1. Description

2. History of research

3. Classification

4. Paleoecology

5. See also

6. References

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 stages in Kazakhstan. Known from one single specimen (now presumed to be lost) which was described in a publication of Russian language with poor illustrations, Unionopterus' affinities are extremely poorly known.

Unionopterus was described in 1948 after the discovery of its only known specimen in the Karaganda Formation of Kazakhstan (at that time part of the Soviet Union), being one of the first discoveries of eurypterids of the Soviet Union and Carboniferous eurypterids in general. It was probably a swimming organism as well as other eurypterids, although with less facilities than these.

There are numerous factors that have made Unionopterus a problematic genus for eurypterid researchers. It has been placed in the Adelophthalmidae family and it has even been doubted whether a species of Adelophthalmus, A. dumonti, actually belongs to Unionopterus, but this can not be confirmed and many times it is chosen to completely ignore the genus during phylogenetic studies, making Unionopterus an enigmatic eurypterid.

Description

The morphology of Unionopterus is relatively well known since its only known specimen is almost completely preserved. The prosoma ("head") was trapezoid and somewhat narrow anteriorly, being close to that of Eurypterus. It was surrounded by a wide 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^[3]). The metasoma (compromising segments 7 to 12) had an ornamentation that consisted of small and frequent scales grouped in rows that overlaped each other,^[1] a system of ornamentation common among the genera of Adelophthalmidae.^[4]

Only four pairs of appendages (limbs) 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).^[1]

History of research

Further, the paleontologists Odd Erik Tetlie and Jason A. Dunlop suggested in 2005 that the Belgian species Adelophthalmus dumonti could actually represent a species of Unionopterus due to similarities in the trapezoidal shape of the prosoma and the width of the marginal rim.^[6] However, this was belied by Tetlie and Peter Van Roy a year later, as a new specimen of A. dumonti was discovered, showing that the carapace shape was in fact parabolic (approximately U-shaped) and with a narrow marginal rim unlike that shown in its original description in 1917.^[4]

Classification

Chernyshev classified Unionopterus as a member of the Pterygotidae family, mainly due to its great similarities to Hughmilleria (genus which he thought should have a separate clade), such as the prosoma and opisthosoma shape, position of the eyes, aerodynamic body, epimera, gradual narrowing of the body and shape of the swimming leg, among others.^[1] Victor P. Tollerton Jr. would classify it in 1989 in the Adelophthalmidae family,^[5] which today is debatable.^[4]

The phylogenetic classification of Unionopterus is not clear due to the poor illustrations given by Chernyshev and the probable loss of the only known specimen of Unionopterus. 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 postabdomen, 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. It has also been suggested that the fossil described by Chernyshev in fact represents another specimen of Adelophthalmus, but since the location of the fossil is still unknown, it is possible that this will never be resolved.^[4]

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).^[7]

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Paleoecology

Compared to other forms with a similar body structure, it is likely that Unionopterus was a benthic (living at the lowest level of the waterbody) organism, with a body and lifestyle capable and adapted to not only swimming, but also crawling, although not as actively as other relatives.^[1]

See also

• List of eurypterid genera
• Timeline of eurypterid research

References

• William S. Burroughs bibliography
• Williams, Ca
• William Scammell
• William Scandling
• William Scarbrough House
• William S. Carlson
• Williams Center
• Williams Center, University of Wisconsin Whitewater
• Williams Center, Whitewater
• William Schabas
• William Schaffner
• Williams Championship
• Williams Chapel A.M.E. Church
• William Schaub
• William Schaus
• William Schenck
• William Scheuneman Kenyon
• William Schey
• William Schickel
• William Schickel & Company
• William Schlesinger
• William Schley Howard
• William Schlich
• William Schluter
• William Schmeisser