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 d ietary shift from frugivory to folivory; Cantius spp. were largely fru givorous, whereas Notharctus spp. and Copelemur were probably more fol ivorous. Here we report on the evolution of molar enamel microstructur e in the notharctid clade and its relationship to adaptive change. Mol ar teeth of ten North American notharctids, Cantius ralstoni, Cantius mckennai, Cantius trigunodus, Cantius abditus, Cantius sp. (>C. abditu s), Cantius venticolus, Copelemur praetutus, Notharctus nuniensus, Not harctus robinsoni, Notharctus sp. (Bridger B), were studied using scan ning 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 N. nun iensus enamels are more complex but prism decussation zones are indist inct and irregular. Later, larger-bodied Cantius (C. abditus, C. sp., C. venticolus) show distinct, regular decussation zones, as do N. robi nsoni and Bridger B Notharctus. However, there is no prism decussation in the smaller middle Wasatchian species, C. praetutus. This distribu tion of enamel types supports the hypothesis that prism decussation ev olved independently within the notharctid clade in conjunction with in crease in body size, but not with diet. Other aspects of enamel struct ure (prism patterns, relative orientations of crystallites) show littl e interspecific variation, and therefore, are of little taxonomic util ity. All notharctids studied are characterized by a combination of clo sed and are-shaped prisms. Are-shaped, Pattern 3 prisms predominate an d closed, Pattern 1 prisms are largely restricted to the most superfic ial 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 he re. The evolutionary change in enamel structure that did occur, the de velopment of prism decussation, was more directly linked to body size change than to the dietary shift fi om frugivory to folivory documente d by changes in their gross molar morphology. (C) 1996 Academic Press Limited