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.

Hunter-Schreger bands (HSB) are seen in teeth that are composed of crossed sets of enamel prisms. They are present in the teeth of man and many other mammals, but absent in most insectivores and multituberculates. It has been suggested that the presence of HSB makes the tooth enamel less likely to split and is associated with chewing ability. We have traced the occurrence of HSB back to the arctonic condylarths of the early Palaeocene (Puercan) age; this must be close to the first appearance of the bands in placental mammals. Our data indicate that the teeth of almost all large mammals since the early Palaeocene have contained these bands, in an orientation that is optimal for limiting the propagation of vertical fractures. The appearance of the bands is associated with the differentiation of herbivores and carnivores from insectivores and our data indicate that their development was critical to the diversification of mammals because it allowed the use of new types of foods.

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