306 — The Only Known Jawed Vertebrate with Four Eyes and the Bauplan of the Pineal Complex

So here’s something I didn’t know about: Many types of lizards have a “third eye” — the pineal organ, which sits at the top of the head (unicorn-horn position) and acts as a basic photoreceptor which aids the animal in keeping track of circadian rhythms and keeping tabs on which way is up. This organ can’t see shape or color, but can detect light.

Saniwa ensidens is a distant extinct relative of the modern monitor. Unlike all other known vertebrates, S. ensidens had a fourth eye — a parapineal — right in front of its pineal.

The skull tissue surrounding these fossilized foramina was very well preserved: Well enough preserved, at least, to definitively confirm that the skull had two distinct and discrete foramina, and not simply a crack or hole due to age or weathering.

So what does it mean to find this specimen?

The presence of both parapineal and pineal organs in this front-to-back organization changes the way evolutionary biologists understood the development of this organ (and as a result, the relationship between the disappearance of the pineal and the evolution of a more powerful or higher resolution eye in certain animals like birds and mammals). For a long time, the “classical” model proposed that these two foramina slowly converged on the top of the head, drifting from an ancient bilateral (i.e. a second pair of eyes) structure. Instead, it’s now clear that these structures evolved centered on the skull.

This has implications for the introduction of blue opsins into the retina — and perhaps ipRGCs in general — in lieu of blue opsin expression taking place predominantly in a third eye.