The 390-million-year-old specimen was found in a German quarry, the journal Biology Letters reports.
The creature, which has been named Jaekelopterus rhenaniae, would have paddled in a river or swamp.
The size of the beast suggests that spiders, insects, crabs and similar creatures were much larger in the past than previously thought, the team says.
The claw itself measures 46cm - indicating its owner would have been longer even than the average-sized human.
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