What looks like a baby dragon, sports a vestigial third eye, and can live over 100 years? Why, the tuatara of course. It might look like a wrinkly, big-headed iguana, but don’t be fooled. Its family, the Sphenodonts (from the Greek “wedge-toothed”), parted ways with the Squamates (lizards and snakes) over 200 million years ago. The strategy proved successful at the time, but today the lonely tuatara is all that remains of a once highly diverse and widely dispersed lineage.
Being the sole ambassador for an entire order of reptiles is a big responsibility. For a while, scientists thought that there were two surviving species of tuatara, but a study published in 2010 indicated that no, it was just the one. While it’s sad that the tuatara is the last of its kind, it’s actually pretty amazing that this persistent Sphenodont managed to get as far as it has. Throughout its time on New Zealand, the unassuming tuatara has lived through an 8°C global temperature drop, glaciation, mountain development, and wildly changeable flora and fauna. While the tuatara has historically been referred to as a primitive “living fossil” because of its unique taxonomic heritage, it has actually had to change a lot in its tens of millions of years of isolation. During this time, it has solved the unique evolutionary problems posed by New Zealand in its own unique way. The little rebel.
Of course the tuatara had to work with the tools it was given. Despite its dynamic history, the tuatara has retained some traits harking back to an ancient and intricate evolutionary branch that has been all but lost, making it a species of great interest to scientists. An example of one of these archaic traits is its third eye. Today, the tuatara only has two visually functional eyes – what an underachiever. These eyes are impressive in their own right, though, with highly adapted night vision and the ability to focus independently of one another. However, modern tuatara also have a vestigial eye on the top of their head, visible in juveniles but covered with a thin layer of scales in adults. Called a “parietal eye,” this third eye sports a retina, lens and degenerated nerves connected to the brain. This means that its current form likely evolved from something that at some point worked like a functioning eye. Today, however, the eye is thought to play a different, less visual role – possibly aiding in thermoregulation or setting circadian rhythms.