Brain tissue is naturally soft. Unlike bones, shells, or teeth, it is rich in fat and rots quickly, and is rarely found in the fossil record.
So when Russell Bicknell, an invertebrate paleontologist at the University of New England in Australia, noticed a white spot on the front of a fossilized horseshoe crab body where the animal’s brain would have been, he was surprised. A closer look revealed an exceptional print of the brain along with other parts of the creature’s nervous system.
Excavated from the Mazon Creek deposit in northeastern Illinois, and dating back 310 million years, it is the first fossilized horseshoe crab brain ever found. dr. Bicknell and his colleagues reported on the find last month in the journal Geology.
“These kinds of fossils are so rare that if you happen to come across one, you would generally be in shock,” he said. “We’re talking about a needle in a haystack of wow.”
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 soft tissue requires special conditions. Scientists have found brains encased in fossilized tree resin, better known as amber, that were less than 66 million years old. They also found brains preserved as flattened carbon films, sometimes replaced or covered by minerals in shale deposits dating back more than 500 million years. Such deposits included corpses of ocean-dwelling arthropods that sank to the seafloor, were quickly buried in mud, and remained protected from immediate decay in the oxygen-depleted environment.
However, the fossilized brain of Euproops danae, which is kept 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 extinct penny-size 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 gathered around the dead creature’s body, forming a fungus. Over time, as the soft tissue decayed, a white-colored 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 recognize the uniquely preserved brain print.
“This is a very different way 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 is remarkable.”
The Extinct Euprops brain showed a central cavity for the passage of a feeding tube and branching nerves that would connect to the animal’s eyes and legs.
dr. Bicknell and his colleagues compared this ancient brain structure with that of Limulus polyphemus, a horseshoe crab species still found along the Atlantic coast, and noted remarkable similarities. Although the horseshoe crabs look slightly different on the outside, the internal brain architecture hadn’t really changed, despite being separated from each other for more than 300 million years.
“It’s as if a set of motherboards has remained constant over geologic time, while peripheral circuits have been modified in different ways,” said Dr. Strausfeld.
Although the E. danae fossil has been examined by other researchers for its shape and size in the past, the brain, which is smaller than a grain of rice, went undetected. “If you’re not looking for that particular feature, you’re not going to see it,” said Dr. Bicknell. “You develop a search image in your head.”
With the lucky discovery of this well-preserved ancient brain, the researchers hope to find more examples in other fossils from the Mazon Creek deposit.
“If there’s one, there must be more,” said Javier Ortega-Hernández, an invertebrate paleontologist at Harvard University’s Museum of Comparative Zoology and a co-author of the study.