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Four-Eyed Ancient Reptile Sheds Light On Lizards' Third Eye

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A long-tailed monitor lizard with four eyes that walked the Earth about 49 million years ago offers researchers new insights about the lizard's "third eye."

Saniwa Ensidens

The remains of the now-extinct animal known as Saniwa ensidens was found nearly 150 years ago in the Bridger Basin in Wyoming, but it was not until recently that a closer look at the fossils revealed its surprising features.

In a study, which was published in the journal Current Biology on Monday, April 2, researchers used computed tomography (CT) scanner to study the reptile's fossils and found that the lizard did not only have a regular pair of eyes.

It also featured a third eye, a structure called the pineal organ, and a fourth eye, also known as the parapineal organ.

Third Eye

The pineal organ, which scientists also called the "third eye" is common in the lower vertebrates such as fishes and frogs that lay their eggs in the water. A fourth eye is not as common. Today, the only known four-eyed creature with a backbone is the jawless lamprey. Saniwa ensidens is in fact the first known jawed terrestrial creature that features both the photosensitive pineal and parapineal organs.

Evolution Of Pineal And Parapineal Organs

The findings provide scientists with insights on the evolutionary history of the pineal and parapineal organs in vertebrates.

Some scientists think that lizards were able to retain the third eye as this structure independently disappeared in other vertebrates such as mammals and birds. Some researchers, however, think that the third eye of the lizard developed from the parapineal.

"We only know that the ancestors of major land-vertebrate groups all had a third eye," said study researcher Krister Smith, from Germany's Senckenberg Research Institute. "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."

Smith said that the four-eyed lizard that featured both the pineal and parapineal organs on the top of the head showed the lizard's third eye is different from those of other vertebrates. The lizards' third eye evolved independently of the third eye seen in other vertebrates.

The researchers also said that in lizards, the parapineal evolved into the third eye and later re-evolved in Saniwa ensidens.

"Here we present evidence that a fourth eye re-evolved from the pineal organ at least once within vertebrates, specifically in an extinct monitor lizard, Saniwa ensidens, in which pineal and parapineal eyes were present simultaneously," the researchers wrote in their study.

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