LiveScience, via FoxNews.com (December 30, 2010)
"Ancient sea scorpions included the largest and arguably most frightening bug-like creatures known to have lived on Earth, but despite their fearsome claws, these giants might actually have been creampuffs, scientists think.
(from William L. Parsons, Buffalo Museum of Science, via FoxNews.com, used w/o permission)
"Sea scorpions, known as pterygotid eurypterids, were arthropods, a group that includes insects and crabs. Though not actually scorpions, many of the animals had tails ending in spikes, hence the name. [image of giant sea scorpion]
"These monstrous pterygotids were believed to be terrors of the seas 470 million to 370 million years ago, long before the dinosaurs appeared, reaching more than 8 feet (2.5 meters) long with large claws laden with sharp spines. [Dangers in the Deep: 10 Scariest Sea Creatures]..."
Big claws, sharp spines, eight feet long: looks impressive. Until someone takes starts looking closely at the mechanics of that sea scorpion's claws. Back to the article:
"...The scientists analyzed claws from a group of one of the largest sea scorpions, Acutiramus, which lived about 416 million to 419 million years ago in what is now New York and Ontario, Canada. They calculated that the pincers could only safely apply no more than 5 newtons of force without damaging themselves. This would make them incapable of penetrating even a medium-size horseshoe crab's armor, which needs 8 to 17 newtons to crack open. (For comparison, research has suggested T. rex's lower jaw could apply 200,000 newtons of force, or enough strength to lift a tractor-trailer.)
"As such, the pincers could not have attacked anything with a hard shell, or any creature that could have put up a significant struggle...."
On top of that, the claws lack what the article calls an "elbow joint," which means they'd have been okay for grasping prey on the seafloor: for catching something swimming above the seafloor? not so much.
At this point, it seems those eight-foot scorpion-things looked fearsome: but were weak and a tad clumsy. A next step would be to check out similar critters of the period, to see how strong and agile they were.
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