Rhynchocephalia - tuatara


Vertebrata; Gnathostomata; Osteichthyes; Sarcopterygii; Tetrapoda; Amniota; Sauropsida;
Reptilia; Diapsida; Sauria; Lepidosauromorpha; Lepidosauria; Rhynchocephalia

The Rhynchocephalia is the sister group to the Squamata (lizards and their relatives), united in the monophyletic clade Lepidosauria by the following synapomorphies:

  • Derived skin structure with shedding mechanisms. Epidermis is periodically lost and replaced in a cyclic switch between the production of α-keratin and β-keratin.
  • An opening, or window, in the pelvis called the thyroid fenestra.
  • Paired male hemipenes - intromittent organs used to deliver sperm to the female during copulation. They are held hidden within eversible pouches in a transverse cloacal slit. Note this is well defined in squamates but only rudimentary in male tuatara (the only extant rhynchocephalian).
  • The possession of fracture planes within tail vertebrae, allowing caudal autotomy - the ability to self-amputate the tail. This is used as a defensive escape mechanism.
  • Extra centres of ossification in the epiphyses of the limb bones.
  • Knee joint in which the fibula fits into a lateral recess on the femur. This is unique within tetrapods.
  • Sexual segment of the kidney.
  • Specialised foot and ankle structure, including:
    • Fused astralago-calcaneun and enlarged fourth distal tarsal - combined, these two features produce a specialised mesotarsal joint that aids in movement over rough terrain.
    • Hooked fifth metatarsal - this acts in a manner analagous to a rudimentary mammalian heel.

Lepidosauria is defined as encompassing the last common ancestor of Rhynchocephalia and Squamata, plus all its descendants. Lepidosauromorpha contains lepidosaurs and stem-lepidosaurs known from fossils (e.g.Kuehneosaurus). If only extant taxa are considered then Lepidosauria is the sister group to the Archosauria (crocodilians, dinosaurs, birds, and possibly turtles). Lepidosauromorpha and Archosauromorpha (Archosaurs plus now extinct stem-Archosaurs) are sometimes together referred to as Sauria. A larger more inclusive grouping, Diapsida, contains Lepidosauromorpha and Archosauromorpha along with various fossil taxa (such as Petrolacosaurus and plesiosaurs). Diapsida is in turn nested within Sauropsida (equal to a monophyletic definition of Reptilia as used by some authors) which contains additional fossil taxa such as procolophonids and captorhinids.

Diversity and Lower Taxonomy:

The Rhynchocephalia is now represented by just a single extant genus, Sphenodon, comprising two species, S. punctatus and S. guntheri (the status of which is questioned). In its nativeNew Zealand, Sphenodon is known as the tuatara - derived from a Māori term meaning "peaks on the back".

This apparently minimal diversity has led many authors to suggest that the Rhynchocephalia is a conservative and relatively unchanged group with uniform morphology; however, there are a number of fossil taxa known that demonstrate variation in body shape, skull structure and tooth morphology.

Distribution and Habitat:

Although globally distributed in the Mesozoic, the living representatives of the Rhynchocephalia (Sphenodon) are now restricted to the terrestrial habitat of New Zealand's offshore islands.

  • Sphenodon punctatus (Northern + Cook Strait tuatara) - present on 33 islands.
  • Sphenodon guntheri (Brothers Island tuatara) - present on only three islands.

Conservation Status (IUCN):

  • Sphenodon punctatus (Northern + Cook Strait tuatara) - Least Concern (LC) - not updated since 1996
  • Sphenodon guntheri (Brothers Island tuatara) - Vulnerable (Vu) - not updated since 1996

Uniting features (synapomorphies) of the Rhychocephalia:

  • Enlarged palatine tooth row - allowing the application of three-point bending to food items. This is a unique feature amongst amniotes.
  • Acrodont dentition - teeth fused to the crest of the jaw bone, with no sockets. These teeth are not usually replaced and tend to be added to the back of the jaw bone as it grows.
  • Posterior extension of the dentary.

