|MadSci Network: Zoology|
Hi Tomas -
You are correct that tuataras have three eyes on their head! It's important to note that the third eye resides on the head, just under the skin, and that it is not explicitly the pineal gland, which is an internal structure/part of the brain in higher vertebrates. However, I'll explain the relationship between the tuatura third eye and pineal gland below.
The tuatura is the only remaining member of an ancient order of reptiles, Rhynochocephalia, which roamed the earth during the early Jurassic period, 140-200 million years ago. They provide a modern window, one that existed prior to the time of the dinosaurs, for examining the phylogeny and evolution of early reptiles.
Study of the tuatura's embryology has investigated development of the third eye. Current data suggests, at this point in vertebrate evolution, an additional invagiation or "wrinkle" formed in the diencephalon, a primitive region of the forebrain. The diecephalon also gives rise to the optic cups which develop into the eyeballs we use for vision. However, the third eye of the tuatura only develops into a partially complete visual organ with limited function. Some scientists have suggested that the developing third eye may not receive necessary developmental signals required to complete its morphogenesis into a fully functional organ, and/or areas of the brain needed to receive the additional visual inputs do not develop. Again, this area is under active investigation.
It appears the third eye attained its most evolved state during the Jurassic period among members of order Rhynochocephalia, and failed to evolve further, regressing into the vestigial, or near-vestigial, structure in mammals known as the pineal gland. In higher mammals, the pineal gland is known for secreting the hormone melatonin, which helps regulate our daily or circadian rhythms. Of interest, melatonin is also produced in the retina of the eye, another link illustrating the evolutionary history of this structure.
As far as I am aware, no other living vertebrate animal has three eyes. Some invertebrates have more than two eyes - scallops, for instance.
Hope this helps answer your question.
Try the links in the MadSci Library for more information on Zoology.