If you lived in what is now Wyoming 49 million years ago, you could have spotted a four-eyed lizard—the one and only known example of such a creature among jawed vertebrates. The species, an extinct monitor lizard called Saniwa ensidens (above), had two standard eyes and also sported so-called pineal and parapineal “eyes” on the top of its head (shown as white dots in the reconstructed image below).
Researchers figured that out by taking a closer look at two S. ensidens fossils that were unearthed from a Wyoming escarpment in 1871. Detailed x-ray scans, generated using computerized tomography, revealed two holes on the top of the lizard’s skull. The holes would have connected the lizard’s brain to eyelike structures, called pineal and parapineal organs, the team reports today in Current Biology.
Many vertebrates alive today—such as some turtles, lizards, and fish—have a third “eye” on the top of their head, which may be important for sensing direction or regulating the animal’s biological clock. But apart from jawless lampreys, the extinct monitor lizard is the only vertebrate known to have two additional eyes. It’s not clear what S. ensidens used those eyes for, but the researchers think the light-sensitive structures may have acted like a compass, helping the lizard figure out what direction it was facing.