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Tuatara


 

The tuatara is the only surviving member of Sphenodontia (or Sphenodontida). Tuatara are endemic to New Zealand, and now live only on a small number of offshore islands. It has been classified as an endangered species since 1895. There are two extant species: Sphenodon punctatus and the much rarer Sphenodon guntheri, or Brothers Island tuatara, which is confined to The Brothers Islands in Cook Strait. These have olive skin with yellowish patches.

Tuatara, like many native New Zealand animals, are threatened by habitat loss, harvesting, and introduced species such as mustelids and rats.

 

Characteristics

Though tuatara resemble lizards (most of all iguanas), the similarity is mostly superficial, since the genus has several characteristics unique among reptiles. The typical lizard shape is very common for the early amniotes.

It is most likely the most unspecialised living amniote. The brain and mode of locomotion resemble that of amphibians and the heart is more primitive than any other reptile's. It has the most primitive skull of all living amniotes, a diapsid skull with two temporal openings bounded by complete arches -- a solid skull construction which doesn't permit a great deal of variation.
The tuatara spine is made up of hour-glass shaped vertebrae, concave both before and behind. This is the usual condition of fish vertebrae and in some amphibians, but is never seen among amniotes except for the tuatara. The tip of the upper jaw is beaklike and separated from the remainder of the jaw by a notch. There is a single row of teeth in the lower jaw and a double row in the upper jaw, with the bottom row fitting perfectly between the two upper rows when the mouth is closed. This is a tooth arrangement not seen in any other reptile; although most snakes also have a double row of teeth in their upper jaw, their arrangement and function is different from the tuatara's. Its teeth are not replaced (monophydont), since they are actually not real teeth but sharp projections of their jaw bone and not separate structures. The jaws, joined by ligament, chew with backwards and forwards movements combined with shearing up and down action, a specialised and unique fore-and-aft movement, with the "false" teeth moving forward like shears when the jaws are closing, causing a grip so strong they cannot be pried open. This arrangement provides a self-sharpening mechanism. Older tuataras have to eat softer prey such as worms, larvae, and slugs as their teeth wear down, and in the end they have to chew their food between their smooth jaw bones. The tuatara usually doesn't chase its prey; instead it just sits and waits until a suitable prey passes by.

Both eyes can accommodate independently, and are specialized with a "duplex retina" that contains two types of visual cells for vision by both day and night. There is also a third eyelid on each eye, the nictitating membrane. The color ranges from olive green to brown to orange-red, and it can change color over its lifetime. Once a year it sheds its skin.

It has no external copulatory organs, and is like caecilians and most birds in transferring the sperm by partially extruding the rear part of its cloaca. It is still not clear if the tuatara evolved from reptiles which never had a penis from the start or if the ancestor of the Lepidosauria lost it at some point during evolution. The pelvis and shoulder girdles are arranged differently than in lizards, as is the case with other parts of the internal anatomy and its scales, another reminder that they are not lizards.

Its limbs are well-muscled, have sharp claws and partially webbed feet, and it can swim well.

It has gastralia, rib-like bones also called gastric or abdominal ribs, a primitive trait. These are found only in some lizards (in lizards they are mostly made of cartilage), crocodiles and the tuatara, and are not attached to the spine or the thoracic ribs. The real ribs are very special too, as small projections, pointing and hooked little bones, are found posterior of each rib (uncinate processes, also seen in birds). The only remaining tetrapode with both well developed gastralia and uncinate processes is the tuatara. Crocodilia has only small and rudimentary cartilaginous remnants of the uncinate processes. In the early tetrapods, the gastralia and ribs with uncinate processes, together with bony elements such as bony plates in the skin (osteoderms) and clavicles (collar bone), would have formed some sort of exo-skeleton around the body, protecting the belly and helped to hold in the guts and inner organs. These anatomical details most likely evolved from structures involved in locomotion even before the vertebrates migrated onto land. It is also possible the gastralia were involved in the breathing process in primtive and now extinct amphibians and reptiles.

Tuataras also show cold weather adaptations that allow them to thrive on the islands of New Zealand. These adaptations are probably unique to tuataras and not inherited from previous sphenodontians (which lived in much warmer climates).



Author: Koen Stein
Last updated: 21/11/2005
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