ASL STEM

Definition

The eurypterids were among the largest and most fearsome marine predators of the Paleozoic. While the smallest were only about 10 centimeters, some reached more than two meters (six feet) in length, making them the largest arthropods that ever lived. They arose in the Ordovician, and the last ones went extinct in the Permian. Most have been found in rocks that were laid down in brackish water or freshwater; the earliest groups may have lived in the sea, and some eurypterids may have spent at least short intervals on land. Eurypterids fossils are known from all continents, and have such amazingly good preservation that their external structure is the best known of all extinct animals. Because of their long tails and the spine-like appendage at the tip, the eurypterids have been called sea-scorpions. And in fact they are closely related to scorpions and other arachnids.

Source: http://www.ucmp.berkeley.edu/arthropoda/chelicerata/eurypterida.html