The Microstructure of Reptilian Tooth Enamel

Book excerpt: A survey of the enamel microstructure of 43 reptilian taxa reveals great structural variety and complexity. With the exception of the agamid lizard Uromastyx, all taxa have enamel without prisms. Reptilian enamel is thus not comparable to that of the Mammalia. The development of a new terminology was necessary to describe reptilian enamel and to address questions of functional vs. phylogenetic constraints on its microstructure. This new terminology was developed using a similar approach to one developed for mammalian enamel. It is hierarchical and free of amelogenetic or phylogenetic connotations. Starting at the smallest structures, five levels of complexity are recognized: crystallite level, module level, enamel type level, schmelzmuster level, and dentition level. Detailed microstructural analysis of the 43 taxa using the new terminology indicates that phylogenetic constraints are of little significance. Only two higher taxa, advanced ornithopod dinosaurs and the Ichthyosauria, show distinctive enamel synapomorphies. Biomechanical constraints on the internal structure of reptilian enamel are even more difficult to discern. The major factor determining microstructure is enamel surface morphology, i.e. the ridges and wrinkles on the surface of many teeth which presumably are adaptive. Reptilian amelogenesis is largely responsible for the creation of this morphology as the boundary plane between the dentin and the enamel (the EDJ) is smooth and not ridged or wrinkled. The concept of amelogenesis as enamel surface morphogenesis is best illustrated by a comparison of durophagous reptiles. Numerous unrelated groups convergently evolved the typical bulbous tooth shape but each group has an autapomorphic enamel microstructure. Other ecomorphotypes such as ziphodont carnivores exhibit convergent evolution of enamel microstructure. The structures observed raise important questions about reptilian amelogenesis. There certainly is no one-to-one correspondence between structural units and the ameloblast matrix as opposed to the situation in mammals. Such a correspondence has been assumed in the current models of the evolutionary origins of mammalian enamel prisms, and its absence invalidates these models.