Eurypterids are an extinct group of arthropods that included the largest arthropods known to have existed with bodies up to 8 feet (2.5 m) in length. They were formidable predators that thrived in warm, shallow marine and fresh water environments from the Ordovician to the Late Permian. Although commonly called “sea scorpions”, only the earliest forms were marine; later ones lived in brackish and fresh water. The typical eurypterid had a large, flat, semicircular carapace followed by a jointed tapering section that terminated with a long spine. Most eurypterids had broad paddles at the end of the carapace that propelled them through the water and for digging in the bottom sediment in search of prey. It also had four pairs of jointed legs for walking with claws on the front two. Similar to modern arthropods, they had one pair of compound eyes and a pair of smaller eyes located between the larger pair. Traditionally, eurypterids have been considered close relatives to the horseshoe crab. They also have been considered to be related to scorpions, which they resemble.