Brain tissue is naturally mushy. Unlike bones, shells, or teeth, it is high in fat and rots quickly and rarely appears in fossils.
When Russell Bicknell, an invertebrate paleontologist at the University of New England in Australia, noticed a white patch near the front of a fossilized horseshoe crab where the animal’s brain would have been, he was surprised. A closer look revealed an extraordinary imprint of the brain along with other parts of the creature’s nervous system.
It was excavated in the Mazon Creek deposit in northeast Illinois and is 310 million years old. It is the first fossilized horseshoe crab brain ever found. Dr. Bicknell and his colleagues reported on the find in Geology magazine last month.
“These types of fossils are so rare that if you happen to come across one one is generally shocked,” he said. “We’re talking about a wow level that’s in a haystack.”
The find 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 the soft tissue requires special conditions. Scientists have found brains that were encased in fossilized tree sap, better known as amber, that were less than 66 million years old. They also found brains that have been preserved as flattened layers of carbon, sometimes replaced or overlaid with minerals in shale deposits that are more than 500 million years old. These debris include corpses of ocean arthropods that sank to the ocean floor, were quickly buried in the mud, and were protected from immediate decay in the deoxygenated environment.
However, Euproops danae’s fossilized brain, which is kept in a collection at the Yale Peabody Museum of Natural History, required other conditions to be preserved.
This arthropod was not a crab, but is closely related to spiders and scorpions. The extinct penny fox ponytail crab was buried in what was once a shallow, brackish sea basin more than 300 million years ago. Siderite, an iron carbonate mineral, quickly collected around the dead creature’s body and formed a mold. Over time, as the soft tissue deteriorated, a white clay mineral called kaolinite filled the void left by the brain. It was this white cast on a dark gray rock that Dr. Bicknell helped identify the uniquely preserved brain print.
“This is a completely different kind of brain preservation,” said Nicholas Strausfeld, a neuroanatomist at the University of Arizona who was one of the first to report on a fossilized arthropod brain in 2012 but was not involved in this study. “It’s remarkable.”
The extinct Euproops brain revealed a central cavity for the passage of a feeding tube and branched nerves that connected to the animal’s eyes and legs.
Dr. Bicknell and colleagues compared this ancient brain structure to that of Limulus polyphemus, a horseshoe crab still found along the Atlantic coast, and found a remarkable similarity. While the horseshoe crabs look slightly different on the outside, the internal brain architecture hadn’t really changed despite being separated by more than 300 million years.
“It’s like a set of motherboards has remained constant over geological time while peripheral circuitry has been modified differently,” said Dr. Strausfeld.
Although the shape and dimensions of the E. danae fossil have been examined by other researchers in the past, the brain, which is smaller than a grain of rice, went unnoticed. “If you don’t look for that particular feature, you won’t see it,” said Dr. Bicknell. “You develop a search image in your head.”
With the fortunate discovery of this well-preserved ancient brain, the researchers hope to find more examples in other fossils from the Mazon Creek deposit.
“If there is one, there must be more,” says Javier Ortega-Hernández, paleontologist for invertebrates at Harvard University’s Museum of Comparative Zoology and co-author of the study.