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Brain tissue is naturally spongy. Unlike bones, shells or teeth, it is high in fat and rots quickly, rarely making an appearance in the fossil record.
So when Russell Bicknell, an invertebrate paleontologist at the University of New England in Australia, noticed a touch of white near the front of a fossilized horseshoe crab body where the animal’s brain was located, he was surprised. Closer examination revealed an exceptional imprint of the brain as well as other pieces of the creature’s nervous system.
Unearthed from the Mazon Creek deposit in northeastern Illinois, and dating back 310 million years, this is the first fossilized horseshoe crab brain ever discovered. Dr Bicknell and his colleagues reported the discovery last month in the journal Geology.
“These types of fossils are so rare that if you came across one you would usually be in shock,” he said. “We’re talking about a needle wow level in a haystack.”
The discovery helps fill a gap in the evolution of arthropod brains and also shows how little they have changed over hundreds of millions of years.
The preservation of soft tissues requires special conditions. Scientists have discovered brains locked in fossilized tree resin, better known as amber, that are less than 66 million years old. They also found brains preserved as flattened carbon films, sometimes replaced or covered by minerals in shale deposits over 500 million years old. These deposits include corpses of oceanic arthropods that sank to the bottom of the sea, were quickly buried in the mud, and remained safe from immediate decomposition in an oxygen-poor environment.
However, Euprops danae’s fossilized brain, which is held in a collection at the Yale Peabody Museum of Natural History, required a different set of conditions to be preserved.
This arthropod was not a crab, but is closely related to spiders and scorpions. The penny-sized horseshoe crab was buried over 300 million years ago in what was once a brackish, shallow sea basin. Siderite, an iron carbonate mineral, quickly accumulated around the body of the dead creature, forming a mold. Over time, as the soft tissue decomposed, a white clay mineral called kaolinite filled the void left by the brain. It was this white cast on a dark gray rock that helped Dr Bicknell spot the uniquely preserved cerebral impression.
“This is a completely different way of preserving the brain,” said Nicholas Strausfeld, a neuroanatomist at the University of Arizona who was among the first to report a fossilized arthropod brain in 2012, but no was not involved in this study. “This is remarkable.”
The extinct Eupoops the brain had a central cavity for the passage of a feeding tube and branching nerves that connected to the eyes and legs of the animal.
Dr Bicknell and his colleagues compared this ancient brain structure to that of Limulus polyphemus, a species of horseshoe crab still found along the Atlantic coast, and noticed a remarkable similarity. While horseshoe crabs look somewhat different on the outside, the internal architecture of the brain has not really changed despite being apart over 300 million years.
“It is as if a set of motherboards has remained constant over geological time, while the peripheral circuits have been variously modified,” said Dr Strausfeld.
Although the The fossil of E. danae has been examined in the past by other researchers for its shape and size, the brain, which is smaller than a grain of rice, has gone unnoticed. “If you don’t look for that particular feature, you won’t see it,” Dr Bicknell said. “You develop a research image in your head. “
With the happy discovery of this well-preserved ancient brain, researchers hope to find more examples in other fossils from the Mazon Creek deposit.
“If there is one, there must be more,” said Javier Ortega-Hernández, invertebrate paleontologist at Harvard University’s Museum of Comparative Zoology and co-author of the study.
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