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Eilenodontines are one of the oldest radiation of herbivorous lepidosaurs (snakes, lizards and tuatara) characterized by batteries of wide teeth with thick enamel that bear mammal-like wear facets. Unlike most reptiles, eilenodontines have limited tooth replacement, making dental longevity particularly important to them. We use both X-ray and neutron computed tomography to examine a fossil tooth from the eilenodontine Eilenodon (Late Jurassic, USA). Of the two approaches, neutron tomography was more successful and facilitated measurements of enamel thickness and distribution. We find the enamel thickness to be regionally variable, thin near the cusp tip (0.10 mm) but thicker around the base (0.15-0.30 mm) and notably greater than that of other rhynchocephalians such as the extant Sphenodon (0.08-0.14 mm). The thick enamel in Eilenodon would permit greater loading, extend tooth lifespan and facilitate the establishment of wear facets that have sharp edges for orally processing plant material such as horsetails ( Equisetum ). The shape of the enamel dentine junction indicates that tooth development in Eilenodon and Sphenodon involved similar folding of the epithelium but different ameloblast activity., (© 2018 The Authors.)

Mammalian enamel, the contact dental tissue, is something of an enigma. It is almost entirely made of hydroxyapatite, yet exhibits very different mechanical behavior to a homogeneous block of the same mineral. Recent approaches suggest that its hierarchical composite form, similar to other biological hard tissues, leads to a mechanical performance that depends very much on the scale of measurement. The stiffness of the material is predicted to be highest at the nanoscale, being sacrificed to produce a high toughness at the largest scale, that is, at the level of the tooth crown itself. Yet because virtually all this research has been conducted only on human (or sometimes "bovine") enamel, there has been little regard for structural variation of the tissue considered as evolutionary adaptation to diet. What is mammalian enamel optimized for? We suggest that there are competing selective pressures. We suggest that the structural characteristics that optimize enamel to resist large-scale fractures, such as crown failures, are very different to those that resist wear (small-scale fracture). While enamel is always designed for damage tolerance, this may be suboptimal in the enamel of some species, including modern humans (which have been the target of most investigations), in order to counteract wear. The experimental part of this study introduces novel techniques that help to assess resistance at the nanoscale., (© 2015 Wiley Periodicals, Inc.)

A review is presented of the mechanical damage suffered by tooth crowns. This has been the subject of much recent research, resulting in a need to revise some of the thinking about the mechanisms involved. Damage is classified here by scale into macro-, meso- and microfracture. The focus is on the outer enamel coat because this is the contact tissue and where most fractures start. Enamel properties appear to be tailored to maximize hardness, but also to prevent fracture. The latter is achieved by the deployment of developmental flaws called enamel tufts. Macrofractures usually appear to initiate as extensions of tufts on the undersurface of the enamel adjacent to the enamel-dentine junction and extend from there into the enamel. Cracks that pass from the tooth surface tend to be deflected by an enamel region of high toughness; if they find the surface again, a chip (mesofracture) is produced. The real protection of the enamel-dentine junction here is the layer of decussating inner enamel. Finally, a novel analysis of mechanical wear (microfracture) suggests that the local toughness of the enamel is very important to its ability to resist tissue loss. Enamel and dentine have contrasting behaviours. Seen on a large scale, dentine is isotropic (behaving similarly in all directions) while enamel is anisotropic, but vice versa on a very small scale. These patterns have implications for anyone studying the fracture behaviour of teeth., (© 2014 S. Karger AG, Basel.)

Dental enamel is prone to damage by chipping with large hard objects at forces that depend on chip size and enamel toughness. Experiments on modern human teeth have suggested that some ante-mortem chips on fossil hominin enamel were produced by bite forces near physiological maxima. Here, we show that equivalent chips in sea otter enamel require even higher forces than human enamel. Increased fracture resistance correlates with more intense enamel prism decussation, often seen also in some fossil hominins. It is possible therefore that enamel chips in such hominins may have formed at even greater forces than currently envisaged., (© 2014 The Author(s) Published by the Royal Society. All rights reserved.)

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.

Primate teeth adapt to the physical properties of foods in a variety of ways including changes in occlusal morphology, enamel thickness, and overall size. We conducted a comparative study of extant primates to examine whether their teeth also adapt to foods through variation in the mechanical properties of the enamel. Nanoindentation techniques were used to map profiles of elastic modulus and hardness across tooth sections from the enamel-dentin junction to the outer enamel surface in a broad sample of primates including apes, Old World monkeys, New World monkeys, and lemurs. The measured data profiles feature considerable overlap among species, indicating a high degree of commonality in mechanical properties. These results suggest that differences in the load-bearing capacity of primate molar teeth are more a function of morphology-particularly tooth size and enamel thickness-than of underlying mechanical properties., (Copyright © 2012 Wiley Periodicals, Inc.)

