"The third eye
The Tuatara's most fascinating feature is its third or pineal ‘eye’. This occurs in many species of lizard too. It is visible in hatchlings, but is covered with scales after 4-6 months. This reason for this 3rd ‘eye’ is not known but it is thought to be useful in absorbing ultraviolet rays to manufacture vitamin D"
vitamin D3 is apsorbing in skin. so 3th eye may have function in sensing better position where is more sun rays or so....
organ like organ has a function in detection. and it can not produce a pictre like sense...
it looks like ,to me, that is something like in primitive one cell organisam just to detect changes in light.