Dentary tooth shape in Sphenodon and its fossil relatives (Diapsida: Lepidosauria: Rhynchocephalia)

Today Rhynchocephalia, the sister tax-on to Squamata (snakes, lizards and amphisbaenians), is only represented by the tuatara (Sphenodon) of New Zealand. However, for much of the Mesozoic, the group was speciose and globally distributed. Historically, the Rhynchocephalia were considered to be homogenous and unspecialized but new fossils and new research are overturning this view. As well as differences in body size, body proportions, habit (aquatic vs. terrestrial), and skull structure, their teeth show variation in shape, size, number, arrangement and enamel thickness. This suggests differences in diet and mode of feeding. The teeth of basal taxa tend to be relatively simple and conical, whereas those of derived taxa possess complex flanges and wear facets.

Dimensions of the dentary tooth bases were measured in apical view for a large sample of rhynchocephalian taxa.

These measurements reveal three general tooth types: small ovoid teeth, large wide teeth, and large elongate teeth.

These three categories correspond to food processing as inferred from tooth wear (puncturing+crushing, grinding+shredding and tearing+cutting, respectively). A phylogenetic signal is also present as the teeth of basal taxa generally conform to the first category. The larger tooth bases of derived taxa provide stronger attachment and contribute to a stouter tooth shape more resistant to loading and torsional forces. This in turn corresponds to skull architecture because the skulls of derived taxa could accommodate larger jaw muscles with a greater leverage relative to basal taxa.