Evolution of molar enamel microstructure in North American Notharctidae (primates)

Evolutionary changes in gross dental morphology and body size of early to middle Eocene North American notharctids is thought to reflect a dietary shift from frugivory to folivory; Contius spp. were largely frugivorous, whereas Notharctus spp. and Copelemur were probably more folivorous. Here we report on the evolution of molar enamel microstructure in the notharctid clade and its relationship to adaptive change. Molar teeth often North American notharctids, Cantius ralstoni, Cantius mckennai, Cantius trigonodus, Cantius abditus, Cantius sp. (>C. abditus), Cantius venticolus, Copelemur praetutus, Notharctus nuniensus, Notharctus robinsoni, Notharctus sp. (Bridger B), were studied using scanning electron and light microscopy. The simple enamel organization of early Cantius (C. ralstoni, C. mckennai) consists of radial enamel and a thin layer of nonprismatic surface enamel. C. trigonodus and M. nuniensus enamels are more complex but prism decussation zones are indistinct and irregular. Later, larger-bodied Cantius (C. abditus, C. sp., C. venticolus) show distinct, regular decussation zones, as do N. robinsoni and Bridger B Notharctus. However, there is no prism decussation in the smaller middle Wasatchian species, C. praetutus. This distribution of enamel types supports the hypothesis that prism decussation evolved independently within the notharctid clade in conjunction with increase in body size, but not with diet. Other aspects of enamel structure (prism patterns, relative orientations of crystallites) show little interspecific variation, and therefore, are of little taxonomic utility. All notharctids studied are characterized by a combination of closed and arc-shaped prisms. Arc-shaped, Pattern 3 prisms predominate and closed, Pattern 1 prisms are largely restricted to the most superficial prismatic enamel. It is likely that this arrangement is primitive for primates. Thus, most aspects of notharctid enamel structure appear to have been stable over the approximately 6 million years sampled here. The evolutionary change in enamel structure that did occur, the development of prism decussation, was more directly linked to body size change than to the dietary shift from frugivory to folivory documented by changes in their gross molar morphology.