Eurypterids (sea scorpions) are an extinct group of arthropods which include some of the largest known arthropods that ever lived. They are members of the extinct order Eurypterida (Chelicerata); which is the most diverse Paleozoic chelicerate order in terms of species.
The name Eurypterida comes from the Greek word eury- meaning "broad" or "wide" and the Greek word pteron meaning "wing", for the pair of wide swimming appendages on the first fossil eurypterids discovered. Eurypterids predate the earliest fishes. The largest, such as Jaekelopterus, reached 2.5 metres (8 ft 2 in) or more in length, but most species were less than 20 centimetres (8 in). They were formidable predators that thrived in warm shallow water, in both seas and lakes, in the Ordovician to Permian from 460 to 248 million years ago. Although informally called "sea scorpions", only the earliest ones were marine (later ones lived in brackish or freshwater), and they were not true scorpions. According to theory, the move from the sea to fresh water probably occurred by the Pennsylvanian subperiod. Eurypterids are believed to have undergone ecdysis, making their significance in ecosystems difficult to assess, because it can be difficult to tell a fossil moult from a true fossil carcass. They went extinct during the Permian–Triassic extinction event 252.2 million years ago, and their fossils have a near global distribution.
About two dozen families of eurypterids are known. Perhaps the best-known genus of eurypterid is Eurypterus, of which around 16 fossil species are known. The genus Eurypterus was described in 1825 by James Ellsworth De Kay, a zoologist. He recognized the arthropod nature of the first ever described eurypterid specimen, found by Dr. S. L. Mitchill. In 1984, that species, Eurypterus remipes was named the State fossil of New York.
The trace fossil trackways produced by eurypterids are placed in the ichnogenus Palmichnium.
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