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.
There are two living species of tuataras, Sphenodon guntheri and S. punctatus. Collectively, they are restricted in their distribution to 30 islands surrounding New Zealand, but S. punctatus is known from Holocene deposits all over New Zealand. It is thought that the arrival of Polynesians roughly 1000 years ago, and of Europeans over the last several centuries, is responsible for the tuatara's disappearance from the mainland. This is because settlers brought rats and dogs, which prey on tuataras, and cattle, which trample them. Sphenodon has been unable to rebound from the impact of humans because of its incredibly limited reproductive potential. Tuataras do not reproduce until they are 12-15 years old, and even then, they reproduce only once every four years. It is estimated that only 55,000 tuataras remain today (Mlot, 1997), despite the fact that Sphenodon was the first reptile in the world to be protected (since 1895, by New Zealand law).
The tuatara, although superficially resembling a lizard, lacks the external tympanum, femoral pores, and paired evertible hemipenes that lizards possess. Cranial features diagnostic of rhynchocephalians include an enlarged tooth row along the maxillary side of the palatine and the loss of the splenial (Gauthier et al., 1988). Both are discernible in the animations above.
About the Species
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.
About this Specimen
The adult specimen was scanned by Richard Ketcham and Matthew Colbert on 19 February 1999 along the coronal axis for a total of 520 slices, each slice 0.225 mm thick, with an interslice spacing of 0.15 mm, for a slice overlap of 0.075 mm. The dataset displayed was reduced for optimal Web delivery from the original, much higher resolution CT data.
Christensen, K. 1927. The morphology of the brain of Sphenodon. University of Iowa Studies in Natural History 12:1-29.
Cree, A., and C. H. Daugherty. 1990. Tuatara sheds its fossil image. New Scientist 1739:30-34.
Dendy, A. 1899. Outlines of the development of the Tuatara, Sphenodon (Hatteria) punctatus. Quarterly Journal of Microscopical Science 42:111-152.
Gauthier, J., Estes, R., and K. de Queiroz. 1988. A phylogenetic analysis of Lepidosauromorpha; pp. 15-98 in R. Estes, and G. Pregill (eds.), Phylogenetic Relationships of the Lizard Families. Stanford University Press, Stanford, California.
Howes, G. B., and H. H. Swinnerton. 1901. On the development of the skeleton of the Tuatara Sphenodon punctatus, with remarks on the egg, on the hatchling, and on the hatched young. Transactions of the Zoological Society of London 16:1-86.
Mlot, C. 1997. Return of the tuatara. Science News 152:300-301.
Osawa, F. 1898. Beiträge zur Anatomie der Hatteria punctata. Archiv für Mikroskopische Anatomie und Entwicklungsgeschichte 51:481-691.
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Sphenodon punctatus on The Animal Diversity Web (The University of Michigan Museum of Zoology)
Tuatara page of the New Zealand Department of Conservation