An extinct species of monitor lizard had four eyes, a first among known jawed vertebrates, a fossil study has found. Today, only the jawless lampreys have four eyes, according to researchers at Senckenberg Research Institute in Germany.
The third and fourth eyes refer to pineal and parapineal organs, eye-like photosensory structures on the top of the head that play key roles in orientation and in circadian and annual cycles.
The new findings published in Current Biology help to elucidate the evolutionary history of these structures among vertebrates, researchers said. The photosensitive pineal organ is found in a number of lower vertebrates such as fishes and frogs, they said. It is often referred to as the “third eye” and was widespread in primitive vertebrates.
“On the one hand, there was this idea that the third eye was simply reduced independently in many different vertebrate groups such as mammals and birds and is retained only in lizards among fully land-dwelling vertebrates,” said Krister Smith from the Senckenberg Research Institute. “On the other hand, there was this idea that the lizard third eye developed from a different organ, called the parapineal, which is well developed in lampreys. These two ideas did not really cohere.”
CT scans for fossil study
By discovering a four-eyed lizard — in which both pineal and parapineal organs formed an eye on the top of the head — the researchers confirmed that the lizard third eye really is different from the third eye of other jawed vertebrates.
The researchers got the idea that the fossilised lizards might have a fourth eye after other experts came to contradictory conclusions about where the lizard’s third eye was located. They turned to museum specimens collected nearly 150 years ago as part of the Yale College Expedition to the Bridger Basin, Wyoming in the US. CT scans showed that two different individuals had spaces where a fourth eye would have been, which, Smith said, “I certainly did not expect!”
Their evidence confirms that the pineal and parapineal glands were not a pair of organs in the way that vertebrate eyes are. They also suggest that the third eye of lizards evolved independently of the third eye in other vertebrate groups. Smith said that while there is “nothing mystical” about the pineal and parapineal organs, they do enable extraordinary abilities.
For instance, they allow some lower vertebrates to sense the polarisation of light and use that information to orient themselves geographically.
