Article: Tooth form, growth, and function in Triassic rhynchosaurs (Reptilia, Diapsida)

The rhynchosaurs (Reptilia, Diapsida) were important medium-sized herbivores in the middle to late Triassic (245-208 Ma). They had a remarkable multiple-row dentition with a powerful precision-shear bite. Their teeth had ankylothecodont implantation—that is, the deeply rooted teeth were fused to bone of attachment which could also invade the pulp chambers, but there was no socket. There was no typical reptilian tooth replacement from below. Detailed analyses of two typical rhynchosaurs, Stenaulorhynchus (middle Triassic) and Hyperodapedon (late Triassic), show that the teeth on each jaw are organized into clear longitudinal Zahnreihen. In each of these Zahnreihen, an ontogenetic series of teeth may be seen clearly from the back to the front of the jaw, ranging from newly ankylosed teeth to fully worn and largely resorbed teeth. The cycle of tooth growth and resorption is controlled by normal jaw growth in which the occlusal area moves back constantly: teeth appear to 'swing' into occlusion at the back and out of occlusion at the front of this area of wear. The multiple-row rhynchosaur dentition effectively 'freezes' ontogeny and it offers important information on vertebrate tooth replacement, especially in view of the fact that the fossil material offers excellent histological detail.