Scurrying around forests about 48 million years ago, at first glance you wouldn’t think much of the ancient monitor lizard Saniwa ensidens. But researchers have just uncovered that this lizard was hiding a pretty weird secret: it had four eyes, and is the first jawed vertebrate known to do so. 

An analysis of partial fossils of S. ensidens held in museums for over 150 years shows that two indents on the top of the skulls were parietal eyes, sometimes called "third" eyes, and may have been able to sense light to regulate daily cycles, short-length (blue) light for geographical orientation, or both. The results of this new study, published in Current Biology, might even have implications for how we think the brains of land vertebrates evolved.

As weird as this sounds, quite a lot of animals effectively have three eyes. In fact, most lizard species – along with many amphibians and fish – still retain a parietal eye, which is located on the top of the head about halfway between the more conventional eyes. This is formed as a paired structure along with the pineal gland during early development.

While in lizards they remain separate and go on to form both the parietal eye and the pineal gland, in many other lineages the early paired structure fuses back together again, and so modern bird and mammals, for example, are just left with a pineal organ and no third eye. But this shows that all land vertebrates must once have had a parietal eye.

“A third eye evolved very early in vertebrate evolution, and stem representatives of almost all major lineages have an opening for a third eye on the midline in the skull roof,” explained Krister Smith, co-author of this latest study, to IFLScience. “However, it was lost repeatedly, particularly in land vertebrates. This includes mammals: stem mammals still had a third eye, but it is now represented only by the pineal in extant mammals.”

Now to make things slightly more complicated, the pineal is actually split into two organs: the parapineal and the pineal proper. While early branching vertebrates such as the lamprey have eyelike structures descended from both the parapineal and the pineal, it has long been thought that soon after the lamprey branched off the parapineal was either massively reduced or simply lost in many following vertebrate lineages.

This suggests that the third eye in stem mammals and lizards alike were both formed from the pineal proper, but this newly recognized fossil monitor lizard questions that assumption. The fact that it has two eyes on the top of its head means that its lineage did not reduce the parapineal and that one of the extra eyes is descended from this organ, and the other from the pineal proper.

What is more, the positioning of both these eyes on the fossils show that in modern lizards, their surviving third eye has likely not originated from the pineal proper but in fact the parapineal. As a result, this could question the origin of the third eye even in mammals, and could completely alter how we thought the brains of land vertebrates evolved.