LiveScience introduces us to Saniwa ensidens, a now-extinct monitor lizard from Wyoming that had an eye on either side of its head and two more on the top of its skull:
S. ensidens‘ third and fourth eyes sat on top of its head, where the lizard’s pineal and parapineal organs were located. These eye-like photosensory structures also played a role in orientation and in circadian and annual cycles.
The photosensitive pineal organ is found in quite a few lower vertebrates, that is, vertebrates such as fishes and frogs that lay their eggs in the water, the researchers noted. This photosensitive organ was so widespread in lower vertebrates that scientists call it the “third eye.”
“On the one hand, there was this idea that the third eye was simply reduced [disappeared] independently in many different vertebrate groups, such as mammals and birds, and is retained only in lizards among fully land-dwelling vertebrates,” study lead researcher Krister Smith, a palaeoanthropologist at the Senckenberg Research Institute in Germany, said in a statement. “On the other hand, there was this idea that the lizard third eye developed from a different organ, called the parapineal, which is well-developed in lampreys. These two ideas didn’t really cohere.”
The newfound eyes in S. ensidens help clear up this mystery.
“By discovering a four-eyed lizard — in which both pineal and parapineal organs formed an eye on the top of the head — we could confirm that the lizard third eye really is different from the third eye of other jawed vertebrates,” Smith said.
Smith and his colleagues made the discovery by taking a second look at two museum specimens of S. ensidens that were collected almost 150 years ago at Grizzly Buttes in Bridger Basin, Wyoming. The researchers popped the 4.3-foot-long (1.3 meters) lizard remains into a computed tomography (CT) scanner, which takes thousands of X-rays and then assembles them into 3D digital images.
The CT scans showed that the ancient monitor lizard, which went extinct about 34 million years ago, had spaces in its skull where a fourth eye would have sat, a finding that “I certainly did not expect!” Smith said.