Enamel microstructure in the phylogeny of the Equidae

Abstract

ABSTRACT Mammalian teeth show a considerable complexity in microstructure. The arrangement of decussated enamel prisms gives enamel the functional advantages of a composite material. This prism arrangement is genetically determined and altered throughout evolution. As the evolution of hypsodont molars implies a drastic alteration of the loading conditions and of the stress pattern in teeth, it can be expected that enamel microstructure is also affected. Equids and several other ungulates evolved the modified radial enamel in adaptation to these altered conditions. As the fossil record for the phylogeny of horses is available in considerable completeness, this group is suitable to investigate the correlation of morphologic evolution and the evolution of enamel microstructure. Indeed, the same patterns of evolutionary change previously known from the morphologic level can also be demonstrated on the microstructural level. Modified radial enamel is characterized by the orientation of the inter-prismatic crystallites, the thickness of the interprismatic matrix, and the radial prism arrangement. The three characters appear step by step in a phylogenetic context. First the orientation of the interprismatic crystallites perpendicular to the prisms occurs in Mesohippus. In Parahippus, all three characters appear together for the first time, but the location and the very small amount of modified radial enamel suggests that a function in strengthening the enamel is very unlikely. In all hypsodont equids, modified radial enamel appears in a considerable amount and obviously is an adaptation to the altered stress distribution in hypsodont molars. Hypohippus shows a further combination of two of the characters; the crystallite orientation and the thickness of the interprismatic matrix is similar to the modified radial enamel, but the prisms are not arranged in radial rows but in decussating layers. Because the characters of modified radial enamel appear stepwise and preadaptively, and because some other lineages that lack the development of these characters did not evolve hypsodont molars, it seems probable that enamel microstructure influences the potential for macromorphologic phylogenetic change.