The humble trilobite, a helmet-headed creature that swam in the ocean hundreds of millions of years ago, hid an extraordinary secret — a “hyper-eye” never seen before in the animal kingdom.
By examining x-rays, researchers found that certain species of trilobite-extinct arthropods distantly related to horseshoe crabs-had “hyper compound eyes” complete with hundreds of lenses, their own neural network for processing and transmitting signals, and multiple optic nerves, according to new research published September 30 in the journal Scientific reports.
Related: Why did trilobites become extinct?
Today’s arthropods, like dragonflies and mantis shrimp, are also known for their powerful compound eyes, which are composed of countless eye facets called ommatidia, each equipped with its own lens, like a disco ball.
But according to the new findings trilobites from the family Phacops had complex eyes that were far larger and more complex than their contemporary arthropod relatives. Each of their eyes (they had one on the left and one on the right) had hundreds of lenses. At almost a millimeter across, these primary lenses were thousands of times larger than a typical arthropod. Beneath them like bulbs in a car headlight sat six (or more) faceted substructures akin to a typically compound eye. “So each of the large Phacopid eyes is a hyper-composite eye with up to 200 compound eyes each,” study author Brigitte Schoenemann, a paleontologist at the University of Cologne in Germany, told WordsSideKick.com in an email.
Trilobites are creatures that lived from the early Cambrian period (521 million years ago) to the end of the Permian (252 million years ago) on the ocean floor. Some may have been predators that hunted aquatic worms, although most were scavengers or plankton eaters. The remains are commonly found in limestone from the Cambrian period. But despite their ubiquity in the fossil record, scientists still have questions about their physiology and evolutionary history.
To answer some of these questions, the researchers used photo-enhancing techniques to examine dozens of archival photos, cross-referencing them with recent findings. In the process, they also resolved a long-running scientific debate: They confirmed that a mysterious series of “fibers” seen in X-rays from more than 40 years ago were actually bundled optical nerves connected to the eyes of the trilobites.
“It’s always difficult to deduce function in old, extinct organisms,” said Nigel Hughes, a trilobite expert at the University of California Riverside who was not involved in the study. In fact, Hughes pointed out, even some unequal features of living beings evade explanation — for example, there is still some debate about the function of narwhals’ long, horn-like tooth, according to Smithsonian Institution.
However, eyes are a little easier to analyze than teeth or horns, Hughes said, because optical systems have only one function: vision. “We know it’s an eye from the structure,” he said, and therefore it makes sense for the attached filaments to be nerves. “I think it’s pretty convincingly argued in the newspaper.” Why a trilobite might need so much visual power is still a mystery.
The X-rays themselves were taken by Wilhelm Stürmer, a professional radiologist and amateur paleontologist from Siemens. In the 1970s, Stürmer mounted an X-ray probe inside his VW bus and created a new method of studying fossils: X-ray paleontology, which allowed him to look through solid rock on the spot and take some of the most sophisticated fossil photos of his day.
After examining the Hunsrück slate, a fossil quarry within driving distance of his home in Munich, Germany, Stürmer revealed a world of petrified creatures embedded in the rock. Remarkably, these specimens – including phacopid trilobites – were so well preserved that even their delicate soft tissues were visible. Stürmer and his collaborator Jan Bergström noted that the trilobites apparently had fossilized “fibers” attached to their compound eyes, as they described in the June 1973 issue of the journal Paleontological Journal.
Related: In pictures: A filter-feeding Cambrian creature
But when Stürmer brought these findings to other paleontologists, “his colleagues in the scientific world laughed at him,” Schoenemann said. The prevailing wisdom at the time was that soft tissues like nerves simply did not fossilize. Stürmer must have confused gel filaments with optic nerve tissue, his critics claimed, according to Schoenemann. However, the radiologist remained firm in his beliefs.
“Stürmer believed in his theory until he died, filled with bitterness in 1986,” Schoenemann said. After almost half a century, Schoenemann and her team feel that they have finally confirmed his work.
Unfortunately, like Wilhelm Stürmer, phacopid trilobites are no longer with us – they became extinct about 358 million years ago at the end of Devonian periodalong with about 75% percent of all life on Earth, Schoenemann said. “But certainly not because of their sophisticated, highly customized eyes.”
Originally published on Live Science.