Gephyrosaurus

Abstract: The skull and lower jaw of a new sphenodontid reptile Diphydontosaurus avonis is described from disarticulated bones. The fossils were recovered from a detrital limestone of Rhaetian age deposited within solutional and tectonically formed fissures within the Carboniferous Limestone of Tytherington quarry near Bristol. The bone remains, numbering over 1000, are exquisitely preserved with intact facets. Diphydontosaurus was the smallest member of an insular fauna, was primarily insectivorous, and probably formed locally high-density populations. Uniquely for a sphenodontid, Diphydontosaurus had pleurodont teeth on the premaxilla and on the anterior regions of the dentary and maxilla. However, Diphydontosaurus also has the series of acrodont teeth alternating in size on the maxilla and dentary which is characteristic of the Sphenodontidae and particularly Sphenodon. An analysis of 49 synapomorphs in the Diapsida emphasises the sphenodontid nature of Diphydontosaurus. A study of these synapomorphs among the other Triassic sphenodontids Clevosaurus and Planocephalosaurus and the eosuchian Gephyrosaurus suggests that the lack of a quadrate-quadratojugal conch and the complete lower temporal bar are secondarily derived in the `living fossil' Sphenodon punctatus. The tuatara is therefore much less archaic than hitherto proposed. Functional reasons are advocated for the loss of the conch and the regrowth of a complete lower temporal bar. Transformation series are described which could have led to the shape of the maxilla, dentary, premaxilla, palatine and to the loss of the lacrimal in Sphenodon.

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.