Monday, 9 April 2018

Extinct monitor lizard had four eyes, fossil evidence shows



Date:  April 2, 2018
Source:  Cell Press

Summary:
Researchers reporting in Current Biology on April 2 have evidence that an extinct species of monitor lizard had four eyes, a first among known jawed vertebrates. Today, only the jawless lampreys have four eyes.

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 help to elucidate the evolutionary history of these structures among vertebrates.

The photosensitive pineal organ is found in a number of lower vertebrates such as fishes and frogs, the researchers explain. It's 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," says Krister Smith at the Senckenberg Research Institute in Germany. "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 didn't really cohere.

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