Fossils of Parahughmilleria have been discovered in deposits of the Devonian and Silurian age in the United States, Canada, Russia, Germany, Luxembourg and Great Britain, and have been referred to several different species.

[2] Parahughmilleria differs from other more basal members of Adelophthalmidae in its reduced spinosity (being spinous) on the appendages, that along with other factors like the large spatulae (a long, flat piece in the operculum) associated with the genital operculum (plate-like segment which contains the genital aperture), suggests a closer relationship with Adelophthalmus.

A rapid contraction was observed in the postabdomen (segments 8 to 12) reminiscent of the scorpion abdomen, which seems to indicate a nepionic (immature) condition.

The only known adult eurypterids to possess it are Carcinosoma scorpioides and several species of Eusarcana, in which the tail was heavily specialized.

The ornamentation of the carapace consists of distinct transverse striations along the previous part in front of the eyes.

[4] In 1957, L. P. Pirozhnikov described two new species of eurypterids, P. matarakensis and Nanahughmilleria schiraensis, and erroneously assigned them to the genus Rhenopterus.

[10] In 1961, Kjellesvig-Waering erected the genus Parahughmilleria (meaning "near Hughmilleria"[1]) and assigned P. salteri as the type species.

In this species, the prosoma was widely rounded, smooth, nearly semicircular and highly arched, and was surrounded by a narrow and marginal rim.

It comprises two opercular flaps (protruding extensions lateral to the genital appendage) rounded at the corners and separated.

The specific name of this species honors John William Salter, an English naturalist and paleontologist,[4] and its maximum size was of 8 cm (3.1 in).

[11] P. major had a longer telson with more lateral concave margins and a broader base, a more slender body and narrower paddles than P. hefteri.

[13] In addition, Markus Poschmann transferred Erieopterus statzi to Parahughmilleria in 2015 based on the proportions of paddle segments which were more related to those of P.

[12] This species is only known by a single specimen that was originally assigned to Eurypterus, but Kjellesvig-Waering classified it to Erieopterus based on the shape of the prosoma lacking of ornamentation and on the highly serrated paddle.

[14] Nine years before Poschmann referred E. statzi to Parahughmilleria, he and O. Erik Tetlie had already suggested this change, which was not done because of the need to study the material.

These eurypterids share a series of characteristics such as enlarged spines on at least one podomere in the appendage V, the presence of epimera in the postabdomen and the long spatulae that has been associated with the genital operculum.

However, Parahughmilleria differs from Adelophthalmus in that the telson is generally shorter or that the cuticular sculpture (ornamentation consisting of small, minute, scales across the back) is much thinner, among other characteristics.

[5] Another genus in the family, Bassipterus, was morphologically close to Parahughmilleria based on the shape of the metastoma and telson, as well as the preabdomen and postabdomen, slightly differentiated from each other.

[5] The cladogram below presents the inferred phylogenetic positions of most of the genera included in the three most derived superfamilies of the Eurypterina suborder of eurypterids (Adelophthalmoidea, Pterygotioidea and the waeringopteroids), as inferred by O. Erik Tetlie and Markus Poschmann in 2008, based on the results of a 2008 analysis specifically pertaining to the Adelophthalmoidea and a preceding 2004 analysis.

[11] Orcanopterus Waeringopterus Grossopterus Eysyslopterus Bassipterus Pittsfordipterus Nanahughmilleria Parahughmilleria Adelophthalmus Hughmilleria Herefordopterus Slimonia Erettopterus Pterygotus Acutiramus Jaekelopterus Fossils of Parahughmilleria have been found from the Silurian deposits of the Llandovery epoch to the Devonian deposits of the Eifelian epoch in North America, Europe and Siberia.

[18] The argillaceous (composed of clay-like materials) Devonian deposits of Alken, Germany, where fossils of P. heftari and P. major have been found, contains various eurypterids such as Rhenopterus macroturberculatus, Jaekelopterus rhenaniae, Alkenopterus brevitelson and Vinetopterus struvei.

Fossils from other organisms have also been found, including the myriapod Eoarthropleura devonica, the chasmataspidids Diploaspis casteri and Heteroaspis novojilovi and the arachnid Alkenia mirabilis.