I think we have been here before but William sent in this link to https://www.yahoo.com/lifestyle/310-million-old-horseshoe-crab-174134689… … a recent discovery of a 310 million year old horseshoe crab with its brain intact has got some paleontologists in a whirl. It seems the central nervous system of this very old creature is closely comparable to that of living horseshoe crabs, even the arrangement of nerves to the eyes and appendages. It also has the same crustal opening for the oesophagus to pass through – which is quite remarkable, they say. A lot of ecological diversification, presumably upheavals in the natural world, have occurred over 310 million years. Yet, horseshoe crabs have managed to survive through thick and thin. In other words, the horseshoe crab's brain box has not changed in all that time.
The fossil crab belongs to the era of the Burgess shale – but there is a difference. Its brain survived as a result of the white colour clay mineral, kaolinite. It formed a mineral cast within the void left by the brain. There was no surviving soft tissue. It is a copy of the brain – in white clay. See also www.livescience.com/310-million-year-old-fossil-brain.html … for soft tissue to survive, or be preserved, special geological conditions are necessary, we are told. A copy of the brain, as a mold, is actually what occurred. It was not the brain itself. It was found in a deposit of the mineral, siderite, an iron carbonate. The brain of the horseshoe crab had decomposed and eventually, it vanished altogether – but it was replaced by kaolinite, a clay mineral. Clays, we may note, have a watery origin.