Scientists have unearthed the oldest known species of a long-extinct group known as sea scorpions, a find that could mean the ancient creatures may have an even older origin than previously thought. Team members have dubbed the newly described predator Pentecopterus after the penteconter, a type of ancient Greek warship that roughly mirrors the creature’s body shape (artist’s reconstruction shown above). More than 150 fossil fragments of the animals, some of them possibly bits of molted exoskeleton, have been excavated from 467-million-year-old rocks in northeastern Iowa—rocks that are about 9 million years older than those that held fossils of the previously oldest known species. (Sea scorpions, more formally known as eurypterids, were arthropods, a group that includes insects, spiders, and crabs as well as their extinct kin such as trilobites.) Adults of the newly described predator were probably about 1.7 meters (5.6 feet) long, but some of the fossil fragments came from juveniles that may have measured between 10 and 15 centimeters (4 to 6 inches) in length, the researchers report online today in BMC Evolutionary Biology. Unlike all other known living or extinct arthropods, some of Pentecopterus’ limbs dramatically changed shape as the individuals grew—which suggests that small-clawed youngsters may have probed through sediments for tiny prey whereas adults grabbed larger, more mobile victims. The antiquity of these creatures suggests two evolutionary scenarios, the scientists say: Either eurypterids diversified quickly during the early stages of the Ordovician period (which began about 485 million years ago), or their lineage, including yet-to-be-discovered ancestors and kin, evolved more slowly and originated even further back in time during the Cambrian period—possibly during the Cambrian explosion, a period of evolutionary diversification sometimes called “life’s big bang,” which began about 542 million years ago.