Tuatara
The tuatara is a species of reptile endemic to New Zealand. Despite its close resemblance to lizards, it is the only extant member of a distinct lineage, the previously highly diverse order Rhynchocephalia. The name tuatara is derived from the Māori language and means "peaks on the back".
The single extant species of tuatara is the only surviving member of its order, which was highly diverse during the Mesozoic era. Rhynchocephalians first appeared in the fossil record during the Middle Triassic, around 244-241.5 million years ago, and reached worldwide distribution and peak diversity during the Jurassic, when they represented the world's dominant group of small reptiles. Rhynchocephalians declined during the Cretaceous, with their youngest records outside New Zealand dating to the Paleocene. Their closest living relatives are squamates. Tuatara are of interest for studying the evolution of reptiles.
Tuatara are greenish brown and grey, and measure up to from head to tail-tip and weigh up to with a spiny crest along the back, especially pronounced in males. They have a second row of upper teeth on the roof of the mouth. They are able to hear, although no external ear is present, and have a number of unusual features in their skeleton compared to lizards, including gastralia in the belly and a completely fused temporal bar at the back of the skull.
Tuatara are sometimes referred to as "living fossils". This term is currently deprecated among paleontologists and evolutionary biologists. Although tuatara have preserved the morphological characteristics of their Mesozoic ancestors, there is no evidence of a continuous fossil record to support the idea that the species has survived unchanged since that time.
The species has between five and six billion base pairs of DNA sequence, nearly twice that of humans.
The tuatara has been protected by law since 1895. Tuatara, like many of New Zealand's native animals, are threatened by habitat loss and introduced predators, such as the Polynesian rat . Tuatara were extinct on the mainland, with the remaining populations confined to 32 offshore islands, until the first North Island release into the heavily fenced and monitored Karori Wildlife Sanctuary in 2005. During routine maintenance work at Zealandia in late 2008, a tuatara nest was uncovered, with a hatchling found the following autumn. This is thought to be the first case of tuatara successfully breeding in the wild on New Zealand's North Island in over 200 years.
Taxonomy and evolution
Relationships of the tuatara to other living reptiles and birds, after Simões et al. 2022Tuatara, along with other now-extinct members of the order Rhynchocephalia, belong to the superorder Lepidosauria, as do the order Squamata, which includes lizards and snakes. Squamates and tuatara both show caudal autotomy, and have transverse cloacal slits.
Tuatara were originally classified as lizards in 1831 when the British Museum received a skull. John Edward Gray used the name Sphenodon to describe the skull; this remains the current scientific name for the genus. Sphenodon is derived from the Greek for "wedge" and "tooth". In 1842, Gray described a member of the species as Hatteria punctata, not realising that it and the skull he received in 1831 were both tuatara.
The genus remained misclassified as a lizard until 1867, when Albert C. L. G. Günther of the British Museum noted features similar to birds, turtles, and crocodiles. He proposed the order Rhynchocephalia for the tuatara and its fossil relatives. Since 1869, Sphenodon punctatus has been used as the scientific name for the species.
At one point, many disparate species were incorrectly referred to the Rhynchocephalia, resulting in what taxonomists call a "wastebasket taxon". Williston in 1925 proposed the Sphenodontia to include only tuatara and their closest fossil relatives. However, Rhynchocephalia is the older name and in widespread use today. Many scholars use Sphenodontia as a subset of Rhynchocephalia, including almost all members of Rhynchocephalia, apart from the most primitive representatives of the group.
The earliest rhynchocephalian, Agriodontosaurus, is known from the Middle Triassic of England, around. During the Late Triassic, rhynchocephalians greatly diversified, going on to become the world's dominant group of small reptiles during the Jurassic period, when the group was represented by a diversity of forms, including the aquatic pleurosaurs and the herbivorous eilenodontines. The earliest members of Sphenodontinae, the clade which includes the tuatara, are known from the Early Jurassic of North America. The earliest representatives of this group are already very similar to the modern tuatara. Rhynchocephalians declined during the Cretaceous period, possibly due to competition with mammals and lizards, with their youngest record outside of New Zealand being of Kawasphenodon, known from the Paleocene of Patagonia in South America.
Fossil material that can be referred to as sphenodontine is known from the Miocene Saint Bathans fauna of Otago in the South Island of New Zealand. Whether these specimens are referable to the genus Sphenodon proper is not entirely clear due to their incomplete nature, but they are likely to be from a taxon that is closely related to tuatara. Therefore, the ancestors of the tuatara were likely already present in New Zealand prior to its separation from Antarctica around 82–60 million years ago.