A comparative study has been made of human and great ape molar tooth enamel. Nanoindentation techniques are used to map profiles of elastic modulus and hardness across sections from the enamel-dentin junction to the outer tooth surface. The measured data profiles overlap between species, suggesting a degree of commonality in material properties. Using established deformation and fracture relations, critical loads to produce function-threatening damage in the enamel of each species are calculated for characteristic tooth sizes and enamel thicknesses. The results suggest that differences in load-bearing capacity of molar teeth in primates are less a function of underlying material properties than of morphology., (Published by Elsevier Ltd.)

Lucas and colleagues recently proposed a model based on fracture and deformation concepts to describe how mammalian tooth enamel may be adapted to the mechanical demands of diet (Lucas et al.: Bioessays 30 2008 374-385). Here we review the applicability of that model by examining existing data on the food mechanical properties and enamel morphology of great apes (Pan, Pongo, and Gorilla). Particular attention is paid to whether the consumption of fallback foods is likely to play a key role in influencing great ape enamel morphology. Our results suggest that this is indeed the case. We also consider the implications of this conclusion on the evolution of the dentition of extinct hominins.

We present results of in situ fracture tests on extracted human molar teeth showing failure by margin cracking. The teeth are mounted into an epoxy base and loaded with a rod indenter capped with a Teflon insert, as representative of food modulus. In situ observations of cracks extending longitudinally upward from the cervical margins are recorded in real time with a video camera. The cracks appear above some threshold and grow steadily within the enamel coat toward the occlusal surface in a configuration reminiscent of channel-like cracks in brittle films. Substantially higher loading is required to delaminate the enamel from the dentin, attesting to the resilience of the tooth structure. A simplistic fracture mechanics analysis is applied to determine the critical load relation for traversal of the margin crack along the full length of the side wall. The capacity of any given tooth to resist failure by margin cracking is predicted to increase with greater enamel thickness and cuspal radius. Implications in relation to dentistry and evolutionary biology are briefly considered.

Tooth enamel is inherently weak, with fracture toughness comparable with glass, yet it is remarkably resilient, surviving millions of functional contacts over a lifetime. We propose a microstructural mechanism of damage resistance, based on observations from ex situ loading of human and sea otter molars (teeth with strikingly similar structural features). Section views of the enamel implicate tufts, hypomineralized crack-like defects at the enamel-dentin junction, as primary fracture sources. We report a stabilization in the evolution of these defects, by "stress shielding" from neighbors, by inhibition of ensuing crack extension from prism interweaving (decussation), and by self-healing. These factors, coupled with the capacity of the tooth configuration to limit the generation of tensile stresses in largely compressive biting, explain how teeth may absorb considerable damage over time without catastrophic failure, an outcome with strong implications concerning the adaptation of animal species to diet.

What kinds of fractures do teeth sustain and how do they resist disintegration? This study involved the mechanical loading of extracted human and sea otter teeth using hard and soft indenters to simulate hard and soft diets. The tests were accompanied by real-time imaging. At least three types of fracture were seen in the enamel--median, radial and margin cracks. Each kind of fracture appears to have a different cause, although the distinction between median and radial cracks blurs as they propagate. Only margin cracks appear to form under soft indenters. Several aspects of tooth form can be described as devices to limit damage to a tooth crown against the onslaught of hard or soft foods. The damage modes of teeth are paralleled by the behavior of some bilayered hard foods., (Copyright (c) 2009 S. Karger AG, Basel.)

Dentition is a vital element of human and animal function, yet there is little fundamental knowledge about how tooth enamel endures under stringent oral conditions. This paper describes a novel approach to the issue. Model glass dome specimens fabricated from glass and back-filled with polymer resin are used as representative of the basic enamel/dentine shell structure. Contact loading is used to deform the dome structures to failure, in simulation of occlusal loading with opposing dentition or food bolus. To investigate the role of enamel microstructure, additional contact tests are conducted on two-phase materials that capture the essence of the mineralized-rod/organic-sheath structure of dental enamel. These materials include dental glass-ceramics and biomimicked composites fabricated from glass fibers infiltrated with epoxy. The tests indicate how enamel is likely to deform and fracture along easy sliding and fracture paths within the binding phase between the rods. Analytical relations describing the critical loads for each damage mode are presented in terms of material properties (hardness, modulus, toughness) and tooth geometry variables (enamel thickness, cusp radius). Implications in dentistry and evolutionary biology are discussed.

The divergent molar characteristics of Pan troglodytes and Pongo pygmaeus provide an instructive paradigm for examining the adaptive form-function relationship between molar enamel thickness and food hardness. Although both species exhibit a categorical preference for ripe fruit over other food objects, the thick enamel and crenulated occlusal surface of Pongo molar teeth predict a diet that is more resistant to deformation (hard) and fracture (tough) than the diet of Pan. We confirm these predictions with behavioral observations of Pan troglodytes schweinfurthii and Pongo pygmaeus wurmbii in the wild and describe the mechanical properties of foods utilized during periods when preferred foods are scarce. Such fallback foods may have exerted a selective pressure on tooth evolution, particularly molar enamel thinness, which is interpreted as a functional adaptation to seasonal folivory and a derived character trait within the hominoid clade. The thick enamel and crenulated occlusal surface of Pongo molars is interpreted as a functional adaptation to the routine consumption of relatively tough and hard foods. We discuss the implications of these interpretations for inferring the diet of hominin species, which possessed varying degrees of thick molar enamel. These data, which are among the first reported for hominoid primates, fill an important empirical void for evaluating the mechanical plausibility of putative hominin food objects.

