Trilobites living 429 million years ago had eyes like modern insects

An animal that lived 429 million years ago had compound eyes almost identical to those of modern insects like bees and dragonflies. The finding implies that the compound eye evolved very early in the history of animals.

“I am quite sure that its roots lie far back in the Precambrian somewhere,” says Brigitte Schoenemann at the University of Cologne in Germany. The Cambrian period, when many of the major animal groups appeared, began about 540 million years ago.

With Euan Clarkson at the University of Edinburgh in the UK, Schoenemann re-examined a fossil of a 1.2-centimetre-long animal called Aulacopleura koninckii. It was a trilobite: a marine animal a bit like a woodlouse, related to insects and shrimp.

Trilobites dominated the oceans for 300 million years, beginning about 520 million years ago. The last ones died out 252 million years ago. Studying trilobites offers clues to the origins of related groups like insects and crustaceans.

Schoenemann and Clarkson found that one A. koninckii specimen still had its left eye. It was a compound eye like those of modern insects, which contains many tiny receptors called ommatidia, each with light-sensitive cells and a lens to focus light. Each ommatidium contributes a single “pixel” to create a mosaic-like image.

The internal structures of its ommatidia were almost identical to those of modern insects. The only difference was that they weren’t quite as densely packed into the eye, probably reducing the amount of detail the animal could see. But to all intents and purposes, it was a modern compound eye, says Schoenemann.

The oldest known compound eye with preserved internal structures belonged to another trilobite, which lived in the early Cambrian over 500 million years ago. Schoenemann’s team described it in 2017. That eye was more primitive though. “There were no distinct lenses,” says Schoenemann. The ommatidia were topped with “a kind of translucent window, without any capacity of focusing”. Also, the eye only had about 100 ommatidia.

It isn’t clear how long compound eyes existed before the Cambrian trilobite, says Schoenemann. Such eyes may have appeared only once, in the earliest ancestors of insects and trilobites.