Gigantic sea scorpions that lurked in the ocean more than 400 million years ago weren't as scary as they sound, a new study suggests.

The massive creatures, known as pterygotids, were the largest arthropods that ever lived, growing to be up to 6.5 feet (2 meters) long, with claws measuring up to about 2 feet (0.6 m). But contrary to what scientists thought, these animals may not have been true top predators.

"These things were almost certainly still predators of some kind, but the imagined notion that they were swimming around terrorizing anything that looked edible is probably an exaggeration," said Derek Briggs, a paleontologist at Yale University in New Haven, Connecticut, and co-author of the new study, published today (July 8) in the journal Biology Letters. [Dangers in the Deep: 10 Scariest Sea Creatures]

Pterygotids were a type of eurypterid, an extinct type of sea scorpion related to arachnids. These ocean-dwelling creatures lived between about 436 million to 402 million years ago, in the Silurian and Devonian periods, Briggs said. Their closest living relatives are horseshoe crabs or modern sea scorpions, he said.

Previously, these spooky sea monsters were thought to be fearsome predators, devouring armored fishes and giant cephalopods (related to modern squids and nautiluses). Their compound eyes and large claws seemed to suggest as much.

But more recently, a study revealed that pterygotid claws wouldn't have been strong enough to break into armored fish or cephalopod shells.

In the recent study, Briggs and his team set out to examine the eyes of these ancient sea scorpions, to determine whether they had good enough vision to be great hunters.

A pterygotid fossil (Image credit: copyright Division of Invertebrate Paleontology, Yale Peabody Museum)

Some of the lenses in the creatures' eyes were big enough for researchers to see them without any help from technology, but others had to be viewed under an electron microscope. The team estimated the angle between the lenses and the size of the lenses, comparing them with the eyes of a smaller eurypterid relative and of modern arthropods.

Briggs and his team concluded that the giant arthropods actually had poor eyesight. They probably lived near the bottom of the sea and likely hunted soft-bodied animals in dark waters or at night, Briggs said. But the fossil evidence limits these interpretations, so it's hard to know for sure how the animals behaved, he added.

After about 35 million years, pterygotids died out, and "it's a good thing they did," Brigg said. "They wouldn't be good company."

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