Eurypterus was the first ever eurypterid discovered. The first one that was discovered was Eurypterus remipes. The person who first discovered Eurypterus thought Eurypterus's front legs were barbels, and that Eurypterus was the first catfish. But later someone classified it as an arthropod, but instead of a eurypterid, he thought it was a branchiopod. Then someone described it as a eurypterid, and that person was right.
Eurypterus, like most other eurypterids, swam "breast stroke," the same way as frogs do today. One eurypterid that probably did not swim breast stroke was Hibbertopterus. It lived in swamps, rivers, and streams, but Hibbertopterus was more of a crawler than a swimmer. It may have crawled up onto land, which Eurypterus probably did not do.
Eurypterus was normally about 5 to 9" long. The largest one ever found was 4.3' long.
Eurypterus had a paddle at the end, with a large spine sticking out. The paddle was probably used for swimming, and the spine was probably used for defense against predators or for injuring its prey. Eurypterus means "wide wing" or "broad paddle."
Eurypterus had spiny legs which were probably used for grabbing prey, because the spines could badly injure the prey. Eurypterus was a generalist species, meaning it would rely on anything it could get for food. There are fifteen species of Eurypterus that are found so far. The reason that Eurypterus was so successful was that it was able to eat anything that contained any nutrients.