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Eurypterids (sea scorpions) are extinct aquatic chelicerates, the same group as modern arachnids, known from environments ranging from open marine to freshwater river systems. They are first known from the Late Ordovician and go extinct in the Late Permian. All eurypterids are thought to be carnivorous, being either active predators or scavengers and sweep-feeders, and are split into two suborders; one bottom crawlers with their posterior appendages retained as a walking leg, the other evolving the posterior appendage into a broad paddle for swimming. Their closest living relatives are thought to be xiphosurans, or horseshoe crabs.

Structure

Eurypterids consist of a carapace with six appendages (which altogether are termed the prosoma) and a body (opisthosoma) consisting of twelve segments. The dorsal side of the carapace has two lateral eyes, which were compound in structure, and two light-sensitive ocelli on the centre of their carapace. The opisthosoma is divided into either the preabdomen (segments 1-7) and postabdomen (segments 8-12) based on contraction of the segments or the mesosoma (segments 1-6) and the ancylosed segments of the metasoma (segments 7-12). At the posterior of the animal is the telson, or tail-spine, which in some eurypterids is modified into a broad rudder.

The prosomal appendages are labelled with roman numerals (I-VI), from anterior to posterior, with each leg segment (podomere) labelled in arabic numerals proximally to distally, the first segment being termed the coxa. The first pair of appendages, the chelicerae, were claws used for holding and tearing food. In most eurypterids the chelicerae were small, but in pterygotoids they were massively enlarged. Appendages II-IV were usually spiniferous and involved in prey capture. Appendage V was usually non-spiniferous, although some eurypterids also evolved spines on it. Appendage VI was non-spiniferous and retained as a walking leg in the suborder Stylonurina and basal Eurypterina while more derived Eurypterina had an expanded appendage VI that acted as a paddle. Trackway evidence shows that eurypterids walked with an octopodous (eight-legged) or hexapodous (six-legged) gait.

From Tetlie & Cuggy 2007