A large monitor lizard that once roamed Earth had four eyes. It is so far the only known land-dwelling jawed animal to ever have this bodily feature. Its fossils were recently unearthed, giving scientists their very first glimpse at the creature.
Called the Saniwa ensidens, this prehistoric creature had two regular eyes and a third light-sensitive organ called the pineal organ and a fourth eye called the parapineal organ mounted on top of its head, reported Gizmodo.
While scientists call them "eyes", these organs did not have an eyeball, eyelids, or other optical structures that are normally associated with sight. Rather, they were sensitive to light, like sensors, and were able to absorb and glean information from it.
"I think the use of the term 'eye' is quite unassailable," said Krister Smith, the lead author of the new study and a researcher at Germany's Senckenberg Research Institute. "It is normal for scientists to refer to the midline photosensory organ of lizards as the 'third eye.' Use of the term goes back to the late 19th century, when the functions of the structure were first debated."
Also, these eyes develop in the same way that normal eyes do — an outward growth of the neural tube that is directly connected to the brain and spinal cord. The extra eyes are spherical as well, having a lens and retina like layers separated by a fluid. The skin over these structures was likely transparent, however.
"The functions of the pineal organ — even when it doesn't form an eye — in lower vertebrates include geographic orientation," said Smith. "There are at least two proposed mechanisms for this, both of which rely on short-wavelength (blue) light. We know that short-wavelength light is rapidly attenuated upon entering the head. So it is plausible that the production of a fourth eye could be related to the improved perception of short-wavelength light and therefore to geographic orientation."
This feature became a necessity when mammals started to take over the world after dinosaur extinction, noted the report, but eventually, it began to fade away. Over time, in most mammals and birds, the pineal glands became a part of the endocrine system.