Visiting Living Fossils pt.1

Limulus polyphemus, the Atlantic Horseshoe Crab

The sun has beating down here the last couple weeks. The beaches are a long drive away and packed with too many humans for my comfort. But, I needed some breezes. So, I headed to a nice, muddy beach along the stinky bay. There's not much here for tourists, but for a naturalist it's got everything I need: sunshine, a nice breeze, enough sand that I won't sink up to my knees in the mud, critters, and fossils - lithified and not.

This stretch of the East Coast is home to one of four species of Horseshoe Crabs left on Earth. For now, they are still plentiful at this beach and an important part of the food chain both in and out of the water. Their numbers are notably slimmer these days as they are used by the locals as fish bait and in other areas are harvested for biomedical research, including the search for a Covid-19 vaccine*.

These arthropods are as alien a creature as you'll find. They have ten optical receptors on their shell and tail, including a pair of compound eyes on the carapace ridges and some that only detect UV light. Their blood is bright blue and copper-based instead of iron-based. When they shed to grow, they leave the lining of the esophagus, crop‐gizzard and rectum with the old shell.  

Most amazing to me is that these weird animals have been around for 450 million years. They are closely related to Eurypterids and more loosely to trilobites, though the three vary in the number of body sections. When the incredibly successful trilobites were ancient history horseshoe crabs crawled ever on, largely unchanged.

On the beach, beneath the crabs' many feet are water-worn pebbles containing Silurain and Devonian fossils. There are rugose corals, bryozoa, crinoids and brachiopods. One of these days  might even find a trilobite. The horseshoe crab predates them all.

A death assemblage including brachiopods, bryozoa and coral, probably either Silurain or Devonian in age, washed up on the horseshoe crabs' spawning grounds.

*https://www.nationalgeographic.com/animals/invertebrates/a/atlantic-horseshoe-crab/