The tuatara is considered 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. Adults are about 50 centimetres (20 in) long and weigh between 0.5 and 1 kilogram (1.1-2.2 lb). They display sexual dimorphism, as the males are larger, weighing up to 1 kilogram (2.2 lb), almost twice the weight reached by females. The spiny crest on their back, made of triangular soft folds of skin, is bigger in males than in females, and can be stiffened for display. The male abdomen is narrower than the female's. The tuatara's color ranges from olive green to brown to orange-red, and it can change color over its lifetime. It sheds its skin once a year.

Skull

In the course of evolution, the skull has been modified in most diapsids from the original version evident in the fossil record. However, in the tuatara, all the original features are preserved: it has two openings (temporal fenestrae) on each side of the skull, with complete arches. In addition, in the tuatara, the upper jaw is firmly attached to the skull. This makes for a very rigid, inflexible construction.
Testudines (turtle and tortoise) skulls were once believed to be the most primitive among amniotes, but newer research suggests this is not the case, as they might have lost the temporal holes in the skull secondarily rather than never having had them.

In the tuatara, two rows of teeth in the upper jaw close over one row in the lower jaw

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 reptiles; although most snakes also have a double row of teeth in their upper jaw, their arrangement and function is different from the tuatara's. The jaws, joined by ligament, chew with backwards and forwards movements combined with a shearing up and down action. The force of the bite is suitable for shearing chitin and bone.[2] The double-row arrangement provides a self-sharpening mechanism. The tuatara's teeth are not replaced, since they are not separate structures like real teeth, but sharp projections of the jaw bone. As their teeth wear down, older tuataras have to switch to softer prey such as earthworms, larvae, and slugs, and eventually have to chew their food between smooth jaw bones.

Sensory organs

In tuataras, both eyes can focus independently, and are specialized with a "duplex retina" that contains two types of visual cells for vision by both day and night , and a tapetum lucidum which reflects on to the retina to enhance vision at night. There is also a third eyelid on each eye, the nictitating membrane.

Tuatara at Hamilton Zoo

The tuatara has a third eye on the top of its head called the parietal eye. It has its own lens, cornea, retina with rod-like structures and degenerated nerve connection to the brain, suggesting it evolved from a real eye. The parietal eye is only visible in hatchlings, which have a translucent patch at the top centre of the skull. After four to six months it becomes covered with opaque scales and pigment.[2] Its purpose is unknown, but it may be useful in absorbing ultraviolet rays to manufacture vitamin D, as well as to determine light/dark cycles, and help with thermoregulation. Of all extant tetrapods, the parietal eye is most pronounced in the tuatara.

Together with turtles, the tuatara has the most primitive hearing organs among the amniotes. There is no eardrum, and the middle ear cavity is filled with loose tissue, mostly adipose tissue. The stapes comes into contact with the quadrate (which is immovable) as well as the hyoid and squamosal. The hair cells are unspecialized, innervated by both afferent and efferent nerve fibers, and respond only to low frequencies. Even though the hearing organs are poorly developed and primitive with no visible external ears, they can still show a frequency response from 100-800 Hz, with peak sensitivity of 40 dB at 200 Hz.

Spine and ribs

Adult tuatara on a rock

The tuatara spine is made up of hour-glass shaped amphicoelous vertebrae, concave both before and behind. This is the usual condition of fish vertebrae and some amphibians, but is unique to tuataras within the amniotes.

The tuatara has gastralia, rib-like bones also called gastric or abdominal ribs, the presumed ancestral trait of diapsids. It is found in some lizards (in lizards they are mostly made of cartilage), crocodiles and the tuatara, and are not attached to the spine or thoracic ribs.

The real ribs are small projections, with small, hooked bones, called uncinate processes, found on the rear of each rib. This feature is also present in birds. The tuatara is the only living tetrapod with well developed gastralia and 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 a 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 primitive and extinct amphibians and reptiles. 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.

This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.