[13] Rhynchocephalians first appeared in the fossil record during the Triassic, around 240 million years ago,[14] and reached worldwide distribution and peak diversity during the Jurassic, when they represented the world's dominant group of small reptiles.

Tuatara are greenish brown and grey, and measure up to 80 cm (31 in) from head to tail-tip and weigh up to 1.3 kg (2.9 lb)[10] with a spiny crest along the back, especially pronounced in males.

Although tuatara have preserved the morphological characteristics of their Mesozoic ancestors (240–230 million years ago), there is no evidence of a continuous fossil record to support the idea that the species has survived unchanged since that time.

[17][18] Tuatara, like many of New Zealand's native animals, are threatened by habitat loss and introduced predators, such as the Polynesian rat (Rattus exulans).

[20] During routine maintenance work at Zealandia in late 2008, a tuatara nest was uncovered,[21] with a hatchling found the following autumn.

[34] Rhynchocephalians declined during the Cretaceous period,[35] possibly due to competition with mammals and lizards,[36] with their youngest record outside of New Zealand being of Kawasphenodon, known from the Paleocene of Patagonia in South America.

[44] 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.

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.

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.

[55] 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.

[56] The tuatara possesses palatal dentition (teeth growing from the bones of the roof of the mouth), which is ancestrally present in reptiles (and tetrapods generally).

[63] It has its own lens, a parietal plug which resembles a cornea,[64] retina with rod-like structures, and degenerated nerve connection to the brain.

[24] Some salamanders have been shown to use their pineal bodies to perceive polarised light, and thus determine the position of the sun, even under cloud cover, aiding navigation.

[67] Animals that depend on the sense of smell to capture prey, escape from predators or simply interact with the environment they inhabit, usually have many odorant receptors.

The vertebral bodies have a tiny hole through which a constricted remnant of the notochord passes; this was typical in early fossil reptiles, but lost in most other amniotes.

[81] The low body temperature results in a slower metabolism.Burrowing seabirds such as petrels, prions, and shearwaters share the tuatara's island habitat during the birds' nesting seasons.

The seabirds' guano helps to maintain invertebrate populations on which tuatara predominantly prey, including beetles, crickets, spiders, wētās, earthworms, and snails.

[87] Female tuatara rarely exhibit parental behaviour by guarding nests on islands with high rodent populations.

[96] Tuatara eggs have a soft, parchment-like 0.2 mm thick shell that consists of calcite crystals embedded in a matrix of fibrous layers.

It is not fully demonstrated, but these genes may be related to the longevity of this animal or may have emerged as a result of the low levels of selenium and other trace elements in the New Zealand terrestrial systems.

[62] The genes of the major histocompatibility complex (MHC) are known to play roles in disease resistance, mate choice, and kin recognition in various vertebrate species.

This diversity is greater than what has been found in other amniotes and in addition, thousands of identical copies of these transposons have been analyzed, suggesting to researchers that there is recent activity.

Recent publications propose that this high level of methylation may be due to the amount of repeating elements that exist in the genome of this animal.

[105] Tuatara were once widespread on New Zealand's main North and South Islands, where subfossil remains have been found in sand dunes, caves, and Māori middens.

All three populations bred in captivity, and after successful eradication of the rats, all individuals, including the new juveniles, were returned to their islands of origin.

In the 1991–92 season, Little Barrier Island was found to hold only eight tuatara, which were taken into in situ captivity, where females produced 42 eggs, which were incubated at Victoria University.

In 1998, 34 juveniles from captive breeding and 20 wild-caught adults were similarly transferred to Matiu/Somes Island, a more publicly accessible location in Wellington Harbour.

[120] The first successful breeding of tuatara in captivity is believed to have achieved by Sir Algernon Thomas at either his University offices or residence in Symonds Street in the late 1880s or his new home, Trewithiel, in Mount Eden in the early 1890s.

Southland Museum and Art Gallery in Invercargill was the first institution to have a tuatara breeding programme; starting in 1986 they bred S. punctatus and have focused on S. guntheri more recently.

The Victoria University of Wellington maintains a research programme into the captive breeding of tuatara, and the Pūkaha / Mount Bruce National Wildlife Centre keeps a pair and a juvenile.