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Parallel Evolution of Derived Enamel Microstructure in Carnivorous Mammals

STEFEN, Clara, Museum of Paleontology and Dept. of Integrative Biology, University of California, Berkeley, CA 94720-4780

Within carnivorous mammals of different systematic affinities the parallel evolution of similar, derived enamel microstructures, Hunter-Schreger bands (HSB), are correlated with similar feeding habits.

HSB consist of layers of parallel prisms; the orientation of the prisms within one layer is distinctively different from the orientations of prisms in the adjacent layers, a decussating pattern. HSB course around the crown of a tooth oriented in generally horizontal planes. According to their morphology in a vertical plane, HSB can be classified as undulating, acute-angled, or zigzag. Undulating HSB, the primitive morphology, have a simple wave form; in acute-angled HSB the wave crests and bases of the troughs are angular; whereas zigzag HSB are characterized by a complex, derived three-dimensional structure of prism decussation.

Among modern mammals, zigzag HSB are found only in species with feeding habits that include bone crushing. The high biting forces required to crack bones induce high tensile stress in the enamel, which, in zigzag HSB, can be dispersed effectively through the complex decussation of the prisms. Pattern of occurrences and theoretical grounds support the interpretation that zigzag HSB are more stress resistant than other enamel types.

A phylogenetically based survey of enamel microstructure of 19 families of living and extinct eutherians, known or thought to include carnivores, included members of the Arctocyonidae, Creodonta, Mesonychidae (Cete), Carnivora, and Entelodontidae (Artiodactyla). It demonstrated that among modern mammals in which the diet could be documented, zigzag HSB are characteristic and limited to the enamel of bone crushing carnivores, e.g., hyaenas. In contrast, the enamel of non-bone crushing carnivores, or piscivorous, and frugivorous species had undulating HSB. The survey also demonstrated that zigzag HSB evolved independently from radial enamel, through the structural stage of undulating HSB, in these five phylogenetically distinct lineages.

75/125 YEARS