The most primitive of these, the Rhenopteroidea, includes several previously enigmatic genera, such as Brachyopterus, Kiaeropterus and Rhenopterus, all united by a rounded posterior margin to the metastoma and the prosomal appendage III bearing single fixed spines.

[3] The superfamilies Mycteropoidea and Kokomopteroidea are sister groups, united by a median ridge on the carapace between the lateral eyes and a distal thickening to the podomeres of the prosomal appendages.

Rhenopteroids, kokomopterids and parastylonurids retained primitive prosomal appendages II-IV that were unsuited for sweep-feeding and likely adopted scavenging, whilst the hardieopterids may have been benthic bottomdwellers living partially buried in the substrate.

[3] Stylonuroids have fixed spines on appendages II-IV which could have been used as dragnets to rake through the sediments and thus entangling anything in their way, whilst the mycteropoids, which have some of the most extreme adaptations, likely were more selective and specialized.

Many historical analyses were limited in scope or resolution and the unique hibbertopterids have even on occasion been suggested to be a separate, but closely related, order to Eurypterida, but have lacked an analysis either proving or disproving such an idea.

[5]Brachyopterus Kiaeropterus Brachyopterella Alkenopterus Rhenopterus Parastylonurus Stylonurella Pagea Stylonurus Laurieipterus Ctenopterus Lamontopterus Kokomopterus Hardieopterus Tarsopterella Hallipterus Drepanopterus Cyrtoctenus Hibbertopterus Woodwardopterus Megarachne Mycterops