? TuataraConservation status: Endangered | ||||||||||||
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| [image] A tuatara on a rock | ||||||||||||
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| Sphenodon punctatus Sphenodon guntheri |
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.
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. Testudinata (Turtle) 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 the remainder of the 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 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 (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 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. 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 at 40 dB at 200 Hz.
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.
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.
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}}??2}}}} |. }} "Phylogeny" In the Shadow of the Dinosaurs: Early Mesozoic Tetrapods.
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}}}}}} |. }} "Late Triassic-Early Jurassic sphenodontians from China and the phylogeny of the Sphenodontia" in Nicholas Fraser & Hans-Dieter Sues (eds) In the Shadow of the Dinosaurs: Early Mesozoic Tetrapods.
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