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Most modern scorpions would fit in the palm of your hand. But in the oceans of the Paleozoic era more than 400 million years ago, animals known popularly as sea scorpions were apex predators that could grow larger than people in size.
"They were effectively functioning as sharks," said Russell Bicknell, a paleobiologist at the American Museum of Natural History.
New research by Bicknell and colleagues, published Saturday in the journal Gondwana Research and relying on Australian fossils, reveals that the biggest sea scorpions were capable of crossing oceans, a finding that is "absolutely pushing the limits of what we know arthropods could do," he said.
What are commonly known as sea scorpions were a diverse group of arthropods called eurypterids. They came in many shapes and sizes but are perhaps best known for their largest representatives, which could grow to more than 9 feet long. With huge claws, a beefy exoskeleton and a strong set of legs for swimming, the larger sea scorpions most likely ruled the seas.
However fearsome these arthropods must have been to Paleozoic prey, they went extinct without much of a bang. The fossil record of eurypterids peaked in the Silurian period, which started about 444 million years ago, and they then abruptly died out after the early Devonian period ended about 393 million years ago.
That sudden turn of fate has left scientists bewildered.
"They appear, they start doing really well, they get very big, and then they go extinct," said James Lamsdell, a paleobiologist at West Virginia University who was not involved in the study. "For a while they were so dominant, and then they just burned out."
Giant arthropods haven't been seen since, and better understanding how widespread gigantic sea scorpions were could lay the groundwork to explain why that is.
Most eurypterid fossils have come from North America and Europe, with a few unearthed more recently in China. While some eurypterid fossils had been previously reported in Australia, they were fragments too broken up and ambiguous to identify whether they were the biggest sea scorpion species.
Bicknell's study documents a new set of eurypterid fossils from Australia. They're fragments, too, but he suspected they could be identified.
Careful examination revealed that the fragments were mostly exoskeleton, including a potential head piece with suggestions of an eye. Bicknell was confident enough in the pieces that he assigned them to two kinds of eurypterid: Pterygotus and the biggest known sea scorpion, Jaekelopterus.
Bicknell's analysis of the rock formations in which the fossils were found showed that they were essentially identical to those where the fossils had been found on the other side of the world: shallow seas and deltas around the shores of continents. One set of fossils was found with armored fish, and another with a bit of fossil droppings containing trilobite fragments, suggesting the sea scorpions could have been chowing down on crunchy prey.
The fragments are better than what had previously been found, although uncovering more specimens in Australia would help solidify the case, Lamsdell said.
If Bicknell is correct, the fossils expand the biggest sea scorpions' range to the ancient supercontinent Gondwana, where they had not been found before. The finding also supports sea scorpions' ability to cross oceans -- which was no small feat. At the time, a journey between Gondwana and the supercontinent Euramerica could have been thousands of miles, depending on the route.
"It's really cool to see that, yeah, they were traveling really far," Bicknell said. "They actually made it to Gondwana."
Bicknell hopes further study will reveal whether giant sea scorpions arrived in a one-time migration event or if they moved seasonally.
That huge sea scorpions made this journey hints that gigantism could have played an important role in migration. It could have simply been that animals needed a big body to survive in the oceans, like oceangoing sharks and whales today, Lamsdell said. Once they got big enough, they could make the trek.
But why sea scorpions' large size would enable such global success and still allow them to go extinct a geologically short while later remains a mystery. Having a better handle on their range will point scientists to new places to hunt for fossils, which in turn could help clear things up.
"Over time, there'll be more discoveries," Lamsdell said. "Then we'll get an even better idea of what's going on with these animals."