A fossil of the monitor lizard’s skull reveals two holes for the photosensory structures
A. Lachmann/Senckenberg Gesellschaft für Naturforschung/Digimorph.org
About 50 million years ago, a monitor lizard in what is now Wyoming perceived the world through four eyes. Saniwa ensidens is the only known jawed vertebrate to have had two eyelike photosensory structures at the top of the head, in addition to the organs we commonly think of as eyes, researchers report April 2 in Current Biology.
The structures are called the pineal and parapineal organs. Among animals alive today, only the jawless fish called a lamprey has both structures. But many modern reptiles have a so-called third eye, the pineal organ.
The researchers examined fossils collected 150 years ago by Yale University students. Scans of the fossils using a technique called X-ray computed tomography revealed spaces in the skull for both the third and fourth eye.
What the ancient lizard did with these organs isn’t known, but some modern vertebrates use the