An extinct giant sea scorpion once thought to be a big, bad predator in ancient seas actually had poor eyesight, possibly making it not so effective at capturing prey, a new study shows.

Thanks to a post-extinction eye exam conducted by Yale University scientists, it turns out the giant pterygotid eurypterid - the largest arthropod that ever lived - was reduced to trolling weaker, soft-bodied animals it stumbled upon at night rather than viciously attacking prey during broad daylight.

"We thought it was this large, swimming predator that dominated Paleozoic seas," lead author Ross Anderson, a Yale graduate student, said in a statement. "But one thing it would need is to be able to find the prey, to see it."

Pterygotids, which could grow more than two meters (6.5 feet) long, roamed shallow, shoreline basins for 35 million years. Due to their massive size, coupled with long-toothed grasping claws in front of their mouth and forward-facing, compound eyes, scientists long believed they were among the ocean's fiercest predators.

Using imaging technology and innovative analysis methods, researchers were able to examine the extinct scorpion's eye lenses without damaging the fossil. They also compared the results with the eyes of other extinct species during the same period, as well as modern-day species such as the horseshoe crab.

"Our analysis shows that they could not see as well as other eurypterids and may have lived in dark or cloudy water. If their claws could not penetrate the armor of contemporary fish, the shells of cephalopods, or possibly even the cuticle of other eurypterids, they may have preyed on soft-bodied, slower-moving prey," explained co-author Derek Briggs.

The results could not discern, however, whether these creatures suffered from nearsightedness or farsightedness. But it did reveal that the bigger the scorpion, the worse the eyesight.

"Maybe this thing was not a big predator, after all," Anderson speculated. "It's possible it was more of a scavenger that hunted at night. It forces us to think about these ecosystems in a very different way."

The findings were published in the journal Biology Letters.