An international team of scientists has analyzed the fossilized remains of a monitor lizard ( Saniwa ensidens ) and have identified a third and fourth eye on the top of the animal’s head . This data represents the first scientific evidence of a species of vertebrate with a jaw that has these features.
Currently, animals such as lizards, toads, tunas, and even some sharks are known to possess a third parietal eye . In fact, it plays key roles in the geographic orientation and circadian cycles (those that regulate changes in physiological characteristics in a 24-hour cycle) of these beings.
However, despite the fact that this organ was common among primitive vertebrates, the truth is that, today, the fourth eye is only possessed by lampreys (a type of fish). Neither birds nor mammals have it.
“We thought that the third eye had been disappearing independently in many groups of vertebrates, such as mammals and birds, and that it was only conserved in lizards among terrestrial vertebrates,” says Krister Smith of the Senckenberg Research Institute (in Germany). “But on the other hand, there was also the idea that the lizard’s third eye developed from a different organ, called a parapineal , which is well developed in lampreys. These two hypotheses did not correspond ”, he emphasizes.
The fossil , found in 1871, belongs to a lineage that lived around 49 million years ago and thanks to this, some first theories about the progressive history of the third and four eyes have been made.
Approach to ocular evolution
As part of an expedition from Yale University (USA), a group of scientists analyzed museum specimens discovered 150 years ago in Grizzly Buttes.
The scans showed surprising results, since several fossil remains had a free space for the fourth eye . Researchers previously knew that these organs provide animals with extraordinary abilities, such as sensing the polarization of light and using that information to orient themselves geographically.
The new findings confirm the value of the museum’s fossils, because despite their small size and condition, they have revealed a great fact. “Our work shows that even small, fragmented fossils can be enormously useful,” says Smith.
Final analyzes have confirmed that the third eye of lizards is actually different from that of other jawed vertebrates , since in the four-eyed lizard, the pineal and parapineal organs form a single eye on the top of the head . However, there is still much to learn about the evolution of these eyes and their functions in living animals.
Reference: Krister T. Smith, Bhart-Anjan S. Bhullar, Gunther Köhler, and Jörg Habersetzer. The Only Known Jawed Vertebrate with Four Eyes and the Bauplan of the Pineal Complex (2018) CellPress. DOI: https://doi.org/10.1016/j.cub.2018.02.021
