A 310 million-year-old horseshoe crab fossil has been discovered, and it includes a fully preserved brain. Researchers believe they’ve found a new specimen that has arthropod brains preserved in a way that they have never seen before.
“In our fossil, the brain of Euproops is replicated by a white-coloured clay mineral called kaolinite,” Paterson says. “This mineral cast would have formed later within the void left by the brain, long after it had decayed. Without this conspicuous white mineral, we may have never spotted the brain,” explained palaeontologist Paterson.
The Mazon Creek deposit is the one place in the world where the fossil of the horseshoe crab, Euproops danae, has been found. Fossils in this deposit are preserved within siderite concretions. This is the first time a siderite deposit proves that it can perfectly preserve the brain. The discovery will help researchers understand the changes the horseshoe crab went through in the past 310 million years.
“We have shown, for the first time, that the Mazon Creek animals were not only moulded by the rapid formation of siderite that entombed their entire bodies, but also that the siderite quickly encased their internal soft tissues before they could decompose,” added Paterson.
The soft parts of animals often decay very rapidly, but the most recent study shows that hard parts can preserve soft tissue with remarkable fidelity. There are two main types of fossil deposits that yield information about prehistoric arthropod brains. Most of the knowledge comes from amber and Burgess Shale-type deposits.
Amber is a natural substance that got formed when resin oozed through tree bark often trapping insects, plants and other organisms inside it. Quick burial in tree resin helped to preserve the intricate anatomical structure and behaviors of these tiny organisms.