Sphenodon punctatus, Tuatara skull

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This specimen, a preserved adult head (YPM 9194), was collected by John Ostrom from an unknown locality between 1960 and 1969. It was made available to the University of Texas High-resolution X-ray CT Facility for scanning by Dr. Jacques Gauthier of Yale University. The specimen was scanned for Dr. Chris Bell of The University of Texas at Austin. Funding for image processing was provided by a National Science Foundation Assembling the Tree of Life grant (EF-0334961), The Deep Scaly Project: Resolving Squamate Phylogeny using Genomic and Morphological Approaches, to Drs. Jacques Gauthier of Yale University, Maureen Kearney of the Field Museum, Mike Lee of the University of Adelaide, Jessie Maisano of The University of Texas at Austin, Tod Reeder of San Diego State University, Olivier Rieppel of the Field Museum, Jack Sites of Brigham Young University, and John Wiens of SUNY Stonybrook.

Sphenodon species, also known as tuataras, are the sole living representatives of Rhynchocephalia, the sister taxon to Squamata (the clade that includes lizards, snakes, and amphisbaenians; e.g., Ctenosaura pectinata, Gerrhosaurus major, Rhineura sp., and Varanus gouldii). By necessity, Sphenodon is usually used to represent the first outgroup to Squamata in phylogenetic analyses. However, there is a large gap in the fossil record of rhynchocephalians, from the early Cretaceous up to a subfossil, S. diversum. Extant Sphenodon is highly derived, and this limits its utility for polarizing characters in the evolution of squamates.