Quirks & Quarks

Why a lizard from 49 million years ago had four eyes

A 49-million-year-old fossil tells the story of a lizard with two eyes for vision and two extra eyes for geographic orientation.
This monitor lizard from 49 million years ago had two additional 'eyes' on the top of its head (Senckenberg Gesellschaft für Naturforschung / A. Lachmann)

The fossil

A 49-million-year-old fossil, found in 1871 in what is now Wyoming, was identified at the time as an extinct monitor lizard. But some strange features of the fossil gave rise to a lasting mystery: a pair of holes in the top of the lizard's head. 

One was identified at the time as a gap for the pineal organ, a photosensory organ known as the "third eye" that lizards and a number of lower vertebrates still have today. 

The purpose of the other remained a mystery until recently when Dr. Krister Smith, a researcher at the Senckenberg Research Institute in Frankfurt, Germany, took a second look. 

The lizard

The fossil is of a monitor lizard about 1.3 metres long that lived in wet, forested areas.

Its species disappeared about 34 million years ago, but it has relatives in monitor lizards today, like the famous Komodo dragon. Because the third eye is widespread in vertebrates, including lizards today, the researchers suggest that the second hole in the fossil represents another similar photosensory organ — a fourth eye. 

Although having four eyes is rare, it is known today in lamprey fish. The scientists believe that in this lizard the fourth eye grew out of the parapineal organ, which is an out growth of the pineal gland.   

Why four eyes?

The purpose of the pineal gland has changed through evolution. In humans, the gland is now buried deep in the skull and produces melatonin, which is related to our sleep cycles. But in lizards and lower vertebrates such as frogs and fishes, the pineal gland is much like an eye in that it has a lens and retina, and is photosensory. 

It plays a key role in orientation, and also in circadian and seasonal cycles. A fourth eye would have simply given this lizard greater capability in those areas.