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Sphenodontidae:

 

The tuatara is a reptile of the family Sphenodontidae, endemic to New Zealand. The two species are the only surviving members of the Sphenodontians. The tuataras resemble lizards, but are equally 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, and for the reconstruction of the appearance and habits of the earliest diapsids (the group that additionally includes birds and crocodiles).

 

The tuatara has been classified as an endangered species since 1895. Tuataras, like many of New Zealand's native animals, are threatened by habitat loss and introduced species, such as mustelids and rats. They were extinct on the mainland, with the remaining populations confined to 32 offshore islands, until the first mainland release into the heavily fenced and monitored Karori Wildlife Sanctuary in 2005.

 

Taxonomy and evolution:

 

Tuataras, and their sister group Squamata (which includes lizards, snakes and amphisbaenians), belong to the superorder Lepidosauria, the only surviving taxon within Lepidosauromorpha. Squamates and tuataras both show caudal autotomy (loss of the tail-tip when threatened), and have a transverse cloacal slit. The origin of the tuatara probably lies close to the split between the Lepidosauromorpha and the Archosauromorpha. Though tuatara resemble lizards, the similarity is mostly superficial, since the family has several characteristics unique among reptiles. The typical lizard shape is very common for the early amniotes; the oldest known fossil of a reptile, the Hylonomus, resembles a modern lizard.

 

Tuataras were originally classified as lizards in 1831 when the British Museum received a skull. The species remained misclassified until 1867, when Albert Günther of the British Museum noted features similar to birds, turtles and crocodiles. He proposed the order Rhynchocephalia (meaning "beak head") for the tuatara and its fossil relatives.

 

During the years since the inception of the Rhynchocephalia, many disparately related species have been added to this order. This has resulted in turning the rhynchocephalia into what taxonomists call a "wastebin taxon". Sphenodontia was proposed by Williston in 1925. Now, most authors prefer to use the more exclusive order name of Sphenodontia for the tuatara and its closest living relatives.

 

Sphenodon punctatus, drawing from unknown periodThere are two extant species of tuatara: Sphenodon punctatus and the much rarer Sphenodon guntheri, or Brothers Island tuatara, which is confined to The Brothers Islands in Cook Strait. The Brothers Island tuatara have olive skin with yellowish patches. Sphenodon punctatus has two subspecies: the Cook Strait tuatara (unnamed subspecies), which lives on other islands in and near Cook Strait, and the northern tuatara (Sphenodon punctatus punctatus), which lives on the Bay of Plenty, and some islands further north.

 

Tuataras have been referred to as living fossils. This means that they have remained mostly unchanged throughout their entire history, which is approximately 200 million years. However, 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 present in extinct sphenodontians, which lived in much warmer climates.

 

Physical description:

 

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 jawThe 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. 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 ZooThe 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. 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 rockThe 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.

 

Natural history:

 

Adult tuataras are terrestrial and nocturnal reptiles, though they will often bask in the sun to warm their bodies. Hatchlings hide under logs and stones, and are diurnal, likely because adults are cannibalistic. Tuataras survive in temperatures much lower than those tolerated by most reptiles, and hibernate during winter. They can maintain normal activities at temperatures as low as 7° C, while temperatures over 28° C are generally fatal. The optimal body temperature for the tuatara is from 16 to 21° C, the lowest of any reptile. The body temperature of tuatara is lower than that of other reptiles ranging from 5.2–11.2° C over a day, whereas most reptiles have body temperatures around 20° C. The low body temperature results in a slower metabolism.

 

Burrowing seabirds such as petrels, prions and shearwaters share the tuataras' island habitat during the bird's nesting season. The tuataras use the bird's burrows for shelter when available, or dig their own. The seabirds' guano helps to maintain invertebrate populations that tuataras predominantly prey on; including beetles, crickets and spiders. Their diet also consists of frogs, lizards and bird's eggs and chicks. Seabirds may provide beneficial fatty acids.

 

Tuatara juvenileTuataras reproduce very slowly; taking ten years to reach sexual maturity. Mating occurs in midsummer; females mate and lay eggs once every four years. During courtship, a male makes his skin darker, raises his crests and parades toward the female. He circles himself around the female while slowly walking with stiffened legs. The female will either submit, and allow the male to mount her, or retreat to her burrow.  Males do not have a penis; they reproduce by the male lifting the tail of the female and placing his vent over hers. The sperm is then transferred into the female.

 

Tuatara eggs have a soft, parchment-like shell. It takes the females between one and three years to provide eggs with yolk, and up to seven months to form the shell. It then takes between 12 and 15 months from copulation to hatching. This means reproduction occurs at 2 to 5 year intervals, the slowest in any reptile. The sex of a 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 an equal chance of being male or female. However, at 22° C, 80% are likely to be males, and at 20° C, 80% are likely to be females; at 18° C all hatchlings will be females. There is some evidence that sex determination in tuataras is determined by both genetic and environmental factors.

 

Tuataras probably have the slowest growth rates of any reptile, continuing 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.