Unionopterus

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Unionopterus
Temporal range:
Ma
Type and only known specimen of U. anastasiae
Restoration of the carapace and first body segment of U. anastasiae
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Order: †Eurypterida
Superfamily: †Adelophthalmoidea (?)
Family: †Adelophthalmidae (?)
Genus: †Unionopterus
Chernyshev, 1948
Type species
†Unionopterus anastasiae
Chernyshev, 1948

Unionopterus is a

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

phylogenetic
studies, making Unionopterus an enigmatic eurypterid.

Description

Unionopterus was a very small

adelophthalmids, such as Adelophthalmus khakassicus of 32 cm (12.6 in) in length.[2]

The

genera of Adelophthalmidae.[4]

Only four out of the six pairs of

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).[1]

History of research

Karaganda is located in Kazakhstan
Karaganda
Karaganda
class=notpageimage|
Location of Karaganda in Kazakhstan, where the only known specimen of Unionopterus has been found

Unionopterus is only known by an almost complete

specific epithet anastasiae to honour his wife, who helped him during the study of Unionopterus.[1]

Unionopterus has been considered as an enigmatic genus whose classification is highly uncertain. Because its only known specimen is probably lost and the original description by Chernyshev was not well-illustrated and in

suborder Eurypterina (swimming eurypterids) along with Adelophthalmus itself.[4]

Further, in 2005 the paleontologists Odd Erik Tetlie and Jason A. Dunlop suggested 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 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.[4]

Classification

Sixth to eight body segments of Unionopterus
Restoration of the possibly related Adelophthalmus

Chernyshev classified Unionopterus as a member of the family Pterygotidae, 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 family Adelophthalmidae,[5] but some later authors consider this classification uncertain.[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 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. It has also been suggested that the fossil described by Chernyshev in fact represents another specimen of Adelophthalmus whose characteristics have been misinterpreted, but since the location of the fossil is still unknown, it is possible that this will never be resolved.[4]

The following

waeringopteroids as outgroup taxa (used as reference groups).[7]

Diploperculata

Paleoecology

Like other forms with a similar body structure, it is likely that Unionopterus was a

waterbody) organism. Its body and lifestyle were capable of and adapted to not only swimming, but also crawling. However, Unionopterus would not have been so active as other relatives.[1]

See also

References

  1. ^ a b c d e f g Chernyshev, Boris I. (1948). "New representative of Merostomata from the Lower Carboniferous". State University of Kiev, Geological Collections. 2: 119–130.
  2. S2CID 91741388
    .
  3. ISBN 978-1125460221
    .
  4. ^
    ISSN 0374-6291
    .
  5. ^
    S2CID 46953627
    .
  6. ISSN 1860-1014
    .
  7. ^ Tetlie, Odd Erik (2004). Eurypterid phylogeny with remarks on the origin of arachnids (PhD). University of Bristol. pp. 1–344.
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