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):
h/t: Gordon
I love tuatara. I’ve seen them a few times when I’ve been to NZ, but I didn’t know about the parietal eye. Such cuties.
If I rename one of my cats “Jerry Coyne” will you come visit
ushim?They have their own order to themselves, they get an extra eye and they have a ridiculously fast super-car named after them. Some species have all the luck.
More than you can imagine! From the Wikipedia article:
“Wild tuatara are known to be still reproducing at about 60 years of age; “Henry”, a male tuatara at Southland Museum in Invercargill, New Zealand, became a father (possibly for the first time) on 23 January 2009, at the age of 111.”
Further comment would be as useless as it would be superfluous…
The downside to Henry’s longevity is that he lives life in the slow lane. Visited him on two separate occasions, two years apart, and I swear he hadn’t moved an inch from his position on the first visit.
The parietal eye photo appears to be of an adult green iguana (Iguana iguana).
I don’t think that the picture showing the parietal eye is of a tuatara. It looks a lot more like the top of a rhinoceros iguana’s head. Most iguanians do bear a parietal eye.
Had a look on the web, and couldn’t find a good picture of a tuatara’s parietal eye. Even pictures of the skull don’t show an obvious parietal foramen, although it’s there, just ahead of where the two temporal arches converge. A quick look in my own library showed that, externally, the eye is actually really quite inconspicuous in tuataras – it’s a lot easier to see in, for example, most iguanians.
…or shares developmental pathways with real eyes.
California fence lizards have a fairly conspicuous parietal eye, as seen here.
A medial parietal eye is basal in craniates – lampreys have them. What we see in tutataras (and lizards) is most likely retained from this basal feature.
Are craniates and sqaumates closely related?
Squamates are craniates, and so are we. Craniates comprise all animals with skulls.
I see. I did not know that. Thank you.
Any idea of the development of the second row of teeth? Sounds similar to a hox gene mutation where entire structures can be duplicated.
I think that the two rows are on separate bones – if you look at the palate, there’s one row on the maxillary (like the rest of us) and it looks as though there is a parallel row on the lateral edge of the palatines.
Teeth on bones other than the maxillary and mandible aren’t uncommon among the squamates.
Was that egg marked “15” or was that just a coincidence? I watched the video and wondered, then saw it was raised in an incubator, so it could be marked. Is there a special way of marking it as to not disturb it?
As a proud New Zealander – and huge evolutionary theory nut – I can’t wait to get my tuatara tattoo finished.
Have you tried to say “tuatara tattoo” quickly?
For those interested here is a link that describes the programme at Victoria University:
The tuatara caught on video is the 255th to be hatched at Victoria as part of the programme
That should have been: “http://www.victoria.ac.nz/news/2014/rare-footage-captured-of-tuatara-hatching”
I took photos of an adult Tuatara in Dec 2007 at the Otorohanga Kiwi House. It came out of a burrow right under my feet and stopped just two metres away. I watched it for about half an hour before I was dragged away by my family.
I don’t know how to post pics here, but if anyone is interested, I have posted one on Twitter. @HeatherHastie
“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.”
I would like to know more about that parietal eye. It evolved from a real eye? Does that mean that some ancestor had three functional eyes? Is that even possible in higher order animals? Seems to me an eye on top of the head would come in real handy in terms of survival. So why would it evolve away?
Can you see the pareital eye?
Yes! The parietal eye of tuatara, as in those lizards that have a distinct one and also regular enlarged head scales, is located within the interparietal scale. In the video you can see this scale as a pale lozenge on the midline, just anterior to the row of prominent vertebral scales that starts on the neck, between dark areas on either side. At 1:08 you can see a dark spot in the middle of the interparietal scale. That’s it.