Three-eyed lizards are not uncommon. Four-eyed ones are a novelty

Apr 05, 2018

Spot the eye in the back of the head

UNLIKE invertebrates, most of which have at least four eyes, vertebrates usually have only two. Yet, there are exceptions. Some fish, amphibians and reptiles have a third, so-called parietal, eye. This organ, a modification of a brain structure called the parapineal gland (itself an associate of the better-known pineal gland, which regulates an animal’s body clock), is usually covered with skin and sits on the top of the head, as the picture below shows. It helps those creatures that possess it to detect low-level illumination, and so aids navigation by moonlight or starlight.

For vertebrates to have four eyes, however, is unusual in the extreme. The only ones known that are so endowed are the lampreys. These have two conventional eyes and two parietals, one derived from the parapineal and the other from the pineal. But lampreys’ exceptionalism in this respect has just been challenged. This week Krister Smith, a palaeontologist at...

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