[2] The rhenopterids were the most primitive stylonurines and the family encompasses many previously enigmatic eurypterids, such as Brachyopterus, Kiareopterus and Rhenopterus itself, all united by a rounded posterior margin to the metastoma and prosomal appendage III bearing single fixed spines.

[1] Unlike the derived stylonurines of the family Stylonuridae and superfamily Hibbertopteroidea, rhenopterids retained primitive Hughmilleria-type prosomal appendages II-IV, which are unsuited to sweep-feeding.

Rhenopterids are in turn rhenopteroids with single fixed spines on the prosomal appendage III and possess a short telson.

[1] Lamsdell, Braddy and Tetlie (2010) assigned one subfamily to the family, the Rhenopterinae, including the genera Alkenopterus and Rhenopterus.

The Rhenopterinae was restricted to the Early to Middle Devonian and were diagnosed as being rhenopterids with a non-spiniferous appendage IV and a caudal postabdomen.