Saniwa ensidens was a varanid lizard that lived during the Eocene, about 49 million years ago, in what is now Wyoming, USA.
Closely related to monitor lizards, it would have grown to about 2m long (~6'6") and looked very similar to modern forms, with a pointed snout, well-developed limbs, and a proportionally long tail.
But it also had something not seen in any other jawed vertebrate — a total of four eyes.
Many lizards have a single prominent parietal "third eye" on the top of their heads, which are light-sensitive and involved with both regulating circadian rhythms and hormone production for thermoregulation. However, Saniwa ensidens had two of these structures, with one formed from the parapineal gland like in other lizards, and the second positioned behind it formed by the pineal gland. The only other vertebrates with this arrangement are the jawless lampreys, suggesting that Saniwa somehow re-evolved a pineal eye hundreds of millions of years after its distant ancestors had lost it.
It's not clear what this extra eye was used for. It would have increased the pineal complex's sensitivity to blue-wavelength light, and may have been involved in orientation and navigation using polarized light similar to some modern lizards — or possibly magnetoreception-based navigation similar to some modern newts.
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