The tuatara’s parietal eye

August 7, 2014 • 10:58 am

JAC: My post on the tuatara parietal eye was short and, for some readers, not informative enough. Where did it come from? What does it look like? (By mistake I published a picture of an iguana and not a tuatara.) Greg answers some of the many questions that have surely been tormenting many of you about this bizarre feature.

by Greg Mayer

The tuatara has long been of interest to us here at WEIT, and just the other day Jerry posted a video of one hatching, along with many interesting notes on their biology, especially on the parietal or ‘third’ eye. Jerry included a picture of the parietal which, as Jon Losos, among others, noted, was not, alas (as a simple google image search indicated), that of a tuatara, but rather that of what looks to me to be a common or green iguana (Iguana iguana— which, if you learn no others, is the one scientific name you should commit to memory). Jon remarked to Jerry that good pictures of a tuatara’s parietal would be hard to find. Well, here’s the best I could find.

The parietal eye of the tuatara (Plate 20 from Dendy, 1911).
The parietal eye of the tuatara (Plate 20 from Dendy, 1911).

This is Plate 20 from Arthur Dendy’s classic 1911 monograph describing the pineal organs (including the parietal eye) of the tuatara. The upper figure is a longitudinal section of the parietal eye, and the lens, retina, and pineal nerve (equivalent to the optic nerve) are readily apparent. The two lower figures provide details of the retina.

Dendy studied both adults and embryos; the above figures are of adults. Dendy, an Englishman, resided in a number of the antipodal parts of the British Empire, and in his monograph records his good fortune in not losing some of his histological sections of tuatara embryos, “… for they were, with most of my Australasian collections, shipwrecked in transmission from New Zealand to South Africa. The boxes containing the sections were, however, salved, and reached me after being soaked for weeks in salt water.”

The following figure, from Angus Bellairs’ still useful Life of Reptiles, is based on Dendy’s top figure, and labels some of the parts for clear identification.

The parietal eye of the tuatara (Figure 114 from Bellairs, 1970).
The parietal eye of the tuatara (Figure 114 from Bellairs, 1970).

Neither of these pictures, of course, shows the parietal from the outside. I’ve read that the parietal is not externally visible in adult tuatara, but I’ve never checked on the preserved tuatara I’ve seen; Jon has seen tuatara live and up close– perhaps he will stop by again here at WEIT and let us know if he has noticed the eye on the ones he’s seen and held.

The parietal eye is also found in many lizards (which, together with snakes, are the tuatara’s closest living relatives, so the sharing of this features is not anomalous.) In vertebrates, there can be a number of evaginations (together known as the pineal complex) from the region of the brain called the epithalamus. One of these forms the pineal gland or organ, while another forms the parietal organ. Both can be photoreceptive.  In lizards and tuatara, the parietal organ can have a lens and a retina, forming the parietal eye. The eye is overlain by a translucent scale, easily visible in many lizards. It cannot, as far as is known, form an image. In lampreys, both the the pineal and parietal can be eye-like, so that some authors refer to them having a pineal eye and a parietal eye (which is why the median eye of lizards and tuatara, though sometimes called the pineal eye, is better called the parietal eye). In lampreys the position of the median eyes is indicated by a whitish, unpigmented, oval on the otherwise dark skin of the middle of the head,  In birds and mammals, the parietal organ is absent, and the pineal organ (now called the pineal gland) is buried deep in the head, and has endocrine functions.

The pineal complex was present in some of the earliest fishes, as indicated by the presence of a single median foramen [JAC: small opening in the bone] in the skull of ostracoderms, placoderms, and others. It is most eye-like in the parietal eye of lizards and tuatara, which suggests that a fully eye-like parietal or pineal was not present in early vertebrates, so that the parietal eye did not evolve from a “real” eye.


Bellairs, A. 1970. The Life of Reptiles. 2 vols. Universe Books, New York.

Dendy, A. 1911. On the structure, development and morphological interpretation of the pineal organs and adjacent parts of the brain in the tuatara (Sphenodon punctatus). Philosophical Transactions of the Royal Society of London 201:227-331, pls. 19-31. pdf (Dendy’s interpretations of homology are no longer all accepted, but the morphological and histological work remains fundamental.)

33 thoughts on “The tuatara’s parietal eye

        1. Ah yes, and the common name for Boa constrictor is the Boa constrictor – and Bison bison is a bison…

          Damn, that would’ve looked more impressive if I knew how to do italics.

          1. Before the text you want italicized, put a tag that consist of a lower-case letter i between a “less than” sign and a “greater than” sign. At the end of the text you want italicized, put the same tag, except with a forward slash in front of the letter i.

            I’d show you how it looks, but the tags would be read, and it would not help at all! I hope I’ve been clear enough.

            1. You can (should ; I’m setting myself up for “HTML FAIL !” here) enter the “less-than” character entity as a piece of text using the “HTML entity” capability. Generically they are [ampersand][entity name][semi-colon]. There are plenty of lists of entity names on the web, most browsers interpreting them correctly.
              So, to produce some italic text, one would write <i>italicised text</i>, or (stuffed with spaces, to break the HTML entities up), & lt ; i & gt ; italicised text & lt ; /i & gt ;
              Incidentally, Firefox has a “view selection source” context menu item, though that can get underwear-knotted on complex pages, such as WordPress ones.
              Well, it passed local testing.

