Among the largest and most abundant aquatic predators during much of the early evolution of vertebrates, eurypterids have long been an iconic and intensely studied group of Paleozoic arthropods. We report a new specimen of the eurypterid Slimonia acuminata, which includes a fully articulated series of tail (postabdominal and telsonal) segments preserved in a tight lateral curve. Such a high degree of apparent lateral tail flexibility has not been previously recognized in eurypterids. From the perspective of hydrodynamics, the dorsoventrally flattened body plan of eurypterids would have limited the effectiveness of lateral tail motion as a means of propulsion. However, the long and serrated terminal tail spine of S. acuminata (and other eurypterids) would have made lateral tail strikes-which would have met a minimum of hydraulic resistance-an effective means of predatory attack and self-defense. Thus, many eurypterids are reinterpreted as substantially better-armed predators than previously supposed.
Keywords: Eurypterida; tail flip; telson.