The horseshoe crab is an interesting critter, remaining relatively unchanged for hundreds of millions of years. One species lives in the Atlantic Ocean along the North American coast, while the other three species live in the waters off Southeast and East Asia. It looks sort of like what you imagine might happen if a facehugger from the Alien movies mated with a large beetle.

You’ve probably never wondered what a horseshoe crab orgy looks like, but if you read past this line, you’re going to find out.

Of course I would find the horseshoe crab fascinating, because I’m into that sort of thing. Also, of course, I would want to learn more about it for the benefit of my reader(s). And whoa, does the horseshoe crab get scary when you dig into its lineage.

Horseshoe crabs are arthropods, a phylum they share with insects, crustaceans, arachnids, millipedes, and so on. Their subphylum, Chelicerata, includes the arachnids, i.e. spiders, scorpions, ticks, etc.

That’s already enough guilt by association for me, but the class to which the horseshoe crab belongs, Merostomata, includes the extinct order Eurypterida, more commonly referred to as sea scorpions.

Picture a scorpion. Now go change your undergarments. When you’re ready, continue picturing a scorpion, but picture one that, if it could stand upright, is several feet taller than you.

Eurypterids (sea scorpions) are an extinct group of arthropods related to arachnids which include the largest known arthropods that ever lived…The largest, such as Jaekelopterus, reached 2.5 metres (8 ft 2 in) in length, but most species were less than 20 centimetres (8 in). They were formidable predators that thrived in warm shallow water, in both seas and lakes, in the Ordovician to Permian from 460 to 248 million years ago.

An eight-inch-long “sea scorpion” sounds terrifying enough, but eight feet just goes to show that we evolved at a very good time in earth’s history. Did I mention that they lived in freshwater lakes???

Only fossils remain now, of course, of monsters like Jaekelopterus and his buddy Pterygotus, but we still have the power of the human imagination to picture this dude coming up to say hi the next time you’re out at the lake, trying to relax:

If a still picture isn’t enough to inspire mortal terror, humans have developed CGI for that very purpose, plus whatever this is:

Photo credits: Asturnut (Own work) [CC BY-SA 3.0, GFDL], via Wikipedia; Ryan Somma (Flickr) [CC-BY-SA-2.0], via Wikimedia Commons.