Fossil of 429 Million-Year-Old Marine Creature’s Eye Reveals Insect-Like Vision

(CN) — Nearly 500 million years ago, life on Earth was very different.  The continents were rocky and dry with little vegetation and the barren poles held no ice. Nearly all plant and animal life existed in the vast, warm oceans covering much of the planet.

Fossilized eye of the Aulacopleura koninckii (Barrande, 1846), specimen. (Photo by Brigitte Schoenemann)

But in the shallow seas surrounding the continents, one creature had already developed a thoroughly modern feature.

One type of trilobites — a group of marine arachnomorph arthropods with sectioned exoskeletons — with compound eyes almost identical to those of modern-day bees, according to a study published Thursday in Scientific Reports.

Aulacopleura koninckii were small and abundant, traversing the balmy Cambrian seas with dark-ringed eyes that enabled them to thrive in bright, shallow waters on the margins of the continents, study authors discovered.

Dr. Brigitte Schoenemann, a natural sciences professor at University of Cologne in Germany, led a research team that examined the internal structure of a 429-million-year-old fossilized trilobite eye. Using digital microscopy to re-examine an A. koninckii fossil discovered in 1846 in what is now the Czech Republic, Schoenemann and her colleagues found features similar to those of the eyes of modern insects and crustaceans, suggesting that “the structure and function of many compound eyes has remained mostly unchanged” since the Palaeozoic era, between 542 million and 251 million years ago.

But what about the trilobite’s eye structure has made its evolutionary design relevant for the last half a billion years? Schoenemann and other researchers probed the fossil for clues.

Named by the French-Czech paleontologist Joachim Barrande, the A. koninckii fossil is no more than 0.8 inches high, with only one of two protruding semi-oval eyes on the back of its head still intact. Among the eye’s distinctive features is a series of identical visual units known as ommatidia “that contain light-detecting cells grouped around a transparent tube called a rhabdom.” A dark ring made from barrier-forming pigment cells surrounds each unit, atop which a thick lens and remnants of a flat crystalline cone allowed light to be focused onto the rhabdom, resulting in a “mosaic-like” vision utilized to this day by bees and dragonflies.

Interestingly, researchers found the trilobite had eye lenses make mainly of calcite, a highly refractive material. While other marine creatures, including brittle stars or chitons, also have calcitic lenses, their eyes function as light detectors, A. koninckii could see, making trilobites “the only anthropods to use calcite in lenses for image-forming vision,” according to the study.

But Schoenemann’s findings reveal more than the mere eye structure of trilobites. The structure itself offers a window into the environment these trilobites flourished in, for such compound eyes would not be useful to creatures who lived in deeper ocean waters.

In a compound eye, each visual unit works separately to provide a single pixel. While the trilobite studies had about 200 of them — giving it mosaic vision that would have allowed it to discern obstacles, its shelter and predators — the modern honey bee has thousands of pixels and a dragonfly has up to 30,000 per eye.

The small size of its eyes indicates A. koninckii lived in bright, shallow waters and was probably active during the day since smaller lenses are better at capturing light in bright conditions.

Trilobites were among the most successful of all early animals, traversing prehistoric oceans for nearly 300 million years. And though they didn’t survive the massive extinction known as the Great Dying that eliminated 70% of land vertebrates and as much as 96% of all marine species, their most important contribution to modern life endures.

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