Unionopterus

Unionopterus Temporal range: Tournaisian-Viséan,
Restoration of the carapace and first body segment of U. anastasiae.
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Class:	†Eurypterida
Superfamily:	†Adelophthalmoidea?
Family:	†Adelophthalmidae?
Genus:	†Unionopterus Chernyshev, 1948
Type species
†Unionopterus anastasiae Chernyshev, 1948

Unionopterus is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils have been registered from the Early Carboniferous period. The genus contains only one species, U. anastasiae, recovered from deposits of Tournaisian to Viséan in Kazakhstan. Known from a single specimen (now presumed to be lost) which was described in a publication of Russian language with poor illustrations, Unionopterus is extremely poorly known.

With the original fossil being impossible to examine and available illustrations making conclusion hard to make, Unionopterus remains an enigmatic genus. The spinosity of the appendages may indicate that it should be placed in the Adelophthalmoidea superfamily, which would make it the only post-Devonian example of not only the superfamily but of the swimming eurypterids (suborder Eurypterina) entirely, alongside Adelophthalmus itself. The taxon has variously been treated as closely related to Adelophthalmus within the Adelophthalmidae, as incertae sedis within the Eurypterida or ignored altogether.

It has been suggested that a species of Adelophthalmus, A. dumonti from Belgium, represents a second species of Unionopterus, which would also reinforce a position of Unionopterus within the Adelophthalmidae. Such a conclusion is now known to be incorrect, as the feature that supposedly indicates A. dumonti as a species of Unionopterus, the trapezoid shape of the carapace, was based on incorrect illustrations. The shape of the carapace of A. dumonti is parabolic, much like other species of Adelophthalmus.

Description

Sixth to eight body segments of Unionopterus.

Unionopterus was a very small eurypterid, with the length of the body being estimated at 2.38 cm (0.9 in), although it should be noted that this measure excludes unknown parts of the body (like the 12th body segment or the tail). Even so, Unionopterus was far from the largest adelophthalmids, such as Adelophthalmus khakassicus of 32 cm (12.6 in) in length.

Unionopterus is a relatively well known genus since its only known specimen is almost completely preserved. The prosoma ("head") was truncately (shortened as by cutting it) rounded and somewhat narrow anteriorly, being close to that of Eurypterus. It was surrounded by a narrow marginal border. The lateral eyes were small (about 1.5 milimetres or 0.06 inches long), reniform (bean-shaped) and close to the prosoma, resembling Nanahughmilleria. The ocelli (simple eye-like sensory organs) occupied a subcentral (almost central) position, immediately behind the eyes. Also known is the metastoma (a large plate that is part of the abdomen), which extended to the second segment, probably shifted downwards. As in the rest of eurypterids, the opisthosoma was divided into twelve segments. The tergites (dorsal half of the segment) of the mesosoma (comprising segments 1 to 6) were rigid and covered with chitin. The first segment was twice as short as the rest, while the fourth was the widest. From this segment, the opisthosoma would begin to narrow gradually. The seventh segment carried two lateral "extensions" (known as epimera) and was separated from the other five by a weakly expressed groove. The pretelson (12th segment anterior to the telson) and telson ("tail") are not known, but due to the opisthosoma structure, the last one was probably the same as in Hughmilleria (styliform and lanceolate). The metasoma (compromising segments 7 to 12) had an ornamentation that consisted of small and frequent scales grouped in rows that overlaped each other, a system of ornamentation common among the genera of Adelophthalmidae.

Only four pairs of appendages are known. 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. According to weak traces, they all possessed spines in their distal ends, which were probably preserved in a different position than during their lifetime's. The rest of the podomeres were hidden under the prosoma, and therefore they are not known. The third pair was more or less similar to the second one, but thicker and longer than the latter, with different proportions and spikes in the podomeres. Few fragments are known from the fourth pair, while the fifth pair is not known at all. In the sixth and last 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. In a shallow groove located on the inner side of the eighth podomere was the ninth one in the form of a small spine. These podomeres form the paddle of the swimming leg. Parts of the coxae (the point of union with the prosoma and the appendages) are known, but all are poorly preserved and in a unnatural position, as well as potential remains of chelicerae (first pair of appendages).

History of research

Karaganda

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Location of Karaganda in Kazakhstan, where the only known specimen of Unionopterus has been found.

Unionopterus is only known by an almost complete fossil from the Karaganda Formation, Karaganda, Kazakhstan (then part of the Soviet Union). The fossil was deposited in a coal seam, and was collected from a borehole. Unionopterus represented the only eurypterid (and merostomate, an outdated group that encompassed eurypterids and xiphosurans) of the entire Soviet Union after Adelophthalmus carbonarius. 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. cabonarius 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. He honoured his wife, who helped him during the study of Unionopterus, in the specific epithet anastasiae.

Classification

The following cladogram is based from a larger study carried out by Tetlie in a 2004 phylogenetic analysis of an unpublished thesis encompassing every eurypterid genus with under two-thirds of missing data. 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).

Diploperculata	Carcinosomatoidea Waeringopteroidea Adelophthalmoidea Parahughmilleria Nanahughmilleria Unionopterus Adelophthalmus Pterygotoidea Hughmilleria Herefordopterus Slimonia Jaekelopterus Pterygotus Erettopterus Acutiramus

See also

• List of eurypterid genera
• Timeline of eurypterid research