Stylonurids, which lived from the Ordovician to Lower Permian periods, were small to very large forms with scales developing into tubercules and knobs. The prosoma (head) exhibited variable shape, with arcuate compound eyes located subcentrally, or anteriorly. Their abdomens were slender. Their walking legs were long and powerful, sometimes characterized by spines. Most genera did not have swimming legs. 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. The prosoma (head) varied from semiovate to subrectangular.
The genus contains two species, one of which is of uncertain placement: Stylonurus powriensis Page, 1856 = Stylonurus ensiformis Woodward, 1864 ?Stylonurus shaffneri Willard, 1933