Extinct 'Fearsome' Sea Scorpion had Bad Eyes for Seeing Prey

The largest arthropod that ever lived is a massive sea scorpion, which once swam through the seas and hunted its prey. Now, though, scientists have found out that these sea scorpions weren't the fearsome predators that they once thought they were; due to poor eyesight, these creatures probably were reduced to eating any weaker, soft-bodied animals they happened to stumble upon at night.

"We thought it was this large, swimming predator that dominated Paleozoic seas," said Ross Anderson, the lead author of the new study, in a news release. "But one thing it would need is to be able to find the prey, to see it."

The sea scorpions, called Pterygotids, could grow more than two meters in length and roamed shallow, shoreline basins for about 35 million years. Because of its massive size, the long-toothed grasping claws in front of their mouth and their compound eyes, scientists once believed that these sea scorpions were fearsome predators.

Yet that might not be the case. A past study called into question these creatures' ability to penetrate armored prey with their claws. And now, it turns out that their eyes weren't all that great at tracking prey, either. The researchers developed a mathematical analysis method to understand the properties of the sea scorpions' eyes. They also used imaging technology with backscattered electrons on a scanning electron microscope to reveal the eye lenses without damaging the fossils.

"Our analysis shows that they could not see as well as other eurypterids and may have lived in dark or cloudy water," said Derek Briggs, one of the researchers. "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."

The findings reveal that these creatures were likely not the top predators that researchers once believed. Instead, it's far more likely that sea scorpions were more like scavengers that hunted at night.

The findings are published in the journal Biology Letters.