Enamel thickness

Teeth represent the hardest tissue in most living vertebrates. Their main function is catching prey and mastication of food. Therefore, they have a unique and delicate ultrastructure, typically with highly mineralized enamel on the outside and sober bone-like dentin inside representing the endodontium.
Bony fish, amphibians, reptiles and mammals (including humans), use calcium phosphate as tooth mineral.The tooth mineral in vertebrates is hydroxyapatite with some carbonate substitutions on phosphate positions, the so-called dahllite. An exception are cartilaginous fish including sharks which use
fluoroapatite as tooth mineral.We have shown recently that shark teeth contain fluoroapatite only in the outer layer, i.e. the enameloid (the enamel-equivalent in sharks, more appropriately called durodentin), but not in dentin. This enameloid is derived from cells of the tooth papilla and is different from true enamel of epithelial origin in bony fish and upper vertebrates. Here we report on a comprehensive study of the teeth of extinct sharks, sauropterygians, mosasaurs and dinosaurs where their ultrastructure and chemical composition were analyzed with high-end chemical and microscopic methods (elemental analysis, scanning electron microscopy, X-ray powder diffraction including Rietveld refinement, infrared spectroscopy). More specific teeth represent the hardest tissue in vertebrates and appear very early in their evolution as an ancestral character of the Eugnathostomata (true jawed vertebrates). In recent vertebrates, two strategies to form and mineralize the outermost functional layer have persisted. In cartilaginous fish, the enameloid is of ectomesenchymal origin with fluoroapatite as the mineral phase. All other groups form enamel of ectodermal origin using hydroxyapatite as the mineral phase. The high abundance of teeth in the fossil record is ideal to compare structure and composition of teeth from extinct groups with those of their recent successors to elucidate possible evolutionary changes. Here, we studied the chemical omposition and the microstructure of the teeth of six extinct shark species, two species of extinct marine reptiles and two dinosaur species using high-resolution chemical and microscopic methods. Although many of the ultrastructural features of fossilized teeth are similar to recent ones (especially for sharks where the ultrastructure basically did not change over millions of years), we found surprising differences in chemical composition. The tooth mineral of all extinct sharks was fluoroapatite in both dentin and enameloid, in sharp contrast to recent sharks where fluoroapatite is only found in enameloid. Unlike extinct sharks, recent sharks use hydroxyapatite as mineral in dentin. Most notably and hitherto unknown, all dinosaur and extinct marine reptile teeth contained fluoroapatite as mineral in dentin and enamel. Our results indicate a drastic change in the tooth mineralization strategy especially for terrestrial vertebrates that must have set in after the cretaceous period. Possibly, this is related to hitherto unconsidered environmental changes that caused unfavourable conditions for the use of fluoroapatite as tooth mineral.

Early observations of cracks protect the teeth. The crack in teeth initiates due to the flaws, defect, or inappropriate fillings design. The brittleness allows the crack to extend from any notches over the enamel due to the lower plasticity. Therefore, in this issue, linear elastic fracture mechanics (LEFM) assumptions will be used instead of the elastic–plastic fracture mechanics (EPFM). Traditionally, the vertical crack in the teeth is predominated. The load distributions over the crown and the cyclic loading will propagate the crack. There are limited works trying to simulate the crack in the teeth. In this work, the crack path (CP) and the fracture behavior of the tooth have been simulated. It was shown that LEFM is sufficient for such simulation.

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 suscep- tible to certain kinds of fractures. At the same time, the base diameter of sea otter !rst 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 !nd 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.

Studies on dental heritability focused on tooth crown size and external structure while studies of enamel thickness and dental tissue proportions permit discussion of taxonomy, phylogenetic relationships or dietary habits. Here we employ a microtomographic (microCT)-based record of teeth from Neolithic individuals of the necropolis of Gurgy (France) to assess biological proximity and heritability from internal tooth structure. Second upper permanent molars from 21 immature and adult individuals of both sexes were scanned using high-resolution microCT at the MRI platform (Univ. Montpellier II, France) on a Skyscan 1076 X-ray equipment. Acquisitions were performed with an isotropic voxel size ranging from 17.93 to 36.18 µm. Semi-automatic threshold-based segmentation was conducted using Avizo v.7 (VSG) and crowns were digitally isolated from roots. For each crown, 15 linear, surface, and volumetric 2D and 3D variables describing enamel thickness and dental tissue proportions were digi...