The parietal third eye is found in a bearded dragons skull, as you might imagine. It is found right in-between its other two visible eyes, which I would call the bridge of the nasal passage. Obviously we can not see this third eye, and it doesn’t work in the same way as eyes usually do. In fact, it can only see shadows and changes in light.
A third eye isn’t as uncommon as you may think. Many lizards, amphibians and fishes can be known to have a parietal third eye.
Just like a normal eye, a parietal eye has a lens and also a retina. However, it lacks an iris, and it’s for this reason it doesn’t appear like a normal eye. The parietal eye is found on the skull, right between the eyes and it is covered by transparent scales. Believe it or not, you can actually see this pretty well on a bearded dragon. They are one of the few species where the third eye can be seen with a closer look.
This can however also be known as a Solar eye, or Pineal eye.
Like anything in the living world, evolution plays its part in the adaptation of ones surrounding. Many years ago, (and I mean a very long time ago) scientist believe that bearded dragons actually had two parietal eyes. Meaning these reptiles theoretically had four eyes.
It is thought that through many years of evolution, one of the parietal eyes began to be pushed over into the middle of the skull. This meant that its new position was perfectly placed so that the forth became unused and completely useless. Through many more years the forth parietal eye became vestigial.
Vestigiality is retention through the process of evolution. So if a body part is no longer needed, over time evolution will no longer produce these parts we no longer need or use.
An example of this on humans would be Wisdom teeth. Over time the human jaw has got smaller, therefore we no longer need as many teeth as we once did. And it’s this same principal with bearded dragons and their third and fourth parietal eyes.