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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

12:19pm, April 5, 2018
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.

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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.

Extra eyes

A study of a fossil skull of S. ensidens revealed two holes at the top of its head. Those holes marked the location of its third and fourth eyes, called the pineal and parapineal organs.

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


K.T. Smith et al. The only known jawed vertebrate with four eyes and the Bauplan of the pineal complex. Current Biology. Vol. 28, April 2, 2018, p. 1. doi: 10.1016/j.cub.2018.02.021.

Further Reading

B. Brookshore. Melatonin and the watery beginnings of sleep. Science News Online. October 7, 2014.

J. Raloff. Night owls may want to dim their lights. Science News Online. January 12, 2011.

R. Ehrenberg. Hormones give lantern sharks the glow. Science News. Vol. 176, December 5, 2009, p. 12.

C. Mlot. Return of the tuatara. Science News. Vol. 152, November 8, 1997, p. 300.

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