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If you think scorpions are scary today, be very glad that you weren’t around 467 million years ago. Today, these stealthy stingers are found on land, but back then, scorpion ancestors ruled the seas.

In a study published today in BMC Evolutionary Biology, scientists from Yale announced the discovery of a new sea scorpion, Pentecopterus decorahensis.

Sea scorpions, or eurypterids are well-represented in the fossil record. They were anywhere between a few inches and 6 feet long, and lived all over the world. Euripterids were probably gifted predators, and at 5 feet, 7 inches, Pentecopterus was no exception.

“This is the first real big predator,” lead author James Lamsdell told the Associated Press. “I wouldn’t have wanted to be swimming with it. There’s something about bugs. When they’re a certain size, they shouldn’t be allowed to get bigger.”

“There’s something about bugs. When they’re a certain size, they shouldn’t be allowed to get bigger.”

Unlike their modern relatives, the ‘stinger’ on Pentecopterus wasn’t used for attacks. It was a navigational aid, like a fish’s tail. The real action on the Pentecopterus happened at the arms, which were used to grab prey.

The fossils were discovered in Iowa, in the Upper Iowa River, which flows through a meteorite crater only a few million years older than the fossils. The Decorah Crater was only discovered a few years ago, and formed around 470 million years ago, when a meteorite slammed into the earth, creating a basin in the (then) shallow seas of North America. Over the next 3 million years, fine sediments settled in the crater, as did the bodies of marine life, like the sea scorpions. The sediments preserved the fossils, leaving them for scientists to discover today…but not without some work. The researchers had to dam part of the Iowa river to recover the specimens.

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