Giant sea scorpions used serrated tail spines to slay prey, Edmonton study finds

Ancient sea scorpions, which once crawled the primordial seas, used serrated, slashing tail spines to overpower their prey, new research from the University of Alberta suggests.

These predators thrived in warm, shallow waters across the globe, long before the evolution of barracudas or sharks, 430 million years ago.

Related to the modern scorpion and horseshoe crabs, sea scorpions — or eurypterids — had thin, flexible bodies. Some species also had pincer claws and could grow up to three metres in length, making them the largest known arthropods to have ever lived.

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"If you saw one, it would be a little bit like a cross between a lobster and a dragonfly," Scott Persons, paleontologist and lead author on the study, said during an interview with CBC Radio's Edmonton AM.

"It has a long segmented body — some of them do have long pinching claws — and then it has got this head, which if it weren't for all the features would actually be pretty cute, because it's got these large eyes, probably important for detecting and zeroing in on its prey."

'Strike, strike and strike again'

A study titled "Sea scorpions strike: New evidence of extreme lateral flexibility in the opisthosoma of eurypterids" concludes the ancient predators had powerful weaponry.

After examining a fossil recently discovered near Lesmahagow, Scotland, researchers believe these creatures attacked and killed their prey with sidelong tail strikes.

"What we envision this critter doing is grabbing hold of things with its segmented legs and mouth parts," Persons said. "So it grapples with it and, if it's something large, it can whip that tail to the side.

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"It can slash it, strike, strike and strike again until whatever it wants to eat is dead and done struggling, and it can shove it into its mouth."

The newly discovered fossil of eurypterid Slimonia acuminate — a particular species no larger than your "typical lobster" — collected from the Patrick Burn Formation, was the specimen that provided the new clues about how these ancient predators hunted for food, Persons said.

"We've known that they've had these serrated tail spines for a long time. What's new about this specimen is that it happens to be preserved with such a strong curve to it," Persons said.

'This thing can really whip around'

"Often when an animal dies, it goes stiff, but in this case, we have it in a nice curve showing the full amount of flexibility that the exoskeleton was capable of, and this thing can really whip around."

Unlike lobsters and shrimps, which can flip their broad tails up and down to help them swim, eurypterid tails were vertically inflexible but horizontally highly mobile.

This meant that these sea scorpions could slash their tails from side to side, without propelling themselves away from an intended target, Persons said.

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"Most predators are limited by what they can eat, based on the size of their mouth, but this is an animal that can hack you into smaller bits," Persons said.

"If you're a little, early vertebrate fish — one of our ancestors — and you're swimming around the lagoon, this might be the critter you're most afraid of."

The research was published in the American Naturalist in March 2017.