The Brain of the Horseshoe Crab

It’s our favourite non-crab crab, back again to surprise us with the wonders of evolutionary history!

The remains of these ancient ‘living-fossils’ are not uncommon in the fossil record because of the survivability of their hard exoskeletons, however the preservation of their soft and squishy bits is much rarer. Which is why the discovery of how little their brains have changed is so amazing and unlikely.

310 million years ago, after the now long-deceased Horseshoe Crab scuttled for the last time, it was encased in a mineral called siderite, which preserved the external shape of the creature. Later however, when the internal organs rotted away into the ether, another mineral called kaolinite filled in the gaps. This mineral, as you can see in the photo appears lighter all these millions of years later.

This visible colour showed researchers almost immediately that the brains of these animals haven’t changed in all this time. This fossil is of a now extinct lineage of Horseshoe Crabs, but its similarity to the animals we find today shows that once they found a brain that worked, they stuck with it.

The little branches that stick out from the sides are the remnants of nerve endings that lead to the eyes and legs and are oriented in an almost identical way to modern Horseshoe Crabs. The hole in the middle, if you were wondering, is where the feeding tube would have passed through.

The fossil record is a patchy one, and has some big ol’ gaps in it, but this recent discovery goes some way to filling in the picture of the history of these amazing and amazingly ancient animals!

P.C: Bicknell et al 2021