An article published in the journal “The American Naturalist” describes a research on Slimonia acuminata (fossil photo ©Ghedoghedo), a species of sea scorpion that lived about 430 million years ago. According to Scott Persons and John Acorn, two scientists from the University of Alberta, this very ancient predator that could reach a length of about 40 centimeters (almost 16″) used its tail as a weapon to hit its prey.
Sea scorpions, also known as eurypterids (Eurypterida), are now extinct. Despite their common name they’re part of the group of chelicerates so they were related to scorpions but because of their characteristics they form a separate group, closer to the so-called horseshoe crabs of the Limulidae family. They probably developed in the Cambrian, about 510 million years ago, and became extinct in the Permian, about 260 million years ago.
Some species of eurypterids reached a remarkable size, with lengths that could exceed two meters (6′ 6″) in the Pterygotus genus, possible due to the greater amount of oxygen present in the Earth’s atmosphere in that era. The 40 centimeters of Slimonia acuminata were far from those records but the discovery of new fossils in today’s Scotland suggests that they were deadly predators.
Paleontologists have long considered that sea scorpions as predators but the knowledge of the techniques they used to hunt is limited, also because there were a lot of species. For example, an article published in the magazine “BMC Evolutionary Biology” in September 2015 described another sea scorpion that perhaps was an important predator in the Paleozoic era.
According to this new research, Slimonia acuminata had a tail useful as a defense weapon but also to attack its prey. Other arthropods can move their tail up and down and for example modern scorpions hit moving it over their head. Instead, Slimonia acuminata’s had no vertical flexibility but was very mobile horizontally. In essence, this sea scorpion could move it laterally and exploit its end, called telson in jargon, which was sharp to hit its prey.
That kind of movement would have encountered a minimum hydraulic resistance during an attack on a sea animal. Slimonia acuminata could use all its tail’s strength to hit its prey, probably while it held it with its front limbs. It’s possible that other sea scorpions could use the same technique and this would make them even more lethal predators than thought.
