Structure and compositional characteristics of caniniform dental enamel in the tuatara sphenodon punctatus (Lepidosauria: Rhynchocephalia)

Abstract

OBJECTIVE: The evolution of dental tissues in relation to tooth function is poorly understood in non-mammalian vertebrates. We studied the dentition of Sphenodon punctatus, the sole remaining member of the order Rhynchocephalia in this light. Methods: We examined 6 anterior maxillary caniniform teeth from adult Sphenodon by scanning electron microscopy, nano-indentation and Raman spectroscopy. Results: The elastic modulus (E) for tuatara enamel was 73.17 (sd, 3.25) GPa and 19.52�0.76 Gpa for dentine. Hardness (H) values for enamel and dentine were 4.00 (sd, 0.22) and 0.63�0.02 Gpa respectively. The enamel was thin (100 �m or less), prismless and consisted of grouped parallel crystallites. Incremental lines occurred at intervals of about 0.5 to 1 �m. There were tubular structures along the enamel dentine junction running from the dentine into the inner enamel, at different angles. These were widened at their base with a smooth, possibly inorganic lining. Enamel elastic modulus and hardness were lower than those for mammals. Conclusions: The presence of enamel tubules in the basal part of the enamel along the EDJ remains speculative, with possible functions being added enamel/dentinal adhesion or a role in mechanosensation.