Peek-a-boo. Creep-crawly trilobites (inset) had compound eyes, much like today’s insects and crustaceans. Just one trilobite eye had dozens of lenses, as seen in the close-up above.

Brigitte Schoenemann

Looking a Trilobite in the Eye

Hundreds of millions of years ago, the Earth's seas teemed with trilobites, hard-shelled critters that resembled spiny aquatic cockroaches. Because their exoskeletons lent themselves to fossilization, scientists know a lot about what the outside of their bodies looked like. Their inner workings, however, have remained mysterious. Now, a new study has revealed the structure of the trilobite eye, bringing researchers one step closer to understanding the evolution of vision.

Like today's insects and crustaceans, trilobites had compound eyes, with many different lenses focusing light onto clusters of sensory cells lying below them. The resulting image was put together a lot like a picture on your computer screen, with each lens producing one "pixel" of the whole. Because the lenses themselves were made of the mineral calcite, they often fossilized along with the rest of the trilobite's tough exoskeleton. The sensory cells underneath the lenses, however, were ephemeral, and scientists had always assumed that they had decayed without a trace.

So imagine Brigitte Schoenemann's surprise when she spotted fossilized versions of these delicate sensory cells while x-raying a long dead trilobite with a computed tomography (CT) scanner. "I expected that we would see [something] in the lens of trilobites, but then suddenly we saw structures of cells below the lens," recalls Schoenemann, a physiologist at the University of Bonn and the University of Cologne, both in Germany. Inspired, she applied to take more fossils to the European Synchrotron Radiation Facility in Grenoble, France, where she could use a particle accelerator's high energy x-rays to peer deeper into the trilobites' eyes. Now, she says, she's created images of the extinct animal's entire visual system, down to the level of fossilized individual cells.

So what does the inside of a trilobite eye look like? A bit like a flower, Schoenemann and a co-author report online today in Scientific Reports. Beneath each lens, round sensory cells are arranged like petals around a diamond-shaped photoreceptor able to pick up the dim light that filtered down through the Earth's ancient oceans. Pigment cells filled in the space between the blooms and likely made trilobite eyes appear brownish-black.

This bouquet of light sensitive blooms buffered by pigment cells is very similar to the structure seen in the eyes of today's horseshoe crabs (Limulus). "If you have an optical system that works, it can last," says Richard Fortey, a paleontologist at the Natural History Museum in London. He hopes the x-ray techniques used in this study will soon be applied to more trilobite species—which may have evolved different visual systems—as well as other types of well-preserved fossils. Modern techniques like synchrotron radiation "produce details you wouldn't have dreamed could be seen a few years ago," he says.

"It's fantastic to see" this new tool being adopted, agrees physicist Uwe Bergmann of the SLAC National Accelerator Laboratory in California, who has also used x-rays from a synchrotron there to study fossils. "It seems as if these x-ray tomography techniques have brought out really new knowledge … about the early evolution of eyes."