For more than 20 years, scientists presumed the long, jagged-edge spine on the tail of the ancient sea scorpion Slimonia acuminata (above) was used for self-defense. But a new study suggests it was also a weapon of primordial destruction. S. acuminata, which grew up to 40 centimeters long, belonged to a diverse group of predators known as eurypterids, whose oldest known species appeared about 467 million years ago. Some of these arthropods (whose closest living kin include insects, spiders, and crabs) were fierce hunters that grew to the size of an adult human, whereas some—such as those without claws or tail spines—likely filtered the water for prey or scavenged the sea floor for food. The new study analyzes a 430-million-year-old fossil found recently in southern Scotland that, unlike previously described fossils, had a tail preserved in a highly curved configuration, suggesting it would have been well-equipped to finish off prey held firm with its small front limbs, the researchers report today in The American Naturalist. And unlike its modern-day namesake, which strikes its prey with an over-the-back movement, the long-extinct sea scorpion would have likely brandished its tail from side to side.