Eurypterids, or sea scorpions, were an extinct class of arthropods related to modern marine chelicerates. They existed for almost half of the total known multicellular life span, with 200 fossil species known. They crawled mostly along the bottom of brackish waters and consumed fish and other arthropods. They went extinct during the Permian-Triassic extinction event, and their death could be considered the end of the success of large arthropods in terrestrial ecosystems.
Eurypterids, also known as sea scorpions, are an extinct class of arthropods related to modern marine chelicerates such as horseshoe crabs and spiders. Eurypterids grew from a few inches to the enormous Jaekelopterus, which reached 2.5 m (8.2 ft) or more in length and vied with the carboniferous myriapod (millipede relative) Arthropleura for the title of largest arthropod of all time. However, a typical eurypterid was only 20 cm (8 in). 200 fossil species are known.
Eurypterids existed for almost half of the total known multicellular life span, from about 510 to 251 million years ago. In contrast, mammals have only existed about half that time and dinosaurs have existed for just over half. Because they emerged near the beginning of the Paleozoic Era and became extinct at its end, eurypterids are considered an iconic organism of the Paleozoic Era. Eurypterids went extinct during the Permian-Triassic extinction event, the most severe mass extinction in Earth’s history. From the point of view of an evolutionary biologist, the death of the eurypterids could be considered the end of the success of the large arthropods in terrestrial ecosystems.
Eurypterids superficially resemble modern scorpions, to which they are related. Instead of having a curved stinger at the ends, they had a long spine that probably didn’t play an important role in hunting. As in horseshoe crabs, it probably helped the animal maneuver through the water and was also used to correct itself if it capsized.
There is no evidence that eurypterids were venomous. Some had claws, but the most primitive eurypterids used only their nutritive parts, the chelicerae, to get food. However, sometimes these chelicerae are casually referred to as “claws.” Eurypterids had a robust carapace attached to paddles that served for swimming and/or burrowing. Under the carapace they had four legs for running. Eurypterids had small, simple eyes for locating prey.
Instead of swimming through the water like some other predators, eurypterids crawled mostly along the bottom. Rare as a fossil, eurypterids lived mostly in brackish waters or lagoons rather than the open ocean. Because they often lived close to 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 trace fossils from the Cambrian, a time period when it is commonly thought that they did not exist life other than microbes).
Eurypterids are thought to have consumed fish and other arthropods such as crustaceans. Their chelicerae exhibit a fine branching structure that would have helped them collect numerous small food particles, like a filter.
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