For this, they must be trapped in amber or very specific geological conditions must be met. Such was the case for this fossil of a Limulid, a family of marine arthropods resembling a horseshoe crab, found in Amazon Creek, Illinois (USA). 310 million years ago, its central nervous system was covered in vesiculitis before it collapsed. The space left by the organ was then filled with a white clay mineral, kaolinite, which allowed all the details to be preserved, like a mould. “Until then, we had never identified an ancient brain in this group of arthropods”, points to Russell Bicknell, a paleontologist at the University of New England, Australia, and co-author of the study. The researchers also noted, with astonishment, that the brain structure of this sample Euproops danae, an extinct species, differs slightly from that of the modern horseshoe crabs.

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