Of all the weird and wonderful early marine animals out there, there’s one oddball that can be a major challenge to study: Eurypterids.

That’s because they’re rare in the fossil record overall. But when we find them, we find BUCKETS full.

And the reason is one of the most classic tales in all of evolution: mating. [♪ INTRO] Let’s start by explaining what a eurypterid even is. They were ancient sea-dwelling invertebrates that first appeared in the late Ordovician period, roughly 467 million years ago, and went extinct by the end of the Permian, around 252 million years ago. They had two big paddle arms, a pair of pincers, and a whole bunch of legs on the front end of their bodies.

And most also had a big needle-shaped stinger at the end of their tails. They look wild. Nothing at all like their closest living relatives, the humble horseshoe crab.

And they came in all sizes, too. The smallest aren’t much bigger than a few centimeters, but the largest is estimated to have been more than two and a half meters long, making it not only the biggest eurypterid, but the largest arthropod to have ever lived. Granted, that’s an estimate, since the only piece they found was a claw, but it's still terrifying.

While they’re sometimes called sea scorpions, there’s evidence that by the end of this group’s existence, they were branching out. They ventured into brackish and freshwater habitats, and a few of them were able to breathe air for short periods of time, kind of like today’s mudskippers. There’s a lot we just don’t know about these creatures, especially the older groups, largely because of that whole “over 400 million years old” thing.

But thanks to the way that certain fossil sites form, we do have a sense of how their lifestyles may have been different from basically any other animal that ever lived. The sites we’re talking about are called Lagerstätte, which is German for “storage place.” But I kind of think that name kind of undersells their significance. They’re fossil sites that are preserved so well that we can sometimes find soft tissue, like skin or even organs.

Most lagerstätte formed in anoxic environments, so basically when a big pile of organisms got covered up with no oxygen for long enough to fossilize. The lack of oxygen prevented decay, which saved the squishy bits and pieces we usually never get to see. The Burgess Shale is one of the most famous examples of a lagerstätte - it’s the fossil site in Canada where many of the icons of the Cambrian Explosion were found.

And the very first eurypterid fossils found came from a lagerstätte called the Bertie Formation, which stretches from New York state up into Ontario, Canada. The Bertie Formation dates to the Silurian Period, so between 423 and 419 million years old. This lagerstatte was discovered in 1818, and the eurypterids became so iconic that they’re now the state fossil of New York.

The ones from the genus Eurypterus are the most well-represented in the fossil record. And at least part of that has to do with this Bertie Formation, where they’re absolutely everywhere. The abundance led researchers to question what it was that caused so many of these guys to gather in one place.

And it turns out, they may have uncovered an epic tale of invertebrate romance. Scientists think the eurypterids would gather en masse at a single spawning location on a regular basis. Once they got there, they immediately shed their exoskeleton, which helped set the mood for step three, which was mating.

This idea is called the mass-molt-mate hypothesis, which also means that these fossil sites probably formed when some cataclysmic event like a mudslide happened, and the party got squashed. Part of the evidence cited for the hypothesis is just how many exoskeletons researchers find in these fossil beds. Paleontologists report that exoskeletons, also called exuviae, make up a substantial portion of the total number of eurypterid fossils.

And that makes sense, since one living eurypterid probably made a whole bunch of cast-off exoskeletons over its lifetime. It can also be hard to tell the difference between the two, since in both cases you’re looking at a flattened object that’s hundreds of millions of years old. So the exceptional preservation of a lagerstätte also comes in handy here in order to tell the difference.

And when they can tell the difference, the exuviae versus body-fossils thing gives researchers critical information about the different behaviors of males and females. Because they find more female exuviae at these sites than they do of the males, paleontologists think that the females stayed behind at the spawning site long after the males had left, probably waiting for the ideal time to lay their eggs. The longer they stayed, the more likely it was that they needed to molt while they were there.

The baby eurypterids probably hung around on those beaches for a while after hatching, to avoid predation while they were still itty-bitty. So the existence of this lagerstätte, and the piles of eurypterids we find there, gives us a really rare glimpse into not just what these animals looked like, but how they lived. And this month’s SciShow Rocks Box subscribers will get to see that for themselves!

The rock of the month will be a eurypterid fossil from an area of the Bertie Formation called Fiddler’s Green. All of these fossils are fragments, which is part of why they’re not in museum collections. And they were all mined ethically and collected with all necessary permissions.

If you’re interested in signing up for the SciShow Rocks Box, well, I gotta tell you, it’s been pretty popular. The first round sold out in just seven hours, and the second round sold out within a day. So if you want to sign up, head to SciShow.

Rocks, and put some pep in your step. Because, unlike the eurypterids themselves, the few available subscriptions will not stick around for long. Personally, I think some cool rocks could make a pretty snazzy holiday gift.

Thanks for watching and for loving rocks as much as we do! [♪ OUTRO]