Unionopterus is a genus of eurypterid, an extinct group of aquatic arthropods commonly known as "sea scorpions". 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). This was one of the first discoveries of eurypterids in the Soviet Union and Carboniferous eurypterids in general. It was probably a swimming organism as well as other eurypterids, although not as good as its relatives. There are numerous factors that have made Unionopterus a problematic genus for eurypterid researchers. It has been placed in the family Adelophthalmidae and it has even been speculated that a species of Adelophthalmus, A. dumonti, actually belongs to Unionopterus, but this cannot be confirmed. Many authors have chosen to completely ignore the genus during phylogenetic studies, making Unionopterus an enigmatic eurypterid.
Description Unionopterus was a very small eurypterid, with the length of the body being estimated at 2.38 cm (0.9 in), although this measure excludes unknown parts of the body (like the 12th body segment or the tail). Unionopterus was far smaller than the largest adelophthalmids, such as Adelophthalmus khakassicus of 32 cm (12.6 in) in length. 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 millimetres 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 opisthosomal segment was twice as short as the rest, and 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 (the posteriormost division of the body) are not known, but due to the opisthosoma structure, the telson was probably similar to the one of 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 overlapped each other, a system of ornamentation common among the genera of Adelophthalmidae. Only four out of the six 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 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
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