Evans, Susan E. Department of Anatomy and Developmental Biology, University College London, London, United Kingdom.
Last reviewed:November 2015
- Extant forms
- Early rhynchocephalians
- Links to Primary Literature
- Additional Readings
An order of lepidosaurian reptiles represented by two extant species, Sphenodon punctatus and Sphenodon guntheri, commonly known as the tuatara. The reptilian members of the order Rhynchocephalia are referred to as the tuatara (also tuataras). There are two surviving species of tuatara: Sphenodon punctatus and Sphenodon guntheri. Sphenodon (a name meaning wedge-tooth) has a fully diapsid skull (Fig. 1a) that was secondarily acquired from an ancestral state represented by Gephyrosaurus (Fig. 1b), the most primitive known member of the Rhynchocephalia, in which the lower temporal bar was incomplete as the result of a loss of contact between the jugal and quadratojugal (Fig. 1), which is a condition unlike that in lizards and basal lepidosauromorphs. The extinct Gephyrosaurus shows some, but not all, of the advanced characters of more typical rhynchocephalians. Most notably, it has a primitive tooth implantation; that is, the teeth are pleurodont (attached to the inside of the jaw). Typically, the teeth of rhynchocephalians are fused to the edges of the jaws (a condition known as acrodont). There are two upper rows of teeth, one of which is enlarged and palatine (on the palate), and one row of dentary (lower jaw) teeth. The dentary teeth bite between the two upper rows in a vertical manner (as in some extinct taxa, for example, Clevosaurus; Fig. 1c) or in a horizontal sliding movement known as propaliny (as in the living species of Sphenodon). See also: Dentition; Diapsida; Lepidosauria; Reptilia; Tooth
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