This ancient lizard may have watched the world through four eyes

A fossil of the monitor lizard’s skull reveals two holes for the photosensory structures

Saniwa ensidens

ANCIENT LIZARD  Four-eyed Saniwa ensidens, which lived 50 million years ago, closely resembled the modern monitor lizard Varanus mertensi, shown here with an overlay of the skull of S. ensidens.

A. Lachmann/Senckenberg Gesellschaft für Naturforschung/

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 amplified photosensitivity they glean from the pineal glands to navigate. S. ensidens may have been able to perceive polarized light and use the angle of the sun like a compass, as some modern lizards do. Or it may have navigated using Earth’s magnetic field, much like some amphibians and migratory birds.

Editor’s note: This story was updated April 11, 2018, to clarify the captions.

Carolyn Gramling is the earth & climate writer. She has bachelor’s degrees in geology and European history and a Ph.D. in marine geochemistry from MIT and the Woods Hole Oceanographic Institution.

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