German Scientists Say They've Discovered an Ancient Lizard With 4 Eyes
Using computed tomography (CT scan) to learn more about lizard fossils from 49 million years ago, researchers at Germany's Senckenberg Research Institute have discovered that the ancient monitor lizard species had two additional eyes located on the top of its head. Known as the Saniwa ensidens, the creature is the only known jawed vertebrate with the physical trait, and the discovery is changing what scientists believe about how the pineal complex in reptiles evolved.
The lizard's third eye was a structure known as the pineal organ, and the fourth a parapineal organ. According to the study published in Current Biology, the organs play "key roles in orientation and in circadian and annual cycles."
Other creatures including frogs and fish have third eyes and a few have been found to have a fourth, but the Saniwa ensidens is the first vertebrate of its kind discovered to have had both.
The photosensitive eyes were not the same as the ones you're using to read this—they did not contain moving eyeballs and could not blink (they could only absorb and detect light)—but Senckenberg researchers argue that the term still applies.
"I think the use of the term 'eye' is quite unassailable," Krister Smith told Gizmodo. "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."
Scientists are not sure why the third eye in some reptiles disappeared over time, but after studying the Saniwa ensidens fossils, Smith and his team believe that the animal evolved in a slightly different way than other creatures.
"The pineal organs show more evolutionary innovation than was generally understood," said Smith. "It's not just that the pineal eye regressed in every major group except lizards, but also that in lizards the parapineal was transformed into the third eye and then later the pineal re-evolved an eye in Saniwa ensidens!"
There are theories and speculation about all of this, including the idea that the third eye in reptiles was used for "geographic orientation," but given that the creature is now extinct and the fossils being studied were found over 150 years ago, the facts are harder to come by. "We only know that the ancestors of major land-vertebrate groups all had a third eye," Smith added. "If we want to understand the course of its evolution, then we need to know when the parapineal assumed its present role, as in lizards."