Stylonurus is a genus of prehistoric eurypterid of the family Stylonuridae.

A previously assigned species, S. ensiformis, is today regarded as synonymous with S.

[1] Stylonurids, which lived from the Ordovician to Lower Permian periods, were small to very large forms with scales developing into tubercles and knobs.

The prosoma (head) exhibited variable shape, with arcuate compound eyes located subcentrally, or anteriorly.

[4] Stylonurus is distinguishable from other stylonurids by their smooth surface, and the greatly elongated 5th (last) pair of walking legs, which reached as far as the telson, which was long and styliform.