The first eurypterid fossils were discovered in 1818 by S. L. Mitchell in Silurian rocks of New York state. Mitchell mistook the fossil for a catfish, and it was not until 1825 that eurypterids were recognized as a group of arthropods. Today, the "sea scorpions" are among the best studied groups of fossils, and their external structure is as well known as for any living group of animals.
Some believe that most eurypterids were marine, but that their soft flexible (and edible) exoskeletons were usually destroyed. Others feel that since most eurypterid fossils are found in brackish or fresh water, the group was only primitively marine.
The oldest fossils come from the Lower Ordovician of New York. The degree of specialization of these early fossils suggests that older fossils may eventually be found in the Cambrian. Sea scorpions were most diverse in the Silurian and Lower Devonian, after which they rapidly declined in diversity. They were less common through the Carboniferous, and finally went extinct in the Permian; the latest fossil was found in the Middle Permian of Kansas.