Cladogram of the position of the tuatara within Sphenodontia, after Simoes et al., 2022:
Species
While there is currently considered to be only one living species of tuatara, two species were previously identified: Sphenodon punctatus, or northern tuatara, and the much rarer Sphenodon guntheri, or Brothers Island tuatara, which is confined to North Brother Island in Cook Strait. The specific name punctatus is Latin for "spotted", and guntheri refers to German-born British herpetologist Albert Günther. A 2009 paper re-examined the genetic bases used to distinguish the two supposed species of tuatara, and concluded they represent only geographic variants, and only one species should be recognised. Consequently, the northern tuatara was re-classified as Sphenodon punctatus punctatus and the Brothers Island tuatara as Sphenodon punctatus guntheri. The Brothers Island tuatara has olive brown skin with yellowish patches, while the colour of the northern tuatara ranges from olive green through grey to dark pink or brick red, often mottled, and always with white spots. In addition, the Brothers Island tuatara is considerably smaller. However, individuals from Brothers Island could not be distinguished from other modern and fossil samples on the basis of jaw morphology.An extinct species of Sphenodon was identified in November 1885 by William Colenso, who was sent an incomplete subfossil specimen from a local coal mine. Colenso named the new species S. diversum. Fawcett and Smith consider it a synonym to the subspecies, based on a lack of distinction.
Description
Tuatara are the largest reptiles in New Zealand. Adult S. punctatus males measure in length and females. Tuatara are sexually dimorphic, males being larger. The San Diego Zoo even cites a length of up to. Males weigh up to, and females up to. Brothers Island tuatara are slightly smaller, weighing up to 660 g.Their lungs have a single chamber with no bronchi.
The tuatara's greenish brown colour matches its environment, and can change over its lifetime. Tuatara shed their skin at least once per year as adults, and three or four times a year as juveniles. Tuatara sexes differ in more than size. The spiny crest on a tuatara's back, made of triangular, soft folds of skin, is larger in males, and can be stiffened for display. The male abdomen is narrower than the female's.
Skull
Unlike the vast majority of lizards, the tuatara has a complete lower temporal bar closing the lower temporal fenestra, caused by the fusion of the quadrate/quadratojugal and the jugal bones of the skull. This is similar to the condition found in primitive diapsid reptiles. However, because more primitive rhynchocephalians have an open lower temporal fenestra with an incomplete temporal bar, this is thought to be a derived characteristic of the tuatara and other members of the clade Sphenodontinae, rather than a primitive trait retained from early diapsids. The complete bar is thought to stabilise the skull during biting.The tip of the upper jaw is chisel- or beak-like and separated from the remainder of the jaw by a notch, this structure is formed from fused premaxillary teeth, and is also found in many other advanced rhynchocephalians. The teeth of the tuatara, and almost all other rhynchocephalians, are described as acrodont, as they are attached to the apex of the jaw bone. This contrast with the pleurodont condition found in the vast majority of lizards, where the teeth are attached to the inward-facing surface of the jaw. The teeth of the tuatara are extensively fused to the jawbone, making the boundary between the tooth and jaw difficult to discern, and the teeth lack roots and are not replaced during the lifetime of the animal, unlike those of pleurodont lizards. It is a common misconception that tuatara lack teeth and instead have sharp projections on the jaw bone; histology shows that they have true teeth with enamel and dentine with pulp cavities. As their teeth wear down, older tuatara have to switch to softer prey, such as earthworms, larvae, and slugs, and eventually have to chew their food between smooth jaw bones.
The tuatara possesses palatal dentition, which is ancestrally present in reptiles. While many of the original palatal teeth present in reptiles have been lost, as in all other known rhynchocephalians, the row of teeth growing from the palatine bones in the tuatara have been enlarged, and as in other members of Sphenodontinae the palatine teeth are orientated parallel to the teeth in the maxilla; during biting the teeth of the lower jaw slot between the two upper tooth rows. The structure of the jaw joint allows the lower jaw to slide forwards after it has closed between the two upper rows of teeth. This mechanism allows the jaws to shear through chitin and bone.
The brain of Sphenodon fills only half of the volume of its endocranium. This proportion has been used by paleontologists trying to estimate the volume of dinosaur brains based on fossils. However, the proportion of the tuatara endocranium occupied by its brain may not be a very good guide to the same proportion in Mesozoic dinosaurs since modern birds are surviving dinosaurs but have brains which occupy a much greater relative volume in the endocranium.