You don't name a sea creature after an ancient Greek warship unless it's built like a predator.
That's certainly true of the recently discovered Pentecopterus, a giant sea scorpion with the sleek features of a penteconter, one of the first Greek galley ships. A Yale University research team says Pentecopterus lived 467 million years ago and could grow to nearly six feet, with a long head shield, a narrow body, and large, grasping limbs for trapping prey. It is the oldest described eurypterid -- a group of aquatic arthropods that are ancestors of modern spiders, lobsters, and ticks.
A detailed description of the animal appears in the Sept. 1 online edition of the journal BMC Evolutionary Biology.
"This shows that eurypterids evolved some 10 million years earlier than we thought, and the relationship of the new animal to other eurypterids shows that they must have been very diverse during this early time of their evolution, even though they are very rare in the fossil record," said James Lamsdell, a postdoctoral associate at Yale University and lead author of the study.
"Pentecopterus is large and predatory, and eurypterids must have been important predators in these early Palaeozoic ecosystems," Lamsdell said.
A NEW EURYPTERID LAGERSTÄTTE FROM THE UPPER SILURIAN OF PENNSYLVANIA
Authors:
Vrazo et al
Abstract:
Eurypterids are generally rare in the fossil record, but occasionally occur in abundance. The genus Eurypterus, in particular, is well known from certain upper Silurian Lagerstätten of the northern Appalachian basin (New York and Ontario), but occurs far less frequently in the central and southern Appalachian basin (Pennsylvania, Maryland, and West Virginia, respectively). The recent discovery of an exceptionally preserved mass assemblage of Eurypterus in the upper Tonoloway Formation (upper Ludlow–Přídolí) of Pennsylvania provides new information on the behavior and life habitat of the genus in this region. Eurypterids at this locality are found in thinly laminated, calcareous shale deposited within the lower intertidal to shallow subtidal zone of a coastal mudflat or sabkha. Rare associated fauna of limited diversity, and evaporitic and desiccation features in associated beds, suggest a stressed environment with variable salinity and possible hypoxic conditions. Most eurypterids are disarticulated and fragmentary, but several fully articulated, exceptionally preserved specimens are present. Exoskeletal features and taphonomic indices values indicate a molt rather than death assemblage, and the presence of arthropod trackways suggests that Eurypterus sp. may have molted en masse in the vicinity of the burial site. Sequence stratigraphic interpretation of the site suggests that preservation of eurypterid remains is the result of occupation of ephemeral environmental (salinity/oxygen) conditions during a transgression. The occurrence of this new Lagerstätte within the upper Silurian succession of the central Appalachians, an interval which had heretofore yielded only rare, fragmentary remains, indicates that eurypterids were more prevalent in this region than previously thought.
Poor peepers are a problem, even if you are a big, bad sea scorpion.
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."
