Microstructural Reinforcement in the Canine Enamel of the Hyaenid Crocuta crocuta, the FelidPuma concolorand the Late Miocene Canid Borophagus secundus

In bone-eating carnivores such as the hyena Crocuta crocuta, the tooth enamel contains a secondary vertical prism decussation phyletically derived from the wavelike horizontal decussation of primitive carnivores. The structure resists fracture under vertical, oblique, and horizontal tensile stresses, owing to the following modifications of the primitive structure. Positions of wave crests and of wave troughs are synchronized in the vertically successive layers of decussating prisms. Prisms in each successive layer run in a common direction at the crests and in a common but reversed direction at the troughs. Between the crests and troughs, prisms in obliquely slanting layers often retain their primitively reversed prism directions. Near the enamel–dentine junction (EDJ), irregular horizontal decussation is retained. In the upper canine of C. crocuta, a consumer of large bones, secondary vertical decussation is largely confined to the labial and anterior sides of the crown toward the tip where modeling of the static stresses predicts the tensile stresses to be highest and aligned vertically. In Puma concolor, which does not consume large bones, secondary vertical decussation is absent, indicating stress magnitude to be a critical factor in the selection for secondary vertical decussation. The canine enamel in Borophagus secundus, an extinct canid with derived aspects of skull and dental shape like those in hyenas, has dental structures similar to those in C. crocuta but which differ in several ways. The wavelike shapes of the decussation planes are better developed in transverse sections in B. secundus than in C. crocuta, suggesting either the folds are less modified or they dip at a steeper angle. Secondary vertical decussation in B. secundus is more extensive around the circumference of the canine than in C. crocuta, related to a difference in cross-sectional shape of the tooth. Vertical prism decussation may have been more frequently attained in carnivorous mammals than in ungulates because of the more random orientation of dental stresses which creates a selective advantage for wavy decussation planes—a structural transition to vertical decussation.