Evolutionary aspects of reptilian and mammalian enamel structure.

The evolution of enamel structure is dealt with here on the basis of fossil reptiles and mammals ranging from the Triassic to the present. The evidence suggests that prismatic enamel had developed in some therapsid reptiles and the mammal, Eozostrodon about 180 million years ago. For the next 100 million years, mammalian evolutionary history is sparingly documented and this is reflected in the poor record of enamel evolution during this period. The few Jurassic reptiles and mammals studied suggest a preprismatic structure. In the Late Cretaceous (80 to 65 million years ago) when the fossil record improves, mammalian enamel investigated from North American localities, are found to be prismatic; allotherian (multituberculate) and metatherian (marsupial) enamels are usually tubular, while eutherian (placental) ones are not. Prism structure in Tertiary mammals in general, conforms to that of their present day descendants, but there are discernible exceptions. The record of evolutionary change in Tertiary mammals is obscured by functional modifications related to biomechanical stresses. Enamel structure may be secondarily modified; similar in phylogenetically unrelated groups (eg., pauciserial enamel of early rodents) or dissimilar at the intra-familial level (eg., rodent families Ctenodactylidae and Ischryomyicae). Prismatic enamel is recorded from the tooth of a hatchling of the gavial, Gavialis gangeticus.