Abstract
During the evolution of the Equoidea, the histological structures of the teeth have become more complex as the molars have become hypsodont in form. The straight Hunter-Schreger bands of Hiracotherium have evolved into a more complex pattern in Equus. The enamel prisms changed from an arched form (about 5μm in diameter) with an alternating pattern in Hiracotherium to an oval form (about 2 μm width) arranged in straight rows in Equus. In Equus the rows of prisms are separated by interprismatic sheets. This pattern may have increased the architectural strength of the enamel, and is related to the later hypsodont molar teeth. With regard to the enamel, it is suggested that the number of ameloblasts increased whereas their size decreased during the evolution of the Equoidea. As to the dentine, on the other hand, the peritubular dentine was little observed in Hyracotherium, but evolved in Mesohippus and Equus. A transitional pattern between these two structures is observed in Mesohippus molars, which are not hypsodont. These changes in the histology of tooth structure may have preceded the changes in molar shape during evolution.
