Eurypterids, also known as sea scorpions, are an extinct class of arthropods that were related to modern-day marine chelicerates such as horseshoe crabs and spiders. Eurypterids grew from a few inches to the huge Jaekelopterus, which reached 2.5 m (8.2 ft) or more in length and competes with the Carboniferous myriapod (centipede-relative) Arthropleura for the title of largest arthropod of all time. A typical eurypterid was only 20 cm (8 in) in size, however. 200 fossil species are known.
Eurypterids existed for almost half the total duration of known multicellular life, from approximately 510 to 251 million years ago. In contrast, mammals have only existed for only about half that time, and dinosaurs a little more than half. Because they emerged almost at the beginning of the Paleozoic era and died out at the end, eurypterids are considered an iconic Paleozoic organism. Eurypterids went extinct during the Permian-Triassic extinction, the most severe mass extinct in Earth's history. From a evolutionary biologist's point of view, the death of the eurypterids might be considered the end of the success of large arthropods in earthly ecosystems.
Eurypterids superficially resemble modern scorpions, to which they are related. Instead of having a curving stinger at their ends, they had a long spine which probably didn't play much of a role in hunting. Like in horseshoe crabs, it probably helped the animal maneuver through the water and was also used to correct itself if it flipped over.
There is no evidence that eurypterids were venomous. Some had claws, but the more primitive eurypterids just used their feeding parts, chelicerae, to obtain food. Sometimes these chelicerae are casually called "claws," however. Eurypterids had a strong carapace connected to paddles which were used for swimming and/or digging. Under their carapace, they had four legs for scuttling along. Eurypterids had small, simple eyes for locating prey.
Instead of swimming through the water like some other predators, eurypterids mostly crawled along the bottom. Rare as fossils, eurypterids mostly lived in brackish water or lagoons rather than the open sea. As they often lived near land, it has been speculated that eurypterids may have spent part of their life cycle on land (a fact supported by the discovery of terrestrial track fossils from the Cambrian, a time period where it is commonly thought there was no terrestrial life besides microbes).
Eurypterids are thought to have consumed fish and other arthropods such as crustaceans. Their chelicerae exhibit a fine branching structure which would have helped them gather numerous small food particles, like a filter feeder.