produce large changes in the stiffness and strength of the leaf (44), with increasing stiffness or toughness in drier material (45, 46). Furthermore, a recent study (37) showed that native phytoliths have lower indentation hardness than phytoliths extracted by dry ashing; therefore, the hydration state of plant tissue could also affect dental abrasion. We thus expected to find differences between fresh and dry forages (smaller volume fraction) that would elucidate the empirical observation that so-called “fresh grass grazers” do not follow the same dental wear patterns as “dry grass grazers” (3).…”

Shape, size, and quantity of ingested external abrasives influence dental microwear texture formation in guinea pigs

“…Type of Abrasive Particles. It is well known that quartz (crystalline SiO 2 , Mohs hardness: 7) is harder than dental enamel (apatite, Mohs hardness: 5) both on Mohs hardness scale and in indentation tests (6,7,13,51,52). Consequently, quartz-containing grit and mineral dust have long been suggested as the main abrasive agents of dental wear (3,(5)(6)(7).…”

“…Controlled feeding experiments with small mammals have shown that phytoliths, even though these endogenous plant silica bodies of 10 up to 100 μm are softer than dental enamel (13), cause distinct dental microwear texture (DMT) patterns (14) and that phytolith-rich diets lead to more dental (macro-)wear than phytolith-poor diets (9,10,15). For exogenous grit and/or mineral dust, however, controlled feeding experiments have produced ambiguous results.…”