What is "stylonuroidea"

Stylonuroidea are an extinct superfamily of eurypterids. They were members of the subphylum Chelicerata.

Stylonuroidea, which lived from the Ordovician to Lower Permian periods, were characterized by their last pair of prosomal (head) appendages, which were developed as walking legs, or less commonly developed as swimming legs with paddles formed by the expansion of the two or three penultimate joints.