Many species of reptiles have a third, or parietal eye on top of their head. It is covered in skin and detects ambient light
The parietal eye is present in most lizards, tuataras, frogs, salamanders, lampreys, sharks, and leather-back sea turtles. Recent research actually suggests that therapsids, the ancestors of modern mammals, also possessed parietal eyes, though they are absent in the mammals of today.
They serve a variety of different purposes, from regulating circadian rhythms to thermoregulation.
Leather-back sea turtles use theirs to help them navigate the ocean by starlight.
Bearded dragons use theirs to detect the passing shadows of predatory birds overhead.
Sources:
https://www.smithsonianmag.com/science-nature/why-leatherback-turtle-skylight-head-180953635/