n. Phylum Arthropoda, Subphylum Chelicerata, Class Meristomata, Order Eurypteridae A type of large, predatory, extinct marine arthropod that flourished during the Silurian Period (438 to 408 million years ago). See sea scorpion.
The eurypterids (sea scorpion) were the largest known arthropods that ever lived (with the possible exception of Anomalocarids). They are members of the extinct class Eurypterida (Arachnomorpha, Chelicerata) and predate the earliest fishes. The largest, such as Pterygotus, reached 2 m or more in length, but most species were less than 20 cm.