The fossilized horseshoe crab Euproops danae (left image) held a perfectly preserved brain mold, which is shown close-up at panel B. Panel C depicts a reconstruction Euproops Danae with the anatomy and position of its brain. (Image credit: Russell Bicknell)A new study has revealed that researchers have discovered a fossilized brain of a horseshoe crab that was 310 million years old. This discovery reveals some surprising facts about the evolution of these crustaceans.The fossilized brain of Euproops Danae, an extinct species, was found at Mazon Creek, Illinois. Its delicate soft tissue was preserved in perfect conditions.Today, there are four types of horseshoe crabs. They all have hard exoskeletons and 10 legs. According to The National Wildlife Federation, these "crabs", although their name might suggest otherwise, are actually arachnids closely related to scorpions, spiders, and other arachnids. Researchers said that horseshoe crab fossils may be common, but little was known about their brains in the past.Similar: 10 brain discoveries that are fascinating and revealing about the brain of 10 people, from dino brains to thought controlLive Science's lead author Russell Bicknell is a paleontologist at University of New England. He stated that this was the only evidence of a brain in a fossil horseshoe claw crab. He said that the chances of finding a fossilized brain is "one in a thousand." Even then, they may be even rarer."Bicknell stated that brain tissue is very susceptible to rapid decay. "They must be kept in very special geological conditions or amber to preserve them."Geology was used in this instance to preserve the brain, or at least a replica of it, and keep the soft tissues in top condition over the years. Bicknell stated that they have a mold for the brain and not the actual brain.Siderite, an iron carbonate mineral that forms concrete mineral precipitations at Mazon Creek, is responsible for the formation of Mazon Creek's deposits. This mineral can rapidly encase and fossilize a dead body. Even though these concretions saved the horseshoe crab's corpse, the brain tissue eventually decomposed and disappeared. The brain was eventually replaced by kaolinite, a clay mineral that forms a cast.Kaolite has a white color and siderite is a dark gray. Bicknell stated that the contrast in color meant that the brain fossil stood out from the rest of fossils more than usual.Now, the hunt continues for other ancient brains that may have been preserved in the same geological conditions that preserved this horseshoe-crab.Bicknell stated that "the Mazon Creek deposit was exceptional." "If we start looking, we might be able to find more [brain fossils] if we start searching."Researchers were able to examine how brains of arachnids have changed over time thanks to this discovery. The researchers were surprised to discover that the Carboniferous period's ancient brain (359 million to 299 millions years ago) was very similar to the modern horseshoe crab.Bicknell stated that "despite 300 million years of evolution the fossil horseshoe crab's brain is pretty much unchanged as modern forms."
