Each of our eyes sees a slightly different view of the world, and our brain combines these signals into a single three-dimensional image. But this only works in one direction, because our eyes face straight ahead and their respective fields of vision only overlap in a narrow zone. But there was once a creature that had binocular vision in a massive arc around its body, not just in front but to the sides as well. It’s called Henningsmoenicaris scutula and it lived around half a billion years ago.
H.scutula lived in the Cambrian period, the part of Earth’s history when most of today’s major animal groups exploded into existence. It was a crustacean, one of the earliest members of the group that includes crabs, prawns and lobsters. It was just a millimetre long and almost totally encased within a bowl-shaped shield. From beneath the shield, weird spike-tipped legs propelled it along, while two stalked eyes, each just half a millimetre across, peered out at the Cambrian oceans.
These eyes are compound ones, made up of several units or ‘ommatidia’. They’ve also withstood the test of time. Their organic tissues have since been converted into the mineral apatite, and the resulting fossils perfectly retain the shape and angle of each ommatidium. The eyes are so well-preserved that Brigitte Schoenemann from the University of Bonn could use them to reconstruct how H.scotula saw the world to a “quite impressive degree”.
Each ommatidium points in a slightly different direction and acts like a pixel on a computer screen. The animal used these separate snapshots to construct an overall view of its world. Schoenemann found that each eye is divided into four different areas that face respectively forward, outwards to the side, backwards, and inwards. Together, they cover every direction. The animal’s only blind spot is the bit of space occupied by its own body.
The front-facing parts of the eyes have the highest resolution and overlapping fields of view, just like our own. The outward-facing parts probably scanned the horizon on either side of the animal. The back-facing parts were the most sensitive; they had the biggest ommatidia and would have captured the most light. They might have been angled downwards to scan the dark ocean depths below the animal.
But what about the inward-facing parts? On a normal head, this wouldn’t make any sense, but remember that this crustacean had eyes on stalks. The inward-facing segments had visual fields that overlapped with those of the outward-facing segments on the opposite eye. This animal would have had excellent depth perception all around its body, rather than just in front of it. As Schoenemann puts it, it has “a three-dimensional visual net that covers not only the front, but extends also far to either side”.
But H.scutula could hardly have been described as sharp-sighted. Each ommatidium acts as a single pixel, and each eye had less than 200 of them. These weren’t eyes that could see the world in high-resolution. They were, however, well adapted for detecting movement.
The overlapping visual fields turn the entire space around the animal into a grid of kite-shaped zones (see the diagram above). Any given position in this grid is covered by a pair of ommatidia, one from each eye. As a prey animal moved around, it travelled from one kite to another, and H.scutula could track its whereabouts, distance and direction, even if it couldn’t see its target clearly.
Even though it was lived almost half a billion years ago, this tiny crustacean has a sophisticated set of eyes that allowed it scan the world around it in almost every direction.
Reference: Schoenemann, Castellani, Clarkson, Haug, Maas, Haug & Waloszek. 2011. The sophisticated visual system of a tiny cambrian crustacean: analysis of a stalked fossil compound eye. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2011.1888
***A visual tour of extraordinary animal eyes***