American and German researchers describe a monitor lizard fossil which, in addition to its two “normal” eyes, has two other eyes on the top of the head. Apart from lampreys, these lizards would be the only vertebrates known to man to have a fourth eye.
Some vertebrates, such as fish, amphibians, reptiles, have a “third eye”, also called a “pineal eye” or “parietal eye”. This third eye, quite common in primitive vertebrates, often derives from the pineal organ. But in lizards, this photosensitive structure does not derive from the pineal gland (the epiphysis), but from the parapinal organ. A new study from Yale University and the Senckenberg Research Institute (Germany) provides an explanation.
The epiphysis (or pineal gland) is a small gland that contains photoreceptors in fish and amphibians. The pineal organ and the parapineal organ are two diverticula of the diencephalon of certain vertebrates (fish and lampreys). The epiphysis and the parapinal organ play major roles in the orientation and biological clock of the animals.
The researchers analyzed two fossils of Saniwa ensidens found in the 1870s in Wyoming. This monitor lizard lived almost 50 million years ago. X-ray scans revealed the presence of two holes at the top of the skull of the lizards. According to the authors, these holes made it possible to connect the brain to two structures resembling the eyes: the pineal organ and the parapineal organ. For the researchers, the two eyes present simultaneously on the head of the lizard did not come from the same organ.
The presence of the fourth eye seems to confirm that the third eye of the lizards derives from the parapineal organ and not from the pineal organ. Krister Smith, the lead author of the study, said in a statement: “In discovering a four-eyed lizard, in which both pineal and parapineal organs formed an eye on the top of the head, we were able to show that the third eye of the lizard is really different from the third eye of other vertebrates.”
This fossil lizard is the only vertebrate fossil with four eyes. The lampreys also have four eyes, considering that the pineal gland and the parapineal organ each form a photosensitive structure resembling an eye.
The advantage of having a pineal organ outside the skull is to amplify the photosensitivity, especially for short wavelengths. In lower vertebrates, this eye could be used for orientation, as the researcher explains: “It is important to recognize that the pineal and parapineal organs are not mystical. They can perceive light and play a role in the endocrine system. However, some abilities conferred by the pineal gland are truly extraordinary. For example, some lower vertebrates may perceive the polarization of light with the third eye and use it to orient themselves geographically. “