Eyes don’t typically appear in the fossil record, but researchers were able to study the well-preserved and fossilized internal structure of a 429 million-year-old trilobite eye. Surprisingly, they found that the structure of the eye was nearly identical to the compound eyes of modern bees, dragonflies and crustaceans, according to the new study.
The study published Thursday in the journal Scientific Reports.
Trilobites lived and went extinct long before dinosaurs roamed the Earth. These animals were tiny marine arthropods and some of the earliest insect-like invertebrates of their kind. Modern arthropods include crustaceans, insects and spiders.
And essentially when these trilobites were alive, their efficient vision suited them and future arthropods to come so well that it hasn’t really changed much over the years. The internal eye structure of this fossilized trilobite can be seen in many modern insects and crustaceans, which means that their vision is based on adaptations made half a billion years ago.
This particular trilobite fossil, Aulacopleura koninckii, was discovered in 1846 in the Czech Republic and has been well studied. To emphasize how tiny trilobites were, this little guy was only between 1 and 2 millimeters high.
Two researchers, Brigitte Schoenemann at the University of Cologne’s Institutes of Biology Education and Zoology and palaeontologist Euan Clarkso at the University of Edinburgh’s School of Geosciences, studied the eye structures of the fossil.
Its two eyes, shaped like semi-ovals, protruded from the back of its head. One of them was already broken off.
Compared to older trilobites, which had slits for eyes, this trilobite is more representative of when they began to explore life in new environments — so these critters needed to be able to watch for predators, obstacles and shelter. The eyes of these trilobites adapted to widen and could look partly upward, Schoenemann said.
Arthropod eyes are very different than human eyes. We see everything at once thanks to the millions of receptors in our retinas. But insects have compound eyes that present 1 pixel per facet (like a diamond), so their visual information is pieced together like a mosaic.
This particular trilobite could see about 200 facets or pixels. Each facet was about 35 micrometers in diameter, which suggests that this trilobite lived in a bright setting and needed to be able to navigate that brightness.
So this trilobite was active during the daytime and lived in shallow parts of the sea that were flooded with light, Schoenemann said. It also had a special mechanism called a palisade to enhance its vision in darker settings — something insects today have, but can be traced back to this early trilobite.
And the eye told scientists something else about the trilobite in general. It was likely translucent, like some modern shrimps. This is the first time researchers have been able to describe a trilobite that was translucent.
The evidence goes back to structures in the eye. The compound eye system works when each facet is isolated from the other by walls, which has been seen in other trilobites. But this one had screening pigments in between the facets instead because the walls would have been translucent.
Translucence would have provided the perfect cover for this trilobite to survive.
“It glided over the ground of the light flooded parts of the seas, eating small organic material, and other useful things it found,” Schoenemann said. “The eyes were not needed to find food – but it could make out the predators. We can see by their eyes which mode of life they followed.”
Previously, another trilobite, Schmidtiellus reetae, was studied in 2017 and it showed some remains of a compound eye. It looked more like a prototype for the eventual compound eye that would be developed by the time Aulacopleura koninckii had a complete compound eye.
Overall, the study of this specimen has suggested that how compound eyes are structured and function has remained the same, more or less, since the Paleozoic era between 251 million and 542 million years ago.
“Fascinating in this research is to see what the eyes can tell you about the mode of life of a creature that had lived such a long time ago,” Schoenemann said.