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.

Reconstruction of where the two extra eyes would have been. Senckenberg Gesellschaft für Naturforschung/Andreas Lachmann/Digimorph.org

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.