Many lizards carry a real third eye on top of their heads
Tuataras, most lizards, frogs, sharks and lampreys have a small light-sensing organ in a hole at the top of the skull. Skin always covers it in living animals, and it senses light through chemical changes rather than rods and cones. Our own ancestors lost theirs by the Middle Triassic.
Franz Leydig, a zoology professor at Tubingen, found the structure in 1872 while dissecting four European lizard species, including the slow worm. He took the cup-shaped bumps beneath their brains for glands and called them frontal organs. In 1886 the Oxford anatomist Walter Baldwin Spencer surveyed 29 lizard species, linked the organ to the hole in the skull roof and to the stalk of the pineal gland, and named it the pineal or parietal eye. Then in 1918 the Swedish zoologist Nils Holmgren spotted it in frogs and dogfish and noticed sensory cells resembling retinal cones, suggesting a primitive light detector.
The eye belongs to the epithalamus, a region at the top of the brain, and works alongside the pineal gland, which keeps daily rhythms and controls hormones involved in regulating body temperature. Its socket, the parietal foramen, is usually framed by the parietal bones. Plenty of ancient vertebrates, from armoured jawless fish to early four-legged animals, have skull openings that seem to have held working third eyes.
Distribution is patchy. Mammals lack it, though extinct relatives on our branch had one; the opening sealed over in a group of mammal-line cynodonts. Birds, crocodilians and turtles have lost it too, and snakes lack it even though they are close cousins of lizards. Lampreys go the other way, with two such eyes one behind the other, possibly the original vertebrate arrangement and useful for noticing danger from above. The extinct monitor-like lizard Saniwa probably had two as well, the only jawed vertebrate known to do so, which suggests it evolved one of them a second time.
What the eye actually does is still being tested. In experiments on the bearded dragon, lizards reacted to a weak low-frequency electromagnetic field, but the response vanished when the third eye was capped with aluminium. A follow-up found that blocking light below 580 nm also stopped the reaction, hinting that the organ helps the animal sense magnetism in a light-dependent way.
Source: Parietal eye
