Tuatara

Template:Taxobox begin Template:Taxobox image Template:Taxobox begin placement Template:Taxobox regnum entry Template:Taxobox phylum entry Template:Taxobox classis entry Template:Taxobox ordo entry
Williston, 1925 Template:Taxobox familia entry
Cope, 1870 Template:Taxobox genus entry
Gray, 1831 Template:Taxobox end placement Template:Taxobox section subdivision Sphenodon punctatus
Sphenodon guntheri Template:Taxobox end The tuatara is a reptile, the only surviving member of Rhynchocephalia, or Sphenodontia/Sphenodontida (for more on this, see classification). 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. Tuatara, like many native New Zealand animals, are threatened by habitat loss, harvesting, and introduced species such as mustelids and rats.

Classification

Together with Squamata (which is its sister group), it belongs to the group Lepidosauria, the only surviver of Lepidosauromorpha. Its origin probably lies close to the split between the Lepidosauromorpha and the Archosauromorpha, making it the closest living thing we can come to a "proto-reptile". Though tuatara resemble lizards (most of all an iguana), the similarity is mostly superficial, since the genus has several characteristics unique among reptiles, as a closer look on its brain, a third eye, skull, ribs, heart, vertebrae and ear and so on will tell. The typical lizard shape are very common for the early amniotes, even the oldest known fossil of a reptile looks like a lizard. 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 reptiles. 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. Testudinata (turtles) 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 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. The tip of the upper jaw is beaklike and separated from remainder of jaw by a notch. There is a 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. 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 move 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. Tuataras usually don't chase their prey, instead they just sit and wait until a suiting prey is passing 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 (tapetum lucidum which reflects light for night vision). There is also a third eyelid on each eye, the nictitating membrane. The color ranges from olive green to brown to orange-red, and they can change color over their lifetime. Once a year their skin is shed. 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. Together with turtles, the tuatara has the most primtive 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 is coming 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 if their hearing organ are poorly developed and primitive with no visible external ears, they can still show a frequency response from 100-800 Hz, with peak sensitivity at 40 dB at 200Hz. Their limbs are well-muscled, have sharp claws and partially webbed feet. The animals can swim well. They have 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. 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. Indeed, tuatara were originally classified as lizards in 1831 when the British Museum received a skull, and the species remained misclassified until 1867, when Gunther (also at the British Museum) noted certain bird-like, turtle-like, and crocodile-like features and proposed the order Rhynchocephalia (beak heads) for the tuatara and its fossile 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 (Fraser & Sues, 1994). A fossil of an ancient reptile called Homeosaurus looks very much like modern tuataras.

The living fossil myth

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 (Wu, 1994) 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 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).

Species

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. The more common of the species can be observed in captivity at the Southland Museum and Art Gallery in Invercargill, which has a successful breeding program.

Name

The name "tuatara" derives from the Māori language, meaning "spiny 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. Māori women are forbidden to eat tuataras.

Natural History

Tuataras thrive in much lower temperatures than are tolerated by most reptiles and are able to maintain normal activities at temperatures as low as 45 degrees Fahrenheit, preferring temperatures of 60–70 °F (16–21 °C), the lowest optimal body temperature of all reptiles; temperatures over 80 °F (27 °C) are fatal. Adults are about 500 mm long and weigh between 0.5 and 1 kg. Adult males weigh 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, are bigger in males than in females, and can be stiffened in display. Tuataras hibernate in winter, but on a warm winter day they will come out to enjoy the sun. Adults are terrestrial and nocturnal, even if they will often bask in the sun to warm their bodies, but hatchlings are arboreal and diurnal (likely because the adults are known to eat younger tuataras). Tuataras are extremely long-lived, with individuals commonly living for over a century. Their metabolic rate is the lowest among reptiles, and they can hold their breath for over an hour. They reproduce very slowly: tuataras take at least ten years to reach sexual maturity, females mates and lay eggs only once every four years (while the males can mate each year), and it takes between 12 and 15 months from copulation for a tuatara to hatch from its egg. During courtship, a male makes himself 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. Mating is pretty rough with tuataras. Sometimes the male gets carried away and he bites the female. Tuatara eggs have a soft, parchment-like shell. 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 has 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. They have probably the slowest growth rates of any reptile. The tuataras continue to grow larger for the first 35 years of their lives.

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 arise as an anterior evagination of the pineal organ or as a separate outgrowth of the roof of the diencephalon. Besides the tuatara, the parietal eye/organ is to be found among lizards, frogs and lampreys, and fish like tunas and pelagic sharks, where it is visible as a light-sensitive spot on the top of their head. It is poorly develeped in salamanders, where it is often called the parapineal gland. In birds and mammals the parietal organ (but not the pineal organ) are absent. In the tuatara the parietal eye is built more like an actual eye, even if it is rudimentary. The organ actually is the remnants 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.

External links

• Animal Diversity Web
• Quickfire Facts - New Zealand Native Animals - Tuatara
• The Reptipage: Rhynchocephalia/Sphenodontia
• Tuatara Sex Debate

References

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