The trilobite Survey of terrestrial seas during nearly all Paleozoic Era (-542 to -251 million years ago), before becoming extinct during the Permian-Triassic Crisis. These marine arthropods were distinguished by elaborate compound eyes. Most species exhibited visual systems resembling flies, with countless hexagons reflecting the image onto a single retina. But the suborder Phacopina did not follow this rule. Their huge eyes gave them a strange look, magnified by large lenses far away from each other. How well can these tools work? Modern imaging techniques have recently succeeded in unraveling the mystery behind ” super eye from Vacubena. To obtain a result similar to that advanced nearly half a century ago by an amateur paleontologist…
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One eye in one eye
In the 1970s, radiologist Wilhelm Stürmer developed a passion for paleontology and decided to take X-ray pictures of Vacubinia. In the images obtained, amateurs believe that they can indeed see strands in the back of their huge eyes. Why not nerves or even a light-guiding system? Unfortunately for Wilhelm, science at the time did not imagine that soft tissues could be preserved during fossilization. “Professionals then prefer to focus their efforts on finding skeletons or calcified like shells,” says Catherine Cronier, a professor of palaeontology at the University of Lille who specializes in Devonian trilobites. It will be necessary to wait for the accuracy of the imaging techniques to improve to be able to confirm the radiologist’s statements.
Today, the use of X-rays or synchrotron radiation is still expensive, but it is proving to be very practical. “It’s kind of non-destructive, like when you do an MRI,” says Catherine Crone. Thanks to this method, an international research team led by Brigitte Schönemann, of the University of Cologne, identified the optic nerve that allows connection between the eye and the brain in Vacubina. In addition, under each of the lenses – with a diameter of at least 1 mm – hide 6 sides, the equivalent of a small compound eye. Knowing that each “super eye” contains approximately 200 lenses, so about 200 compound eyes make up the entire optical system…a completely unique discovery in the animal kingdom!
Advantage… but which one?
What could be the purpose of such an optical device? “During the Devonian period, approximately 390 million years ago, trilobites occupied mainly continental seas,” says Catherine Cronier. These relatively warm bodies of water are, as the name implies, part of the continental shelf. Over the first 50 meters of depth, these predators dominate over hundreds of millions of years … the bottom, which means more comfort on the bottom of the water, these predators present a whole range of eye sizes depending on their environment. Those who live outside have smaller eyes with fewer lenses, as more lenses are not necessary yet away from daylight. On the contrary, more coastal specimens have huge eyes with one of the many lenses, which are able to capture the slightest ray of sunlight. But what about vacupina?
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These animals have the peculiarity of combining two features: large and small eyes. It would have made their lives easier in low light or rapidly changing environments, by giving them good clarity. “This mix of species could change the idea we have about Phacopina’s adaptation to ambient light,” says Catherine Cronier. Not to mention that Phacopina’s “super eye” may also have played a role in the quality of contrasts, or even the perception of different colours. However, it is still difficult to provide a definitive answer to the evolutionary advantage of these eyes. Researchers can only make hypotheses by observing crabs or horseshoe crabs that, we imagine, behave similarly to their distant “cousins,” the now-extinct trilobite.
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