Extinct lizard had two ‘third eyes’
WE learned in high school biology that the tuatara has a
third eye. This pineal eye or
parietal eye is clearly visible in juvenile tuatara. It occurs in the midline on the top of its skull, and is an outgrowth of the brain called the epithalamus. This pineal eye is present in lampreys, sharks, bony fish, frogs, tuatara, iguanas and monitor lizards. All lizards that retain a pineal eye are coldblooded. The third eye responds to the presence of light and regulates circadian rhythms — body processes and behaviours based on a cycle of 24 hours. The third eye also regulates hormones for heat regulation. The pineal body is an outgrowth of the forebrain, primitively forming two lobes, one behind the other, although in most vertebrates only one remains. In the lamprey, the third eye is very eyelike, with both lens and retina.
This month’s Current Biology features an extinct Eocene monitor lizard from Wyoming, US, that throws light on the evolution of the brain of land vertebrates. It is the first jawed vertebrate found to have two pineal eyeholes on the top of its skull.
The pineal eye appeared early in vertebrate evolution but was lost repeatedly in land vertebrates, including mammals. Early mammals had a pineal eye but living mammals have lost the opening in the skull, and only the pineal organ remains. The pineal body is divided into two organs, the parapineal and the pineal. The lamprey, a ‘‘living fossil’’ that evolved before sharks, has
(unlike sharks) two pineal openings.
After lampreys evolved, around 360 million years ago, one of the two parietal eye openings (long thought to be the parapineal) was lost in younger vertebrate lineages. It was therefore astounding to discover that the monitor lizard Saniwa ensidens, although only 48 million years old, had two openings in the midline on the top of its skull, one for the parapineal organ, the other for the pineal proper, as in a lamprey. The positions of the two midline eyes on the fossil monitor show that in modern lizards, the third eye is derived from the parapineal, not, as was previously thought, the pineal organ. It similarly brings into question the origin of the pineal organ in mammals. The discovery demonstrates the remarkable capacity of vertebrates to reevolve longlost structures — a comparable example would be the reevolution of teeth in birds. (See Smith, K.T. et al. 2018, The only known jawed vertebrate with four eyes and the Bauplan of the pineal complex. Current
Biology 28 (7): 11011107.)
(In the drawings from last week’s column, the thrips’ name should have been attached to the other illustration).