Sphenodon, The Living Fossil

(Order Rhynchocephalia)


Albert Günther (1867) of the British Museum proposed the order name Rhynchocephalia, meaning “beak headed” for the tuatara and its fossil relatives. The common name “tuatara” is derived from the Maori language, which means “peaks on the back”. Tuatara have been referred to as living fossils, which is a term used for any living species of organisms that appears to be the same as a species only known from fossils and which has no close living relatives.

Sphenodon or hatteria is the last survivor of the reptilian order Rhynchocephalia, which evolved in the early Mesozoic era, about 200 million years ago, but is now survived by only two species, which are found on the islands off the New Zealand coast and in Karori Wildlife Sanctuary, Wellington, New Zealand. In 1895, the country of New Zealand awarded the tuatara strict legal protection.

Sphenodon punctatus.The Northern Tuatara colour ranges from olive green through grey to dark pink or brick red, often mottled and always with white spots. Males measure 61 cm in length and weigh up to 1 kg and females are 45 cm long.

Sphenodon guntheri.Brothers Island tuatara is confined to North Brother Island in Cook Strait, with a population of approximately 400. It is smaller, weighing up to 660 grams and has olive brown skin with yellowish patches. It inhabits low forest, scrub areas and rock stacks on Brothers Island and is found up to 300 meters above sea level where the climate is cool. The island is also occupied by petrel and shearwater birds, which provide the tuatara with many benefits.

Habitat. The geographic range of tuatara are 32 offshore islands which are generally cliff-bound, frequently exposed to strong winds and which support a natural, often stunted, salt and wind tolerant vegetation. Most islands are also home to several species of sea birds, whose nutrient-rich guano helps support the island’s ecosystem. Tuataras usually inhabit the breeding burrows of small petrels. They feed on small insects. The total population of tuatara of all species and subspecies is estimated to be greater than 60,000 but less than 100,000. A rodent species called kiore (Polynesian rat, Rattus exulans) has recently established on several islands and has threatened the survival of tuatara.

Morphology. The olive coloured, yellow-speckled tuatara reaches a length of 2 ft (60 cm) or more. It is very lizard like in external form, with a crest of spines down its neck and back. The male tuatara displays a striking crest down the back of the neck, and another down the middle of the back. Its internal anatomy, its scales and teeth are quite different from those of lizards. Like certain primitive lizards, it possesses a vestigial third eye on top of its head. Its teeth are not replaced as they are acrodont projections of jaw. They have a single row of teeth in the lower jaw, and a double row in the upper jaw, the bottom row fitting between the two upper rows. The jaws chew with backwards and forwards movements combined with a shearing up and down action.

Vertebrae are amphicoelous with remnants of notochord in the cavities between the centra. In addition to atlas and axis there is another small median bone, the proatlas in the occipital region of the skull. Ribs are single headed and some of them possess uncinate process. There is a median sternum with coracoid and interclavicle attached at the anterior end.

There is a T-shaped intercalvicle and a clavicle. There are eleven carpal bones; four, including a pisiform, in the proximal row, two centrals and five in the distal row. In the pelvis, pubes are united in symphysis, in front of which is a cartilaginous epipubis. A cartilaginous hypoischium is attached to the ischia behind.

Biology. It takes between 10 and 20 years for a tuatara to reach sexual maturity. The female lays between 5 and 18 eggs only once every 4 years. Mating occurs from mid-summer to early autumn and the eggs are laid the following spring or early summer. Incubation takes from 12 to 15 months, with the development of the embryo stopping during the winter months. It has an average lifespan of 60 years and reaches sexual maturity in a decade.

It lays eggs in burrows that it shares with seabirds, and has the longest incubation period of all reptiles (up to 15 months). The sex of the offspring is determined by the temperature: warm soil temperatures produce males, while cooler soil produces females. Adult tuatara is terrestrial and nocturnal reptile, though it will often bask in the sun to warm their bodies and hibernate during winter.

Their long life is the product of having an extremely low metabolic rate, which is due to their tolerance to extremely cool weather. Indeed the activity levels of the tuatara peak at body temperatures of 12 to 17 degrees Celsius, which is the lowest for any reptile. This is probably why they have been able to survive in New Zealand’s temperate climate for so long. Tuatara has the slowest growth rate among reptiles and continues to grow larger for the first 35 years of their lives. The average lifespan is about 60 years but they can live to be over 100 years old.

Food. Diet of tuatara consists of arthropods, earthworms, snails, bird eggs, small birds, frogs, and lizards, and a native cricket-like insect called a weta.

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 preyed on tuataras and cattle, trampled them. Sphenodon has been unable to tolerate the impact of humans because of its incredibly limited reproductive potential.