An extinct Paleozoic group of aquatic arthropods, belonging to the subphylum Chelicerata and class Merostomata and thus related to the living marine xiphosurans (horseshoe crabs) and terrestrial arachnids (spiders, scorpions). Although most eurypterids were less than 10 in. (25 cm) in length, some members of the group were the largest arthropods of all time, reaching sizes up to 6 ft (2 m). See also: Merostomata
The anterior portion of the eurypterid body is the prosoma. The prosoma is covered dorsally by the carapace, which bears a large pair of lateral compound eyes and a small pair of median simple eyes or ocelli (see illustration). Visible on the ventral surface of the prosoma are six pairs of appendages. The first pair, adjacent to the mouth, comprises small pincers known as the chelicerae. This feature unites the eurypterids with the xiphosurans and the arachnids. The chelicerae in some forms are enormous. The next four pairs of appendages were probably used in walking and food gathering. The distal portions of the sixth pair of appendages in many eurypterids are flattened and laterally expanded. They closely resemble the “swim paddles” of living blue crabs and were probably used to generate thrust and lift for swimming. The metastoma is a large plate which lies ventral to the bases of the sixth pair of appendages. It may represent a fused seventh pair of appendages and is unique to the eurypterids.
The posterior opisthosoma consists of 12 unfused segments and the terminal telson or tail spine. The opisthosoma can be divided into the anterior six-segmented mesosoma, which bears appendages, and the posterior six-segmented metasoma, which lacks appendages. The five pairs of platelike mesosomal appendages cover gills located on the ventral body wall. The first two mesosomal appendages carry the sexually dimorphic external genitalia. The opisthosoma can also be divided into a broad seven-segmented preabdomen and a narrower five-segmented postabdomen.
The telson of most eurypterids is styliform and closely resembles that of horseshoe crabs. In some forms the telson is distinctly curved, whereas in others it forms a broad flat plate, which may have functioned as a rudder during swimming.
Eurypterid fossils are found worldwide. The chitinous body of eurypterids is not mineralized, and so fossil remains are rare and generally restricted to a limited number of ancient aquatic environments. Almost all of the approximately 300 described species, belonging to about 65 genera, are from single localities or regions. The earliest definite eurypterids are found in the Ordovician, in both shallow and deep marine settings. The group diversified during the Silurian, peaked during the Late Silurian, and then declined during the Devonian. Silurian and Devonian eurypterids are found in both marine and nonmarine environments. Carboniferous (Mississippian Pennsylvanian), and Permian eurypterids are rare and are exclusively nonmarine (for example, coal swamps). The group became extinct sometime during the Permian. See also: Devonian; Mississippian; Ordovician; Pennsylvanian; Permian; Silurian
Eurypterids were almost exclusively aquatic, although some forms may have been amphibious. A number of forms were dominantly benthonic, but most were active and agile swimmers. Most eurypterids were probably carnivores, although they lacked the ability to crush heavily armored prey.
Although the eurypterids have no living descendants, they are very similar to (and are often found with) the earliest scorpions. The two groups probably share a common ancestor. See also: Arthropoda