The generic name derives from the penteconter, a warship from ancient Greece, and the suffix -pterus, which means "wing" and is often used in other genus of eurypterids.

Pentecopterus is diagnosed as a megalograptid retaining a single pair of spines on the third podomere of the third prosomal appendages, a short appendage V with a serrated distal margin of podomeres; prosomal ventral plates widening anteriorly, posterolateral pretelson lacking expansion and xiphos-like shaped telson, with a margin laterally ornamented with scales.

Other samples were collected from blocks eroded during flooding, which are assumed to have been sourced from the uppermost 2–3 m. The total number of fossils found is over 5,000, of which around 6.6% belong to eurypterids.

[3] The genus was named by James C. Lamsdell, Derek E. G. Briggs, Huaibao P. Liu, Brian J. Witzke and Robert M. McKay in 2015 and the type species is Pentecopterus decorahensis.

Pentecopterus shares with the other megalograptids a series of characteristics such as the randomly-oriented armature on the swollen podomeres of appendage IV and the narrow gnathobase bearing multiple rows of small teeth on the coxa of appendage V. It also shares with Megalograptus the rows of enlarged scales running down the center of the opisthosomal tergites, however, these differ from those of Megalograptus because they are not situated on pronounced ridges.

Pentecopterus also lacks the cercal blades that occur in Megalograptus, where they have been interpreted as functioning as a biological rudder, like the pterygotid telson.

Bore hole data indicate that the total thickness of the Winneshiek Shale is about 18 m at the outcrop locality, but only the upper 4 m was systematically collected during the excavation.

This shale is confined to a circular basin about 5.6 km in diameter in the Decorah area that probably originated from a meteorite impact.

Palaeogeographic and paleoenvironmental studies suggest that this crater was located in marginal to nearshore marine conditions with little oxygen, possibly in a brackish water environment within the tropical southern Laurentia.