Temporal range: Ordovician–Permian
|Eurypterid from Ernst Haeckel's
Kunstformen der Natur (1904)
The largest, such as Jaekelopterus, reached 2½ metres in length, but most species were less than 20 cm (8 inches).
The move from the sea to fresh water probably occurred by the Pennsylvanian period.
They went extinct during the Permian–Triassic extinction event 251 million years ago, and their fossils have a near global distribution.
The typical eurypterid had a large, flat, semicircular carapace, followed by a jointed section, and finally a tapering, flexible tail, most ending with a long spine at the end (Pterygotus, though, had a large flat tail, possibly with a smaller spine). Behind the head of the eurypterids were twelve body segments. These segments are formed by a dorsal plate, called a tergite, and a ventral plate, called a sternite. The tail, known as the telson, is spiked in most eurypterids.