Tuatara

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Tuataras

For the experimental music band, see Tuatara (band)

The tuataras are two species of reptile found in New Zealand. They are the only surviving members of the Rhynchocephalia, or Sphenodontia/Sphenodontida and have been classified as endangered species since 1895. Tuataras, like many native New Zealand animals, were threatened by habitat loss, harvesting, and introduced species such as mustelids and rats, and were extinct on the mainland with the remaining populations confined to 32 mammal free offshore islands, until a first mainland release into the heavily fenced and monitored Karori Wildlife Sanctuary in 2005.

They resemble lizards, but are actually equally distantly related to lizards and snakes, which are their closest living relatives. For this reason, they are of great interest in the study of the evolution of lizards and snakes, as well as efforts to infer the appearance and habits of the earliest diapsids.

Taxonomy and evolution

Together with its sister group Squamata (lizards, snakes and amphisbaenians), the tuatara belongs to the group Lepidosauria, which is the only surviving taxon within the Lepidosauromorpha. The origin of the tuatara probably lies close to the split between the Lepidosauromorpha and the Archosauromorpha, making it the closest living thing we can find to a "proto-reptile". 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; even the oldest known fossil of a reptile looks similar to a fossilized modern lizard.

Tuatara were originally classified as lizards in 1831 when the British Museum received a skull, and the species remained misclassified until 1867, when Robert Gunther (also at the British Museum) noted certain bird-like, turtle-like, and crocodile-like features (see Anatomy section) and proposed the order Rhynchocephalia (meaning "beak head") for the tuatara and its fossil relatives.

Unfortunately, during the years since Gunther's inception of the Rhynchocephalia, many disparately related species (including the similarly named archosaurian rhynchosaurs) have been added to this order. This has resulted in turning the rhynchocephalia into what taxonomists call a "wastebin taxon". Nowadays, most authors prefer to use the more inclusive order name of Sphenodontia for the tuatara and its closest living relatives. Sphenodontia was proposed by Williston in 1925, and thus has priority use over the similarly named Sphenodontida, which was proposed by Estes in 1983. A fossil of an ancient reptile called Homeosaurus looks very much like modern tuataras.

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. Sphenodon punctatus has two subspecies: the Cook Strait tuatara, which lives on other islands in and near Cook Strait, and the northern tuatara (Sphenodon punctatus punctatus), which lives on the islands from the Bay of Plenty north.

Tuataras and sphenodontians in general (along with sharks and crocodilians) have been referred to as living fossils. This essentially means that such animals remained unchanged throughout their entire tenure on this planet (~200 million years, for sphenodontians). However, recent taxonomic work on Sphenodontia has shown that this group has undergone a variety of changes throughout the Mesozoic. Many of the niches normally associated with lizards, were instead held by sphenodontians. There was even a successful group of aquatic sphenodontians known as pleurosaurs, which differed markedly from living tuataras. Tuataras 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).

Physical description

The tuatara 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. Adults are about 500 mm long and weigh between 0.5 and 1 kg; the species displays sexual dimorphism; the males are larger weighing up to 1,000 grams, 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 in display. The male abdomen is less pear shaped that the females. Males do not have a penis, so it can be difficult to ascertain the sex of a tuatara until it is several years old. The tuatara's 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.

Skull

The tuatara 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. 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.

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 (acrodont dentition).Cree, Alison. 2002. Tuatara. In: Halliday, T. and Adler, K. (eds.), The new encyclopedia of reptiles and amphibians, Oxford University Press, Oxford, pp. 210-211. 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. Its teeth are not replaced (monophydont), since they are not separate structures like real teeth but instead are sharp projections of their jaw bone. 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 very strong grip. This arrangement provides a self-sharpening mechanism. Older tuataras have to eat softer prey such as earthworms, larvae, and slugs as their teeth wear down, and in the end they have to chew their food between their smooth jaw bones.

Sensory organs

In tuataras, 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 (tapetum lucidum which reflects light for night vision). There is also a third eyelid on each eye, the nictitating membrane.

The tuatara has a famous third eye on the top of its head (called the parietal eye). It is a part of the epithalamus, which can be divided into two major parts; the epiphysis (the pineal organ, or pineal gland if mostly endocrine) and the parietal organ, often called the parietal eye, or third eye, if photoreceptive. It arises as an anterior evagination of the pineal organ or as a separate outgrowth of the roof of the diencephalon. In the tuatara the parietal eye is similar to an actual eye, even if it is rudimentary. The organ is the remnant of a real eye inherited from some very ancient and remote ancestor. It has its own lens, cornea, retina with rod-like structures and nerve connection to the brain (even if it is degenerated), a feature not found in any other tetrapods. This unique eye is visible only 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. Its purpose is so far unknown. It may be useful in absorbing ultraviolet rays to manufacture vitamin D, as well as to determine light/dark cycles, and help with proper thermoregulation.

