old four-eyes – Nix Illustration

Saniwa

Saniwa ensidens was a varanid lizard that lived during the Eocene, about 49 million years ago, in what is now Wyoming, USA.

Closely related to monitor lizards, it would have grown to about 2m long (~6’6″) and looked very similar to modern forms, with a pointed snout, well-developed limbs, and a proportionally long tail.

But it also had something not seen in any other jawed vertebrate — a total of four eyes.

Many lizards have a single prominent parietal “third eye” on the top of their heads, which are light-sensitive and involved with both regulating circadian rhythms and hormone production for thermoregulation. However, Saniwa ensidens had two of these structures, with one formed from the parapineal gland like in other lizards, and the second positioned behind it formed by the pineal gland. The only other vertebrates with this arrangement are the jawless lampreys, suggesting that Saniwa somehow re-evolved a pineal eye hundreds of millions of years after its distant ancestors had lost it.

It’s not clear what this extra eye was used for. It would have increased the pineal complex’s sensitivity to blue-wavelength light, and may have been involved in orientation and navigation using polarized light similar to some modern lizards — or possibly magnetoreception-based navigation similar to some modern newts.

Haikouichthys

Haikouichthys ercaicunensis was one of the earliest known vertebrates, living in what is now southwestern China during the Cambrian, around 518 million years ago.

About 2.5cm long (~1″), it had a streamlined body with a long fin along its back and tail. It lacked limbs and jaws, but it did have a flexible “upper lip” that could potentially have been used to manipulate food particles, either engulfing them on the seafloor or stirring them up into the water for filter-feeding.

And a recent study revealed a big surprise — it had four eyes! Along with its already-known large sideways-facing pair of eyes, it also had another smaller upwards-facing pair in the middle of its face, giving it a much wider field of vision and the ability to watch out for predators from multiple angles at once.

The extra pair of eyes may be the evolutionary precursors to the pineal complex in later vertebrates — represented in modern forms by the pineal gland and the parietal eye.

Pygmaclypeatus

Pygmaclypeatus daziensis was a small early trilobitomorph arthropod that lived during the Cambrian, about 518 million years ago, in what is now southwestern China.

It had a wide flat carapace, about 14mm long (~0.5″), with a single pair of antennae, fourteen pairs of limbs, and a short segmented “tail”.

It also had an unusual arrangement of four eyes — one pair of fixed-in-place trilobite-like eyes on top of its headshield, and a second pair of crustacean-like mobile stalked eyes on the underside. Other four-eyed Cambrian arthropods are known, but Pygmaclypeatus is currently unique for having two completely different compound eye systems.

Its well-developed limbs with paddle-like branches indicate it could swim well, and probably also burrow into soft seafloor sediment. Its upper eyes seem to have been adapted to sensing motion in dim daylight conditions, suggesting they were used to keep a lookout for predators in shallow murky water. Its lower eyes were more sensitive, and may have been used to locate food items such as smaller soft-bodied invertebrates or organic detritus on the seafloor.