Extinct giant sea scorpion gets an eye exam, with surprising results

One minute, you're an imperious predator, scouring the shallow waters for any prey in sight. The next, thanks to a post-extinction eye exam by Yale University scientists, you're reduced to trolling for weaker, soft-bodied animals you stumble upon at night.

Such is the lot of the giant pterygotid eurypterid, the largest arthropod that ever lived. A new paper by Yale paleontologists, published in the journal Biology Letters, dramatically re-interprets the creature's habits, capabilities, and ecological role. The paper is titled "What big eyes you have: The ecological role of giant pterygotid eurypterids."

"We thought it was this large, swimming predator that dominated Paleozoic seas," said Ross Anderson, a Yale graduate student and lead author of the paper. "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 long, roamed shallow, shoreline basins for 35 million years. Because of the creatures' size, the long-toothed grasping claws in front of their mouth, and their forward-facing, compound eyes, scientists have long believed these sea scorpions to be fearsome predators.

But research by Richard Laub of the Buffalo Museum of Science cast doubt on the ability of pterygotids' claws to penetrate armored prey. Yale's eye study further confirms the idea that pterygotids were not top predators.

"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," said Derek Briggs, the G. Evelyn Hutchinson Professor of Geology & Geophysics at Yale and curator of invertebrate paleontology at the Yale Peabody Museum of Natural History. Briggs co-authored the paper.

Victoria McCoy, a Yale graduate student, developed an innovative mathematical analysis method to understand the properties of the sea scorpions' eyes. Yale also used imaging technology with backscattered electrons on a scanning electron microscope to reveal the eye lenses without damaging the fossils. The team 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.

Although the data couldn't be used to determine nearsightedness or farsightedness, it revealed a basic visual acuity level for the sea scorpions, which had thousands of eye lenses. "We measured the angle between the lenses of the eye itself," Anderson said. "The smaller the angle, the better the eyesight."

Unfortunately for pterygotids, their eyesight proved less than exceptional, note the researchers. In fact, their vision worsened as they grew larger. It certainly wasn't on par with high-level arthropod predators such as mantis shrimp and dragonflies, said the scientists.

"Maybe this thing was not a big predator, after all," Anderson said. "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 Yale team's vision testing methodology may prove instrumental in understanding how other species functioned, as well. "You could use it on a number of different organisms," according to Anderson. "It will be particularly useful with other arthropod eyesight examinations."

Former Yale postdoctoral fellow Maria McNamara of University College Cork also co-authored the paper. The research began as a project in a fossil preservation class Briggs taught at Yale.