Hard tissue anatomy of the cranial joints in Sphenodon (Rhynchocephalia): sutures, kinesis, and skull mechanics

Abstract

The anatomy of the extant lepidosaur Sphenodon (New Zealand tuatara) has been extensively examined by palaeontologists and comparative anatomists because of its phylogenetic status as the only living member of the Rhynchocephalia. It is also of interest because of its sophisticated feeding apparatus and a prooral (anteriorly directed) mode of shearing used to rip food apart. However, despite several detailed descriptions of the skull, the three-dimensional relationship between individual bones of the skull has generally been ignored. Here we provide the first joint by joint description of the hard tissue anatomy for almost every cranial suture in the skull of Sphenodon. This survey shows that most joints involve either abutments (e.g., along the midline) or extensive overlaps (e.g., more peripheral areas) but there are others that are heavily interlocked (e.g., postorbital-postfrontal) or involve a notable amount of soft tissue (e.g., vomer-premaxilla). There is variation in facet surface texture (e.g., smooth, ridged, pitted) but extensive interdigitation is uncommon and generally restricted to one plane. The joints do not appear suited to promote the marked intracranial movement reported in lizards such as geckos. However, it is possible that the base of the premaxillae would have been able to pivot slightly when loaded or impacted by the lower jaw during shearing. The extensive overlapping joints probably serve to maximise the surface area available for soft tissues that can dissipate and redistribute stress while maintaining the rigidity of the skull. These joints are larger in adults which bite more forcefully and may feed on harder prey.