A fossilized 429 million-year-old trilobite provides insight into Paleozoic eyes

Dr. Brigitte Schoenemann has her eyes on evolution.

To see the ancient eyes clearly, Schoenemann uses a digital microscope. Looking through the lens can take her way, way back — 429 million years ago.

It is, she said, “breathtaking.”

Based in the zoology department at the University of Cologne, Schoenemann's research focused on fossils. She got a lucky break when she purchased a triolobite fossil that stood out to her. 

“...I simply liked this trilobite with its big head, and big eyes. So I bought one, and the collector was so kind to give me some more fragments he did not need for his collection,” she told Current Science Daily. “When I looked through the microscope then, it was breathtaking what I saw.

“The great surprise was to see a complete visual unit of a compound eye (ommatidium), which is (almost) identical to that of modern bees, dragonflies, many diurnal insects and crustaceans , and that it was already equipped with very sophisticated mechanisms to make the eyes more sensitive (palisades),” she said.

“Not long ago people still thought that in fossils just teeth, bones, etc., could be preserved, but never cellular structures. This has changed obviously.”

She explained the importance of her discovery in an article she co-wrote that was published in Scientific Reports. The creature she had purchased, which was discovered in 1846 near Lodenice in the Czech Republic, had a pair of eyes sticking out of the back of its head. One of the semi-oval eyes had been dislodged over the eons.

“We have found amazing parallels in the visual system of an ancient arthropod to modern insects,” she said. “We studied the compound eyes of a trilobite, Aulacopleura koninckii. Trilobites are extinct arthropods that dominated the Paleozoic faunas of the oceans, half a billion years ago and a bit later. The internal structure of this eye is almost identical to that of modern bees, dragonflies, crustaceans and many other day active arthropods living today.

“...The ancient eye is almost the same as the modern ones of bees, for example. That means that this system has been proved as very effective over a long time. There is a paper from 2017 (Schoenemann et al. 2017) showing that the essential structure, the arrangement of the sensory cells grouped around the light perceiving structure, is more than half a billion years old.”

Some differences existed, however, Dr. Schoenemann said.

“This eye had no lenses, and all groups of sensory cells sat in a kind of cellular baskets — bees and Aulacopleura have a lens, and no baskets,” she said. 

The little creature studied is about 1.2 cm long, 0.8 cm wide and just 2-3 millimeters high, and it has a “very, very thin shell,” Schoenemann said.

“It has been a bottom dweller, gliding over the ground of the seas, eating organic material and other useful things it found. In this type of eye each facet of the compound eye contributes to the seen image just a point, like a pixel does to a computer graphic,” she said. “All older eyes of the Cambrian are just slitformed, scanning the horizon — this one here opens the field of view upwards, and with c 200 ‘pixels’ per eye it nicely can distinguish obstacles in the environment, shelters, but especially the dangerous predators of its time approaching, the cephalopods.

“Another story of the eye is simple, though much physics is hidden in behind. Large facets can capture much light to make the system below work efficiently,” she said. “So compound eyes with large facets often work in deeper waters, in twilight or in the dark. Small facets need an intense ‘package’ of light to work. So our small trilobite has facet diameters of c 35 micrometers, quite small facets.

“This allows to say that the trilobite was day-active, and lived in shallow, lightly flooded waters,” Schoenemann said. “Probably shore regions. And there is a last point. We found dark pigments in the eyes of the trilobite, isolating the optical units against each other. By several reasons one may deduce now, that the thin shell of the trilobite was translucent.

“We have that in modern shrimps for example, where it is a splendid camouflage on the sea ground,” she said. “Shrimps you just can see, if they start to move. It would be the first trilobite to be described as translucent.”

Schoenemann admits to some unexpected moments during her eye exam.

“One must not forget that we work with rocks, stones, and then to see formed there a complete, lovely ommatidium, 429 years old, was breathtaking. And then, when continuing to analyze it, to find even the receptor cells, the palisades, the pigment screen ... and then finally to understand that A. konickii with its thin shell probably was as translucent like a shrimp ...”

She can’t finish the quote. It was a powerful moment, a sense of peering into the ancient world.

“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. “The news actually is that directly in the fossil record you can see how these ancient systems worked. To be able to understand, how a several hundred-million-year-old eye worked, to be able to deduce, what the little creature had seen, and under which environmental conditions it lived, just by looking into its eyes, is breathtaking.”

The implications are far-reaching.

“This is not an indirect mode of investigation — we are at the source here — and it might be exemplarily to ask fossils for evolutionary history,” she said.