Eurypterina is one of two suborders of eurypterids, an extinct group of chelicerate arthropods commonly known as "sea scorpions". Eurypterine eurypterids are sometimes informally known as "swimming eurypterids".[1] They are known from fossil deposits worldwide, though primarily in North America and Europe.
Seventy-five percent of eurypterid species are eurypterines; this represents 99% of specimens.[2] The superfamily Pterygotioidea is the most species-rich clade, with 56 species, followed by the Adelophthalmoidea with 43 species; as sister taxa, they comprise the most derived eurypterines. Pterygotioidea includes the pterygotids, which are the only eurypterids known to have a cosmopolitan distribution.[3]
Though more numerous both in specimens and taxa, the eurypterines have the shorter temporal range of the two eurypterid suborders. They first appeared around the same time as the Stylonurina in the Middle Ordovician. The suborder faced a slow extinction during the Middle and Late Devonian, possibly tied to the emergence of jawed vertebrates.[4] Every Eurypterine genus and lineage went extinct before the Carboniferous save for Adelophthalmus which would go extinct in the Early Permian, millions of years before the Permian-Triassic extinction event that ended the stylonurines.[5]
The Stylonurina and Eurypterina are most easily distinguished by the morphology of the posteriormost prosomal appendage. In the Stylonurina, this appendage takes the form of a long and slender walking leg, lacking a modified spine (termed podomere 7a). In the Eurypterina, the leg is most usually modified and broadened into a swimming paddle and always includes a podomere 7a.[3]
Swimming eurypterines represent the absolute majority of both known eurypterid species and known specimens, though the morphology of the walking stylonurines is almost as diverse in appearance, and the fossil record of the eurypterines may therefore simply be more complete than that of the stylonurines, possibly due to varying habitat preferences.[3]
The most basal eurypterines with swimming legs, the genus Onychopterella, are known from the east coast of Gondwana close to the equator (a region that today is South Africa) from the Late Ordovician. It is not known whether or not the swimming forms originated here or not, but it is speculated that they migrated from Laurentia, since most stylonurines and basal swimming forms are predominantly known from Laurentia and Gondwana otherwise completely lacks basal swimming forms.[3]