A 390-million-year-old trilobite fossil with a set of tiny eyes inside its eyes

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Fossilized trilobites first studied by an amateur paleontologist half a century ago have provided researchers with an entirely new way of seeing the world, in a very literal sense.

A second look at X-ray images taken of ancient arthropods in the early 1970s revealed an eye structure unlike any animal before or since.

As the head of radiology at Siemens, Wilhelm Stürmer knew a thing or two about using X-rays to reveal hidden secrets. This was especially true when it came to studying fossils, a passion that fueled him by outfitting a minibus with X-ray equipment to take it to paleontology sites.

Despite his expertise in radiology, Stormer was not a paleontologist, so few took seriously his claim of discovering optic nerves inside the 390-million-year-old fossil Phacops geesops.

“At the time, the consensus was that only bones and teeth, which are the hard parts of living things, could be seen in the fossils, but not the soft parts, such as the intestines or nerves,” says Brigitte Schoenemann, a paleontologist at the University of Cologne.

In addition to the nerves, there was an arrangement of “fibers” that strangely looked like photoreceptors called omatidia. Only in this case, were they strangely elongated, about 25 times their diameter; So tall that it makes sense as a light-gathering structure.

A lot has changed since then. Today, paleontologists are comfortable with the idea that soft tissue structures can leave a visible imprint in fossilized materials. Super-long ommatidia has since been discovered in the compound eyes of aquatic arthropods.

With that in mind, Schönemann and her colleagues went back to Stürmer’s original photos for a closer look. After verifying the fossil again with modern computed tomography technology, they determined that the leads he discovered were almost certainly optical nerve fibres.

But what the foam fiber nest was associated with is what really caught the researchers’ attention: What looked like two compound eyes were actually hundreds, divided across groups of right and left.

“Each of these eyes is made up of about 200 lenses that are up to a millimeter in size,” Schoenemann says.

“Under each of these lenses, at least six sides are prepared, each together again forming a small compound eye. So we have about 200 compound eyes (one under each lens) in one eye.”

Trilobites in one form or another have dominated the oceans for hundreds of millions of years, adapting to fill a wide variety of water outlets with a variety of weird and wonderful body plans.

One of their smartest inventions was a visual system of unprecedented complexity. Although relatively simple in modern terms, their version of the eyes gave them an advantage in hunting or hiding and in detecting the tiniest changes in brightness and movement.

Although the anatomy of their eyes came in many forms, the structures most familiar to most zoologists today are easy to identify, which consist of a neatly arranged pattern of lenses that work together to convert scattered light into a highly pixelated map of their surroundings.

Modern insects and other arthropods continue to rely on compound eyes like these to great effect. What this split view lacks in precision it easily makes up for in simplicity and adaptability, evolving to overcome limitations with some modifications to anatomy.

However, as for the astonishing diversity of three-lobed eyes, those on certain members of the suborder Vaccupina have baffled paleontologists.

In what is known as a schizophrenic eye, each lens is located a short distance from its neighbor, leaving plenty of empty space that can be exploited to capture more light.

We now know that what appears to be a single lens is, in fact, a single compound eye within two “highly attentive eyes”.

While it doesn’t tell us why these eyes evolved, it does change the questions we need to ask about this unusual arthropod.

Instead of thinking about wasting space between each lens, biologists can now speculate on the benefits of hundreds of tiny eyes in adapting to low light, or responding to rapid changes in lighting conditions over a wider area.

“It is also possible that individual components of the eye may perform different functions, allowing, for example, to improve contrast or the perception of different colours,” Schoenemann says.

Stormer must have suspected there was something worth looking into the eyes, having drawn an arrow with a red pen pointing directly at the half-dozen dials under one of the lenses.

Sadly, the radiologist died in the 1980s, long before his discovery was given the verification it deserved.

Like the Trilobites, Stormer was clearly a visionary ahead of his time.

This research has been published in Scientific Reports.

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