Nearly 500 million years ago, life on Earth was very different. The continent was rocky, dry, with little vegetation, and the barren poles were ice-free. Almost all plants and animals live in vast, warm oceans that cover much of the planet.
But in the shallow seas that surround the continent, there is a creature that has developed a completely modern compound-eye visual feature.
According to a study published last Thursday in Scientific Reports, there is a trilobite , the Aulacopleura koninckii , a marine spider-shaped arthropod that lived 429 million years ago , whose compound eyes are almost identical to those of modern bees!
This discovery suggests that compound eyes evolved very early in animal history, and that they also had fairly intact visual systems.
Along with Euan Clarkson of the University of Edinburgh in the United Kingdom, a team of researchers led by Dr Brigitte Schoenemann, a professor of natural sciences at the University of Cologne in Germany, re-examined a 1.2-centimeter-long fossil of an animal known as Aulacopleura koninckii. It is a trilobite: a marine animal that resembles a tree louse and is related to insects and shrimp.
The researchers found that the kong seabream was small and numerous, and they crossed the warm Cambrian ocean with dark rings on their eyes, which allowed them to survive in bright shallow water on the edge of the continent.
Using a digital microscope, they examined the internal structure of the fossilized trilobite's eye. The fossil was discovered in what is now the Czech Republic in 1846, and Schoenemann and her colleagues found that the seabream's left eye was still there, functioning similar to that of modern insects and crustaceans. They say the structure and function of many compound eyes have barely changed since the Paleozoic era (542 million to 252 million years ago), providing a new understanding of the ancient life of trilobites.
But how did the trilobite's eye structure correlate its evolutionary design to the past 500 million years? Schoenemann and other researchers studied the fossils for clues.
Named after the French-Czech paleontologist Joachim Barrande, the fossil seabream is no more than 0.8 inches tall and has only two protruding semi-oval eyes on the back of its skull intact.
Among the salient features of the eyeball are a series of identical visual units about 35 microns in diameter, which are miniature receptors called "ommatidia", each with photosensitive cells and lenses that focus light. It also contains light-detecting cells, which congregate around a transparent tube called "rhabdom." A dark ring of pigment cells formed by a barrier surrounds each cell, and above it is the remnants of a thick lens and a flat crystal cone that allows light to focus on the lines, creating a "mosaic"-like visual effect.
The internal structure of its ligands is almost identical to that of modern insects. The only difference is that they don't cluster so densely in the eyes, which may reduce the amount of detail the animal can see. But in essence, it's a modern compound eye. To this day, bees and dragonflies still use this vision.
Interestingly, the researchers found that the eye lens of the trilobite is mainly made of calcite, a highly refractive substance. According to the study, while other marine organisms, including starfish and stone turtles, also have calcified lenses, their eyes act as light detectors, and the seabream can see, making trilobites "the only animals that use calcite in the lens to form image vision."
But Schoenemann's team found that these are more than just the structure of the eye. The structure itself provides a window into the thriving environment of these trilobites, as this compound eye is of no use to creatures living in the deep sea.
In a compound eye, each visual unit works individually to provide a single pixel. In the trilobites' study, there are about 200 pixels — which gives them mosaic vision to distinguish between obstacles, shelters, and predators — while modern bees have thousands of pixels, and dragonflies have up to 30,000 pixels per eye.
The small size of the seabream eye suggests that it lives in bright shallow water and may be active during the day, as smaller lenses are more likely to capture light in bright environments.
Trilobites were one of the most successful early animals, ruling the oceans for 300 million years from about 520 million years ago, with the last extinction occurring 252 million years ago (this "mass extinction" wiped out 70 percent of terrestrial vertebrates and 96 percent of marine species). Although they did not survive mass extinctions, their most important contributions to modern life persisted, providing clues to the origins of related taxa such as insects and crustaceans.
It's unclear how long the compound eye existed before the cambrian trilobite appeared, Schoenemann said. Such eyes may have appeared only once, on the earliest ancestors of insects and trilobites.
Compilation/Prospective Economist APP Information Group
References: https://www.newscientist.com/article/2251851-trilobites-living-429-million-years-ago-had-eyes-like-modern-insects/
https://www.courthousenews.com/fossil-of-429-million-year-old-marine-creatures-eye-reveals-insect-like-vision/