An article published in the journal “Current Biology” describes a research on a species of reptile called Saniwa ensidens (bottom photo ©Smokeybjb), related to the modern monitor lizards that lived about 49 million years ago in today’s Wyoming, USA. A team of researchers studied fossils of these animals discovered almost 150 years ago, concluding that they had four eyes. The two extra eyes were connected to the pineal and parapineal glands. This is the first discovered case of jawed vertebrate with four eyes.
The third and fourth eye are so called because they’re light-sensitive organs that in various animal species have important roles in orientation and in various cycles such as the circadian and the annual ones. Their evolution in vertebrates is not yet clear and there are different hypotheses about their origin with some scientists who think that they atrophied independently in groups such as mammals and birds while others think that in lizards the third eye developed from the parapineal gland and not from the pineal one.
What’s certain is that the so-called third eye connected to the pineal gland is present in vertebrates such as fish and frogs. The discovery of a kind of lizard that lived in the Eocene period in which both the pineal and the parapineal glands formed an eye on the top of its head is a confirmation that in lizards the third eye is really different from that of other jawed vertebrates.
Saniwa ensidens is considered a close relative of monitor lizards, to which it looks like, and had a length between 1.3 and 2.1 meters (4.3 to 6.9 feet). The first fossils were discovered in 1870 near the town of Granger, Wyoming, the first animal related to lizards to receive a name in North America. The fossils studied by the team led by Dr. Krister Smith of the Senckenberg Research Institute in Germany were collected in 1871 and their conditions were not good. However, the use of modern technologies enabled the team to study them in depth.
The top image (courtesy Senckenberg Gesellschaft für Naturforschung / Andreas Lachmann / Digimorph.org. All rights reserved) shows a reconstruction of the head of a Saniwa ensidens and in the box the holes, or forami, parietal and pineal. In that area a CT scan showed that two different Saniwa ensidens specimens had a space where there would be a fourth eye. For Dr. Krister Smith it was a surprise that showed that in this species the pineal and parapineal glands were not a pair of organs in the way the eyes of vertebrates are.
According to the researchers, the presence of these extra eyes in Saniwa ensidens indicate that they evolved in the lizards related to this species independently from other vertebrate groups. However, it doesn’t indicate which other species of ancient monitor lizards and lizards had a third and/or a fourth eye. However, it’s an excellent result considering the fact that the fossils examined were not in good condition.