If you’re a biologist–at least one with an interest in natural history–you’ll know that the lizard-like tuatara of New Zealand (Sphenodon punctatus) is a rare and evolutionarily unusual beast. Like many New Zealand endemics, it’s highly threatened.
The tuatara is the only species in the order Rhynchocephalia; to show you how unusual it is to have an order containing a single species, its sister group (the most closely related order) is Squamata, which includes all lizards and snakes: more than 9,000 species. The two groups split, according to Timetree, about 240 million years ago.
The tuatara, in other words, looks like a lizard but isn’t one, although it was misclassified as a lizard until 1831. Wikipedia notes some of its unique traits:
Though they resemble lizards, the similarity is superficial, because the group has several characteristics unique among reptiles. The typical lizard shape is very common for the early amniotes; the oldest known fossil of a reptile, theHylonomus, resembles a modern lizard. R.L. Ditmars, Litt.D, says; “The Tuatara resembles in form stout-bodies modern lizards, which we might call iguanas; this resemblance is further intensified by a row of spines upon the back. It is dark olive, the sides sprinkled with pale dots. The eye has a cat-like pupil. Large specimens are two and a half feet long. While superficial resemblance might tend to group this reptile with lizards, its skeleton and anatomy show it to belong to a different part of a technical classification.”
. . . Tuatara are greenish brown and gray, and measure up to 80 cm (31 in) from head to tail-tip and weigh up to 1.3 kg (2.9 lb) with a spiny crest along the back, especially pronounced in males. Their dentition, in which two rows of teeth in the upper jaw overlap one row on the lower jaw, is unique among living species. They are further unusual in having a pronounced photoreceptive eye, the “third eye”, which is thought to be involved in setting circadian and seasonal cycles. They are able to hear, although no external ear is present, and have a number of unique features in their skeleton, some of them apparently evolutionarily retained from fish. Although tuatara are sometimes called “living fossils“, recent anatomical work has shown that they have changed significantly since the Mesozoic era.
And what most biologists know it for:
The 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.
Here’s the remnant of the pareital eye of an adult tuatara; it’s also seen in other “herps” (reptiles and amphibians), but not in a form as pronounced as that of the tuatara.
UPDATE: Jon Losos of Harvard, who should know, tells me the photo below is of an iguana, not a tuatara, and that it’s hard to find good pictures of tuataras that clearly show the third eye.
But lo, a baby hatches. This is extremely rare footage of a tuatara hatching, filmed at Victoria University of Wellington and posted on August 1. The YouTube notes give information:
The egg was one of 23 being incubated in captivity as part of a joint initiative with the Department of Conservation and Ngati Manuhiri that has saved a threatened population of tuatara from extinction.
Can you see the pareital eye?
And you’ll want to know this:
Tuatara, like many of New Zealand’s native animals, are threatened by habitat loss and introduced predators, such as the Polynesian rat (Rattus exulans). 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 Sanctuaryin 2005.
During routine maintenance work at Karori Sanctuary 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 on the New Zealand mainland in over 200 years, outside of captive rearing facilities.
Here’s an adult tuatara; perhaps I’ll get to see one on my bucket-list trip to New Zealand to tend the kakapos (and, of course, visit Jerry Coyne the Cat):