A review is presented of the mechanical damage suffered by tooth crowns. This has been the subject of much recent research, resulting in a need to revise some of the thinking about the mechanisms involved. Damage is classified here by scale into macro-, meso- and microfracture. The focus is on the outer enamel coat because this is the contact tissue and where most fractures start. Enamel properties appear to be tailored to maximize hardness, but also to prevent fracture. The latter is achieved by the deployment of developmental flaws called enamel tufts. Macrofractures usually appear to initiate as extensions of tufts on the undersurface of the enamel adjacent to the enamel-dentine junction and extend from there into the enamel. Cracks that pass from the tooth surface tend to be deflected by an enamel region of high toughness; if they find the surface again, a chip (mesofracture) is produced. The real protection of the enamel-dentine junction here is the layer of decussating inner enamel. Finally, a novel analysis of mechanical wear (microfracture) suggests that the local toughness of the enamel is very important to its ability to resist tissue loss. Enamel and dentine have contrasting behaviours. Seen on a large scale, dentine is isotropic (behaving similarly in all directions) while enamel is anisotropic, but vice versa on a very small scale. These patterns have implications for anyone studying the fracture behaviour of teeth. © 2014 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

The threat of wear to dental enamel from hard particles of silica or silicates may have exerted great selective pressure on mammals. Increasing the hardness of enamel helps to forestall this, but capacity for variation is small because the tissue is almost entirely composed of hydroxyapatite. Hard though it is, enamel also displays considerable toughness, which is important in setting the sharpness of particles (defined as an attack angle) necessary to wear it. Added to the threat from environmental silica(tes) are phytoliths, particles of opaline silica embedded in plant tissues. We show here that phytoliths have very different properties to grit and dust and are unlikely to wear enamel. However, phytoliths would tend to fracture between teeth under similar conditions, so resembling natural agents of wear. In this context, we suggest that phytoliths could represent an example of mimicry, forming an example of a feeding deterrent operating by deceit. [ABSTRACT FROM AUTHOR]

Teeth are brittle and highly susceptible to cracking. We propose that observations of such cracking can be used as a diagnostic tool for predicting bite force and inferring tooth function in living and fossil mammals. Laboratory tests on model tooth structures and extracted human teeth in simulated biting identify the principal fracture modes in enamel. Examination of museum specimens reveals the presence of similar fractures in a wide range of vertebrates, suggesting that cracks extended during ingestion or mastication. The use of 'fracture mechanics' from materials engineering provides elegant relations for quantifying critical bite forces in terms of characteristic tooth size and enamel thickness. The role of enamel microstructure in determining how cracks initiate and propagate within the enamel (and beyond) is discussed. The picture emerges of teeth as damage-tolerant structures, full of internal weaknesses and defects and yet able to contain the expansion of seemingly precarious cracks and fissures within the enamel shell. How the findings impact on dietary pressures forms an undercurrent of the study. [ABSTRACT FROM AUTHOR]

The article discusses a study which investigated the association between tooth chipping and the diet and bite forces of fossil hominins. The researchers applied a simple fracture equation to estimate peak bite forces directly from chip size as many fossil hominins had antemortem chips on their posterior teeth. Photographs of hominin, ape, monkey and peccary enamel chips were obtained from original specimens or casts of museum specimens.

Abstract: The large, bunodont postcanine teeth in living sea otters (Enhydra lutris) have been likened to those of certain fossil hominins, particularly the ’robust’ australopiths (genus Paranthropus). We examine this evolutionary convergence by conducting fracture experiments on extracted molar teeth of sea otters and modern humans (Homo sapiens) to determine how load-bearing capacity relates to tooth morphology and enamel material properties. In situ optical microscopy and x-ray imaging during simulated occlusal loading reveal the nature of the fracture patterns. Explicit fracture relations are used to analyze the data and to extrapolate the results from humans to earlier hominins. It is shown that the molar teeth of sea otters have considerably thinner enamel than those of humans, making sea otter molars more susceptible to certain kinds of fractures. At the same time, the base diameter of sea otter first molars is larger, diminishing the fracture susceptibility in a compensatory manner. We also conduct nanoindentation tests to map out elastic modulus and hardness of sea otter and human molars through a section thickness, and microindentation tests to measure toughness. We find that while sea otter enamel is just as stiff elastically as human enamel, it is a little softer and tougher. The role of these material factors in the capacity of dentition to resist fracture and deformation is considered. From such comparisons, we argue that early hominin species like Paranthropus most likely consumed hard food objects with substantially higher biting forces than those exerted by modern humans. [Copyright &y& Elsevier]