Fossil taxa and rhynchocephalian phylogeny:

Thought to have originated in the Early Triassic (~250-240 mya), rhychocephalians achieved global distribution in the Early Mesozoic, and were a major part of faunal assemblages for a large proportion of this era. In addition to the phylogenetically basal-most taxa, such asGephryosaurus, there are 5 distinct, apparently monophyletic lineages of derived rhynchocephalians: the clevosaurs, pleurosaurs, sapheosaurs, sphenodontines, and eilenodontines. These groups - whose phylogenetic interrelationships are uncertain - are all extinct, except the sphenodontines, which include the extant Sphenodon. The phylogenetic tree below illustrates rhynchocephalian interrelationships based on Jones (Journal of Morphology, 2008):


"Basal taxa" - known from the Triassic through to the Early Jurassic, this is a paraphyletic assemblage, consisting of taxa that originated early in the rhyncocephalian lineage - includingGephryosaurus, Diphydontosaurus, and Planocephalosaurus. The "basal taxa" possess many relatively simple maxillary and dentary teeth, with a conical form suitable for piercing small, invertebrate prey. The dentition perhaps demonstrates transition from an ancestral state, as they are pleuro-acrodont (in the ancestral pleurodont condition, teeth are set into the inner sides of the jaw bones). All other rhychocephalians have a fully acrodont dentition, and some of the more derived forms discussed below show a trend towards fewer, stouter teeth, that are more resilient to loading and bending forces, thus suitable for seizing larger prey. The shape of the jaw joint indicates that some sliding movement of the lower jaw could occur during food processing.

Clevosaurs - a group of carnivorous ryhnchocephalians known from the Mid Triassic to the Early Jurassic. Clevosaurs possess a highly specialised dentition, with a blade-like morphology that were used in conjunction with a precise vertical (orthal) cutting action like that of a pair of scissors. With this dentition and a skull size of 20-40+ mm, they were able to seize large invertebrates and probably even some small vertebrates.

Pleurosaurs - one of the two groups of aquatic rhynchocephalians - the other being the sapheosaurs. This aquatic habit is thought to have evolved independently in the two lineages. The pleurosaurs, known from the Early Mid to the Late Jurassic, possess a flattened, elongate skull (60-80mm), trunk and tail specialised for their aquatic mode of life, and have a dentition similar to that of the clevosaurs.

Sapheosaurs- the second group of aquatic rhynchocephalians - known only from around the Jurassic-Cretaceous boundary (~206 mya). The sapheosaur fossil record is poorly known and little is known about their specialisations. The dentition has been described as blade-like and may resemble that of the clevosaurs or pleurosaurs.

Sphenodontines- known from the Early Jurassic onwards, this is the group that includes the extant genus, Sphenodon. The sphenodontines possess only a single enlarged palatine tooth row, running parallel to the maxillary teeth. Each maxillary and palatine tooth bears a posterior flange. The teeth on the dentary are conical with an anteriorly placed apex. Following wear they bear two small anterior flanges or “shoulders”. When the jaws close the lower jaw slides forward (protraction) to allow a shearing action analogous to that of a steak knife. This is termed a propalinal, or more specifically prooral, jaw action. This allows members of this group to process tougher, more complex prey, such as arthropods and small vertebrates but in a different way to clevosaurs and pleurosaurs.

Eilenodontines - a group of highly specialised rhynchocephalians, that may form the sister group to the sphenodontines. Known from the Late Jurassic to the Mid Late Cretaceous, the eilenodontines were the largest group of rhynchocephalians, with skull sizes reaching between 80-150mm and robust jaws. The dentition of the eilenodontines is in many ways like that of the sphenodontines - they have similar maxillary teeth, have a single enlarged palatal tooth row parallel to the maxillary teeth, and utilise some kind of propalinal jaw action. However, the dentary teeth are wider, with a thickened layer of enamel. When worn (which they usually are) each tooth possesses a flat wear facet bounded by sharp enamel edges. These resemble the teeth of mammalian herbivores and are therefore considered suitable for processing plant material, and thus indicate a herbivorous diet for the group.


Jones, MEH. 2008. Skull shape and feeding strategy in Sphenodon and other Rhynchocephalia (Diapsida: Lepidosauria). Journal of Morphology 269: 945-966.

Evans, SE. 2003. At the feet of the dinosaurs: the early history and radiation of lizards. Biological Reviews 78: 513-551.


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