Jones, M.E.H.; (2006) Skull evolution and functional morphology in Sphenodon and other Rhynchocephalia (Diapsida: Lepidosauria). Doctoral thesis , University of London.

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Abstract

The tuatara, Sphenodon, is the sole extant representative of the Rhynchoccphalia, a group of diapsid reptiles that were extremely widespread during the Mesozoic. Traditionally, Sphenodon was considered to be "primitive", and its fossil relatives are frequently disregarded as conservative. However, a detailed review shows that the group was diverse in terms of both morphology and lifestyle. In particular, it demonstrates a range of different tooth morphologies and arrangements. Geometric morphomctric analysis shows that differences between the skull shape of different taxa is related to feeding (e.g. muscle volume, jaw joint position). Derived taxa possess stouter teeth, an increase in space for adductor musculature, a larger skull size and in turn a greater potential bite force. A survey of suture morphology reveals that by comparison to basal taxa (Diphydontosaurus, Gephyrosaurus), derived taxa (e.g. Clevosaurus, Sphenodon) have more complicated sutures including extensive overlaps. These observations correspond with research indicating that sutures are important for controlling and reducing stresses within the skull. Variation in sutures is also found between different derived taxa. For example in Clevosaurus the most complex sutures are found in the palate by contrast in Sphenodon. the most complex sutures surround the postfrontal bones. These differences are probably related to the extent and distribution of forces experienced by the skull. A contributing factor is the different mode of shearing mechanism employed by each taxon: a precise orthal scissor-like cut in Clevosaurus and a prooral rip in Sphenodon, each of which required a specific muscle arrangement. The Rhynchocephalia as a whole demonstrate a progressive evolutionary trend in their diet toward larger and harder food items this allowed at least one clade to become herbivorous. This to some extent echoes Sphenodon ontogeny. The rhynchocephalian skull is highly integrated suture complexity increased in parallel with increasing complexity of feeding apparatus. Key Words: skull design, functional morphology, bite force, jaws, teeth, palaeoecology.

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