Giant scorpions once ruled seas — and may have traversed entire oceans 

As It Happens6:21Giant scorpions once ruled seas — and may have traversed entire oceans

Asked to rank giant sea scorpions on a scale of one to terrifying, Russell Bicknell puts them at about an eight.

"I suppose it depends how you define terrifying," Bicknell, a paleobiologist at the American Museum of Natural History, told As It Happens guest host Susan Bonner.

"For me, I mean, I think they're pretty cool."

Sea scorpions lived in the oceans more than 400 million years ago. Some species were larger than humans, and their powerful claws and thick exoskeletons would have made these apex predators a force to be reckoned with. 

As if that wasn't impressive enough, new research suggests they also were globe-trotting endurance swimmers.

The study, by Bicknell and his colleagues, was published Saturday in the journal Gondwana Research. The findings expand what we know about the ancient creatures' range, and may provide some clues as to what made these formerly fearsome sea bugs go extinct.

A mighty migration

Eurypterids, commonly known as sea scorpions, were a group of more than 200 species of arthropods that dwelled in the oceans until they abruptly went extinct about 393 million years ago.

Their fossils have primarily been found in what is now North America and Europe, and some were found China as recently as last year. 

While some eurypterid fossils have been found in Australia before now, Bicknell says, they have been too fragmented to classify with certainty. But he and his team have studied two new sets of fossils, both found in Australia. 

While still fragmented, the specimens were intact enough for the scientists to identify them as the Pterygotus and Jaekelopterus — or what Bicknell describes as "the really, really big ones."

Pterygotus could reach 1.7 metres in length, while Jaekelopterus could grow as long as 2.5 metres.

"Like, substantially larger than a person," Bicknell said. 

The findings expand the known range of giant sea scorpions, and mark the first evidence they existed in Gondwana, an ancient supercontinent.

Because giant sea scorpions' fossil record is largely based in the U.S. and Europe, Bicknell and his colleagues surmise they made "quite a substantial trek" of thousands of kilometres to end up in what is now Australia, where they were found fossilized with large fish and trilobites — most likely their dinner. 

"That tells us that these animals were actually really built quite effectively for travelling really, really far. So they were probably traversing a similar amount of distance as some whales do," Bicknell said.

"And that's really, really exciting because it's really, really hard for us to know how these animals were able to migrate, or if they did migrate, without these kinds of fossils."

It's not clear whether this migration was a one-time thing, or if they travelled back and forth seasonally. But their remnants were found in similar ocean environments as other large eurypterid fossils. 

"The animals had basically managed to migrate to the oceans around Australia and then was sort of moving in and out of these environments that we think are kind of like very, very big deltas — kind of freshwater, kind of marine environments," Bicknell said. 

Why did they die out?

Scientists don't know what killed off the once-dominant eurypterids, and Bicknell's research doesn't solve that mystery. But it may offer a starting point for further research. 

"It allows us to understand the scope of this idea called gigantism — or things getting really, really big — and allows us to understand what gigantism as an evolutionary process can produce," he said. 

Sea scorpions, he said, appear to have evolved to get bigger and bigger until they hit about 2.5 metres, "and then they just kind of drop off."

Why, he says, is unclear. 

"It may be environmental. It may be ecological, so they're sort of competing with different animals. Or they may have just pushed themselves too far, and that resulted in their extinction," he said. "That's sort of an open question."

James Lamsdell, a paleobiologist at West Virginia University, who was not involved in the study, told the New York Times that the fossils Bicknell and his team studied are in better shape than those previously found in Australia.

But he said finding more specimens in Australia would strengthen their case that giant sea scorpions roamed the coasts of ancient Gondwana.

The study, he says, could help scientists figure out where to find more eurypterid fossils in Australia, which could, in turn, help solve the mystery of why these seemingly evolutionarily successful creatures disappeared so suddenly. 

"Over time, there'll be more discoveries," he said. "Then we'll get an even better idea of what's going on with these animals."