Microstructure of dental hard tissues and bone in the Tuatara dentary, Sphenodon punctatus (Diapsida: Lepidosauria: Rhynchocephalia)

Abstract

The Tuatara, Sphenodon, is a small reptile currently restricted to islands off the coast of New Zealand where it feeds mainly on arthropods. A widely held misconception is that ‘Sphenodon does not have real teeth’ and instead possesses ‘serrations on the jaw bone’. One hatchling and one adult dentary were examined under SEM. Two longitudinal ground sections 100-μm thick were prepared through a lower canine tooth and its supporting tissues. There was clear evidence of aprismatic enamel (primless enamel) containing dentine tubules crossing the EDJ, dentine, cementum and a basalbone attachment. Enamel increments averaged ~3 μm/day and extension rates were ~30 μm/day. The base of the tooth consisted of basal attachment bone that graded from few cell inclusions to lamella or even Haversian-like bone with evidence of remodeling. A string of sclerosed pulp-stone like structures filled the pulp chamber and were continuous with the bone of attachment. Bone beneath the large central nutrient mandibular (Meckel’s) canal was quite unlike lamella bone and appeared to be fast growing and to contain wide alternating cell-rich and cell-free zones. Bone cells were rounded (never fusiform) and had few, if any, canaliculi. The dentine close to the EDJ formed at about the same rate as enamel but also contained longer period increments ~100 μm apart. These were spaced appropriately for monthly lunar growth bands, which would explain the basis of the banding pattern observed in the fast growing basal bone beneath the Meckel’s canal.