Ordovician-Permian | image = Haeckel Eurypterid.jpg | image_caption = Eurypterid from Ernst Haeckel's Kunstformen der Natur, 1904. | regnum = Animalia | phylum = Arthropoda | subphylum = Chelicerata | class = †Eurypterida | subdivision_ranks = Suborders | subdivision = †Stylonurina Diener, 1924 †Eurypterina Burmeister, 1843 }} Eurypterids (sea scorpions) are an extinct group of arthropods related to arachnids, which include the largest known arthropods that ever lived. They're… (More on Eurypterid)