This brain remains intact in a 310-million-year-old fossil

Brain tissue is inherently soft.

Unlike bones, valves or teeth, it is high in fat and rots quickly, so it rarely appears in the fossil record.

So when Russell Bicknell, a paleontologist specializing in invertebrates at the University of New England (Australia), observed that in the anterior part of the body of a fossilized horseshoe crab, there was a white spot where the animal’s brain was, he was surprised.

Further examination revealed a rare brain imprint with other parts of the nervous system of the creature.

Unearthed from the Mazon Creek field in northeastern Illinois and dating from 310 million years ago, is the first fossilized horseshoe crab brain ever found.

Dr Bicknell and his colleagues reported the discovery last month in the journal Geology.

“These types of fossils are so rare that if you come across one you’re usually shocked,” he said.

“We are talking about a level of wonder like that of a needle in a haystack“.

Discovery helps fill a gap in arthropod brain evolution and also shows how little they have changed for hundreds of millions of years.

Other conclusions

The preservation of soft tissues requires special conditions.

Scientists have discovered brains locked in fossilized tree resin, better known as amber, 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 carcasses of oceanic arthropods that sank to the sea floor, quickly buried themselves in the mud, and remained protected from immediate decomposition in an oxygen-poor environment.

However, the fossilized brain of Euprops danae, which is kept in a collection of the Yale Peabody Museum of Natural History, required the preservation of a number of different conditions.

This arthropod was not a crab, but is closely related to spiders and scorpions.

Preservation

The extinct horseshoe crab, the size of a penny, was buried over 300 million years ago in what was a shallow, brackish sea basin.

The sideritis, an iron carbonate mineral, quickly accumulated around the body of the dead creature, forming a casting.

Over time, as the soft tissue breaks down, 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 detect the exceptionally 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 one of the first to report a fossilized arthropod brain in 2012, but was not involved in this study.

“It is notable”.

The brain of the missing Euproops it showed a central cavity for the passage of a feeding tube and branching nerves that connected with 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 on the Atlantic coast, and they observed a remarkable similarity.

Although horseshoe crabs appear somewhat different on the outside, the internal architecture the brain hadn’t really changed despite being separated by more than 300 million years ago.

“It is as if a set of base plates has remained constant throughout geological time, while the peripheral circuits they have been modified in various ways, ”said Dr Strausfeld.

Although 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 at the Mazon Creek site.

“Yes 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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