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Brain tissue is naturally soft. Unlike bones, shells or teeth, it is rich in fat and decays rapidly, rarely appearing in the fossil record.
Invertebrate paleontologist Russell Bicknell of the University of New England in Australia was surprised to notice a white eruption near the front of a fossilized horseshoe crab body where the animal’s brain used to be. A closer look revealed an extraordinary trace of the brain, along with other parts of the creature’s nervous system.
Extracted from the Mazon Creek bed in northeastern Illinois and dating back 310 million years, this is the first fossilized horseshoe crab brain ever found. Dr. Bicknell and colleagues reported to find Last month in the journal Geology.
“These types of fossils are so rare that if you come across one, you’re usually in shock,” he said. “We’re talking about a level of wow like a needle in a haystack.”
The finding helps fill a gap in the evolution of arthropod brains, and also shows how little they’ve changed over hundreds of millions of years.
The preservation of soft tissues requires special conditions. Scientists have found brains more than 66 million years old, covered in fossilized tree resin, better known as amber. They also found brains preserved as flat carbon films that are sometimes replaced or overlapped by minerals in shale deposits that are more than 500 million years old. Such deposits contain the corpses of ocean-dwelling arthropods that sink to the seafloor, quickly sink into the mud, and are protected from sudden deterioration in a low-oxygen environment.
However, the fossilized brain of Euproops danae, held in a collection at the Yale Peabody Museum of Natural History, required a different set of conditions in order to be preserved.
This arthropod was not a crab, but closely related to spiders and scorpions. The extinct penny-sized horseshoe crab was buried more than 300 million years ago in what was once a shallow, brackish sea basin. Siderite, an iron carbonate mineral, quickly builds up around the dead creature’s body, forming a mold. Over time, as the soft tissue decayed, a white clay mineral called kaolinite filled the space left by the brain. Dr. It was this white mold on a dark gray rock that helped Bicknell notice the uniquely preserved brain impression.
“It’s a completely different mode of brain preservation,” said neuroanatomist Nicholas Strausfeld of the University of Arizona, who was one of the first to report a fossilized arthropod brain in 2012 but was not involved in the study. “This is extraordinary.”
extinct Euproops The brain showed a central space for the passage of a feeding tube and branching nerves to connect with the animal’s eyes and legs.
Bicknell and colleagues compared this ancient brain structure to that of Limulus polyphemus, a species of horseshoe crab still found on the Atlantic coast, and noticed a remarkable similarity. While horseshoe crabs may look slightly different from the outside, their internal brain architecture hasn’t really changed despite being separated for more than 300 million years.
Dr. “It’s as if a set of motherboards have remained constant over geological time, while the peripheral circuits have been altered in various ways,” Strausfeld said.
despite The E. danae fossil had been studied by other researchers in the past because of its shape and size, and the brain, which was smaller than a grain of rice, was overlooked. Dr. “If you’re not looking for this particular feature, you’re not going to see it,” Bicknell said. “You develop a search image in your head.”
With the lucky discovery of this well-preserved ancient brain, researchers hope to find more specimens in other fossils in the Mazon Creek deposit.
“If there is, there must be more,” said Javier Ortega-Hernández, an invertebrate paleontologist at Harvard University’s Museum of Comparative Zoology and co-author of the study.
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