Together with turtles, the tuatara has the most primitive and original 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

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 in some amphibians, but is never seen among amniotes except for the tuatara.

It has gastralia, rib-like bones also called gastric or abdominal ribs, the presumed ancestral condition of diapsids - this is only found 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 remarkable too, as small projections, pointing and hooked little bones, are found posterior of each rib (uncinate processes, also seen in birds). The only remaining tetrapod with both well developed gastralia and uncinate processes is the tuatara. Crocodilia have 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 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 now 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, another reminder that they are not lizards.

Natural history

Adult tuatara are terrestrial and nocturnal, although they will often bask in the sun to warm their bodies. Hatchlings are arboreal and diurnal, likely because the adults are known to eat younger tuataras. Tuataras thrive in temperatures that are much lower than those tolerated by most reptiles, and they are known to hibernate in winter. They are able to maintain normal activities at temperatures as low as 7° C, preferring temperatures of 16–21° C, the lowest optimal body temperature of all reptiles; temperatures over 28° C are generally fatal. The tuatara has a lower metabolic rate than other lizards as is indicated by differences in body temperature. The body temperature of tuatara is lower that that of other lizards ranging from 5.2–11.2° C over a day, whereas other lizards have body temperatures around 20° C.

Reproduction

Tuatara reproduce very slowly; they take at least ten years to reach sexual maturity, females mate and lay eggs only once every four years (while the males can mate each year). Mating occurs in midsummer; during courtship, a male makes his skin darker, raises up his spiny crest and parades toward the female. He circles himself around the female while slowly lifting his body up and down. If the female is impressed, she nods her head showing that the male is suitable to father her eggs.

Tuatara eggs have a soft, parchment-like shell. It takes between 12 and 15 months from copulation for a tuatara to hatch from its egg. The sex of the hatchling depends on the temperature of the egg, with warmer eggs tending to produce male tuataras, and cooler eggs producing females. Eggs incubated at 21° C have a 50/50 chance of being born male or female, but at 22° C, 80% are likely to be males. At 20° C, 80% are likely to be females; at 18° C all hatchlings will be females. There is some evidence that sex in tuataras is a complex trait determined by both genetic and environmental factors (incubation temperature).

They have probably the slowest growth rates of any reptile, continuing to grow larger for the first 35 years of their lives.

Conservation status

Tuatara, like many native New Zealand animals, were threatened by habitat loss, harvesting, and introduced species such as mustelids and rats, and were long confined to 32 mammal free offshore islands, until a first mainland release into the heavily fenced and monitored Karori Wildlife Sanctuary in 2005. Sphenodon guntheri is present naturally on one small island with a population of approximately 400, and has been reintroduced to two others. Sphenodon punctatus naturally occurs on 29 islands and its population is estimated to be over 60,000.

The more common species, Sphenodon punctatus, can be observed in captivity at the Southland Museum and Art Gallery in Invercargill, the first to have a tuatara breeding programme, Hamilton Zoo and Wellington Zoo, all of which now also breed tuatara for release into the wild. The Victoria University of Wellington maintains a research programme into the captive breeding of tuataras, and the National Wildlife Centre at Pukaha Mount Bruce keeps a pair and juvenile. The WildNZ Trust has a tuatara breeding enclosure at Ruawai.

Several zoos have tuataras on display but do not breed them. These include Chester Zoo, San Diego Zoo, Saint Louis Zoo and Natureland Zoo at Nelson, New Zealand.

Etymology and cultural significance

The name "tuatara" derives from the Māori language, meaning "peaks on the back". Tuataras feature in a number of indigenous legends. They are held as ariki (God forms). Tuataras are regarded as the messengers of Whiro, the god of death and disaster, and Māori women are forbidden to eat tuataras. The tuatara is featured on one side of the New Zealand 5 cent coin, to be phased out in October 2006.

Sphenodon comes from the greek roots for "wedge" (sphenos) and "tooth" (odon(t)); punctatus is Latin for "spotted".

References

Cited references

General references

External links

Lepidosaurs | New Zealand reptiles | cryptic animals | Living fossils

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