Our study, published today in Geology, shows how even the intricate brains of ancient aquatic arthropods (invertebrates with jointed legs) can be preserved in remarkable detail.
The discovery of a 310 million-year-old horseshoe crab in the US, complete with its brain intact, adds to a recent string of fossil finds which have unearthed some of the oldest arthropods with a preserved central nervous system.
The horseshoe crab fossil we document in our study sheds new light on how these fragile organs — typically prone to very rapid decay — can be preserved with such fidelity.
(...) The specimen of the horseshoe crab, Euproops danae, comes from the world-famous Mazon Creek deposit of Illinois, in the US. Fossils from this deposit are preserved within concretions made of an iron carbonate mineral called siderite.
(...) Notably, the brain of Euproops is replicated by a white-coloured clay mineral called kaolinite. 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.
One of the challenges of interpreting ancient arthropod anatomy is the lack of close modern relatives available for comparison. But luckily for us, Euproops can be compared to the four species of living horseshoe crabs.
Even to the untrained eye, a comparison of the fossil’s nervous system with that of a modern horseshoe crab (below) leaves little question that the same structures are found in both species, despite them being separated by 310 million years.
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