              1. I forgot the link to the Wikipedia list of entities.
                and I should have read Haggis’s message below. Bowmore!

              1. I could tell you, but then I’d have to shoot you! OK, to show a <, you need to type “& # 6 0 ;” without the spaces and without the quotation marks. For >, type “& # 6 2 ;”, as before. As you can see, lots of scope for getting it wrong.

                Now, where do you live…

  1. I know that posts like this one don’t generate the number of comments that posts of a more incendiary nature typically do. But I just want to state how great it is that someone with a very pedestrian knowledge of biology is exposed information like this. I’ve learned much more about biology, ecology and certainly evolution in just a few months of being a regular reader here than I ever did in my formal education, so much so that the amount of it could only be accurately measured in terms of sheer tonnage. I haven’t sat in a classroom as a student in over a decade, but I’m going to start taking some night courses in biology at the local University after the first of the year. There is way too much fascinating information of which I am ignorant and my brain hates that. It is precisely these types of posts that have spurred me to want to study again and for that I will always owe a debt of gratitude to WEIT.

    1. That is so cool, and I’m jealous. I want to take some biology classes for the same reason. I have to quit my day job first though (ie, retire). That is soon. Congrats.

    2. People ask me now, “What would you do for work if you had it to do over?” (I’m an engineer.)

      After reading Jerry’s book and Dawkins’ books and: Why We Get Sick, At the Water’s Edge, Parasite Rex, and many others, I would want to do evolutionary biology.

      I too love what’s posted here, especially including science, travel, food, drink, cats, and boots (and readers’ wildlife photos!).

  2. “In birds and mammals, … the pineal organ (now called the pineal gland) is buried deep in the head, and has endocrine functions”.

    The pineal gland secretes melatonin unless inhibited by light (bright light in humans). As the two crown groups have lost their pineal and parietal eyes, there is a roundabout neural circuit from retina to suprachaismatic nucleus to paraventricular nucleus (both in the hypothalamus), thence to the spinal cord sympathetic nuclei and from there back to the pineal.

    This system is critical for entraining behavior to the light dark cycle and malfunctions of the system cause sleep disturbances. I’d be interested to know if the tuatara (and other species with direct pineal photoreceptors) have this same indirect circuit or if it developed at the same time as the loss of the extra eyes. My guess is the former, but information would be wonderful.

  3. Many years ago, I heard a seminar on the pineal eye. We were shown drawings or photographs of fossil amphibian (as I recall)skulls which showed evidence of paired separated pineal eyes. Then other drawings or photographs of fossil skulls showing a gradual merger of the separated paired eyes. I found it fascinating at the time. Pretty nice transitional fossils, I thought. Anyone know about this/

    1. I’ve seen such images too. But I don’t have a reference.
      Eryops ??
      No, not Eryops. How about PLatecarpus (Fig 1 of linked article) ? Or figures 3 and Supplementary fig3 (left) of Swartz, 2011?

  4. It is most eye-like in the parietal eye of lizards and tuatara, which suggests that a fully eye-like parietal or pineal was not present in early vertebrates, so that the parietal eye did not evolve from a “real” eye.

    So is it reasonable to conjecture, then, that the light-sensing functions of the parietal eye (in those species that have it) evolved by the activation of existing eye-forming genes in previously non-visual brain tissue? Is this even a sensible question?

    1. Yes it is sensible. I cannot see how the pineal eye could have evolved without recruitment of key genes for eye development to be expressed in this additional location.
      There is an interesting experimental parallel. The ‘master’ control gene for eye development is known as eyeless in Drosophila. If this single gene is expressed in ANY location, such as on a developing wing, leg, or back, a pretty good compound eye forms in that location. Just Google ‘Drosophila eyeless’ and you will see some pretty freaky pix.

  5. I looked around a little and found fossil placoderms with paired pineal eyes. My recollection was of larger and more widely spaced eye sockets.

  6. Great illustrated explanation! I knew of parietal eyes but never stopped to think of their internal anatomy. Cool stuff!

  7. Thanks Greg, this is really cool. I had no idea!

    I guess Jesus thought they needed an additional eye!

    Is this a hox gene thing (like digits) or is it driven by something else?


  8. There is a certain strain of thought in India that would take this as proof that Shiva really existed . (Shiva has a third eye, that unleashes destruction when he opens it)

  9. A median parietal/pineal foramen is present in many lizards as noted above (between paired parietal bones, within the single fused parietal, on the parietal-frontal suture, or even between the paired frontals), but absent in all basal lineages of snakes including Cretaceous fossils, scolecophidians, pythons, boas etc. Then in the deeply-nested clade Colubroidea (which happens to include >80% of extant snake species) you typically get not one but a pair of foramina in the parietal. This is hardly ever mentioned in the literature, and I’m not aware of any investigation of the relationship between paired foramina and soft-tissue structures, but it’s curious.

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