(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.
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.
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.