In the second pair, the eighth podomere (leg segment) had the appearance of a thin spine (1.01 mm or 0.03 in long), while the seventh, sixth and fifth only differed slightly in thickness and length.

In the sixth pair of appendages, also known as swimming legs, the seventh podomere was long and very expanded towards the end, the eighth was elliptical and large, but not as much as the previous one.

This specimen was collected by the Russian geologist Leonty Leontiyevich Halfin and sent to the Ukrainian paleontologist Boris Isidorovich Chernyshev, who described it in 1948 and also described A. carbonarius in 1933.

Due to its temporal rank (Lower Carboniferous, one of the first finds from this period), Chernyshev considered the genus as worthwhile for the knowledge of Eurypterida and its evolution, placing it in the family Pterygotidae.

Unionopterus has been considered as a genus close to Adelophthalmus, as incertae sedis in the whole Eurypterida order or directly ignored by eurypterid researchers.

[4] Victor P. Tollerton, Jr. considered Unionopterus as part of his new family Adelophthalmidae, and placed the genus in it in 1989 based in the possession of spines in its appendages and their similar appearance with those of Adelophthalmus,[5] with which some authors agree.

If this was the case, Unionopterus would represent not only the only post-Devonian form of the family Adelophthalmidae, but of the whole suborder Eurypterina (swimming eurypterids) along with Adelophthalmus itself.

[6] However, this was contested by Tetlie and Peter Van Roy a year later, following the re-analysis of a specimen of A. dumonti which showed that its carapace in fact had a parabolic (approximately U-shaped) shape and a narrow marginal rim, unlike that shown in its original description in 1917.

There are several factors that help the inclusion of the genus in Adelophthalmidae, such as the possession of spines in the appendage or epimeras in the metasoma, but the trapezoidal shape of the prosoma is different from all adelophthalmids.

In addition, the affinities within Adelophthalmidae of Unionopterus are also uncertain, as it seems to share characteristics with Adelophthalmus but also with the more basal ("primitive") Nanahughmilleria and Pittsfordipterus.

[4] The following cladogram is based on a larger phylogenetic analysis carried out by Tetlie (2004) in an unpublished thesis encompassing every more or less well known eurypterid genus.

It has been simplified to only show Adelophthalmoidea and the closely related Pterygotoidea, with Carcinosomatoidea and the waeringopteroids as outgroup taxa (used as reference groups).