eurypterid

eurypterid, member of an extinct order (Eurypterida) of unusual arthropods rarely preserved as fossils. The eurypterids appeared at the beginning of the Ordovician Period (about 488 million years ago) and became extinct at the end of the Permian Period (about 251 million years ago). Frequently referred to as giant scorpions, most eurypterids were small animals, although Jaekelopterus rhenaniae (also called Pterygotus rhenanius or P. buffaloenis), a species from the Silurian Period (about 444 to 416...