Functional and phylogenetic interpretation of enamel microstructure in rhinoceroses

Abstract

Worn enamel surfaces of the cheek teeth in fossil and recent rhinoceroses are characterized by fine, parallel ridges aligned perpendicular to the enamel-dentin interface. We show that these ridges result from an unusual enamel ultrastructure in which a primitively horizontal layering of the prisms has become vertical. The new structure apparently appeared between early and middle Eocene, at the time when the superfamilies of perissodactyls were rapidly diverging. Similar modifications of the enamel structure occurred in certain parts of the cheek teeth in tapiroids, chalicotherioids and brontotherioids, but hardly at all in the equids. The modified enamel structure, where it occurs in groups other than rhinocerotoids, is associated with lophs but not cusps. Experimental evidence shows that the modified enamel is more resistant to wear than the unmodified enamel. The consistent association with thin lophs rather than cusps suggests that the modified enamel evolved to prolong the life of the lophs, where occlusal pressures are highest and attrition greatest. The dominance of modified enamel in rhinocerotoids correlates with the higher degree of compression of the cusps and extreme lophodonty in this group. The absence of modified enamel structure in the equids, even in the ectoloph, correlates with the lesser importance of the ectoloph in equids relative to brontotherioids, chalicotherioids and rhinocerotoids.

Microstructural variation in conodont enamel is a functional adaptation

“…Analysis of microwear demonstrates that mechanical abrasion is a signi¢cant factor in Idiognathodus P 1 platforms (Purnell 1995;Donoghue & Purnell 1999b). This equates with a surfacenormal crystallite orientation that is optimal for abrasion resistance (Rensberger & von Koenigswald 1980). The asymmetrical orientation (in blade-parallel section through the platform) of the crystallite domains (¢gure 3d) places the component crystallites directly perpendicular to the main direction of force in the power stroke of the elements, which Donoghue & Purnell (1999b) identi¢ed as ventral to dorsal.…”

“…Furthermore, where there is an anisotropic distribution of a surfaceparallel microstructure, it can be observed to occur preferentially along occlusal surfaces (¢gure 1d ). The alignment of (immediately) subsurface crystallites parallel to inferred shearing forces appears counterintuitive as this is the orientation in which the crystallites are least resistant to wear (Rensberger & von Koenigswald 1980). However, evidence suggests that this may be an adaptation to the functional development of a £atter occlusal surface with sharper cutting edges.…”

Tooth wear and its relation to kind of food: The repercussion on age criteria in Capra pyrenaica

“…The occurrence of cusps on the molar surface can be explained by differential wear of its surface, depending on tooth design and composition (relation dentine /enamel; Rensberger 1973, Rensberger andKoenigswald 1980). Cusps increase the crushing capability of the molar teeth ( Van Valen 1960) but they wear off with age and disappear and processing capacity decline as the tooth wear out (Lanyon and Sanson 1986).…”

Evolução dos Toxodontia da América do Sul durante o Cenozoico: aspectos dentários, paleoclimáticos e paleoambientais

“…O esmalte radial é o mais primitivo, encontrado na maioria dos mamíferos mesozoicos, no qual os prismas distribuem-se na zona mais externa, radialmente em relação à junção esmalte-dentina (Koenigswald & Clemens 1992), e está relacionado à resistência a forças abrasivas, parecendo reduzir seus efeitos, especialmente quando superfícies oclusais opostas deslizam umas sobre as outras (Rensberger & Koenigswald 1980, Boyde & Fortelius 1986. Embora primitivo, o esmalte radial é efetivo na manutenção de superfícies cortantes pontiagudas.…”