Onychopterella (/ˌɒnɪkɒptəˈrɛlə/ ON-ə-kop-tə-REL-ə, from Ancient Greek: ὄνῠξ (ónyx), "claw", and πτερόν (pteron), "wing") is a genus of predatory eurypterid ("sea scorpion"), an extinct group of aquatic arthropods.

Onychopterella is also the type genus of the basal ("primitive") family of eurypterines Onychopterellidae together with Alkenopterus and Tylopterella, characterized by the presence of spines in the second to fourth pair of appendages and a lack of them in the fifth and sixth (except occasionally one on the distal end of the swimming leg), as well as the lanceolate (lance-shaped) or styliform (pen-shaped) form of the telson and other characteristics.

[6] In 1896, paleontologists Samuel Almond Miller and William Frank Eugene Gurley described a new species of Eurypterus, E. kokomoensis, based on four specimens, three of them well-preserved and a fragmentary one,[3][5] collected at the Waterlime Group at Kokomo, Indiana, in the United States.

[5] In 1948, paleontologist Erik Norman Kjellesvig-Waering concluded that Onychopterus warranted generic rank due to the undifferentiation of its fifth appendage and the possession of a claw in the sixth one.

[9] In 1951, paleontologist and geologist Leif Størmer noticed that the name Onychopterus had been used previously by a bird genus introduced in 1850 by the ornithologist Ludwig Reichenbach.

[10] In 1916, researcher and geologist Thomas Edmund Savage erected the new species Eurypterus pumilus to accommodate one single well-preserved specimen showcasing the ventral side of the body[6] from the Edgewood Limestone near Essex, Illinois, in the United States.

[9] It differed from the type species by slimmer prosomal (of the prosoma) appendages, a tapering preabdomen, the lack of epimera in the pretelson and the placement of the eyes in a more forward position.

[11] In 1995, paleontologists Braddy, Richard John Aldridge and Johannes N. Theron described a well-preserved eurypterid from the Soom Shale Member of the Table Mountain Sandstone, Cape Province, South Africa, and named it O. augusti.

The holotype (GSSA C373, housed at the Geological Survey of South Africa in Pretoria along with the paratype) was discovered by August Patrick Pedro, honoured in the specific epithet augusti.

In 2007, paleontologists Odd Erik Tetlie and Michael B. Cuggy interpreted Onychopterella as such in a phylogenetic analysis due to the more stylonurine-like dimensions of the fourth and fifth podomeres of the swimming leg in O. augusti than in O. kokomoensis.

[13] In 2011, Lamsdell recovered Onychopterella as monophyletic (a group consisting of all the descendants of the last common ancestor, i.e. a valid genus), considering Moselopteroidea the most basal eurypterine clade.

[7] Alkenopterus was assigned to Onychopterellidae three years later because of the detection of a movable spine in the swimming leg, rather than a simple projection as previously thought.

[14] Before the creation of Onychopterellidae, Onychopterella had been classified in the family Erieopteridae since 1989 by paleontologist Victor P. Tollerton, initially together with Erieopterus and Buffalopterus based on similarities of the morphology of the appendages and the opisthosoma.

[7] Brachyopterus stubblefieldi Rhenopterus diensti Parastylonurus ornatus Stoermeropterus nodosus Stoermeropterus latus Stoermeropterus conicus Vinetopterus martini Vinetopterus struvei Moselopterus ancylotelson Moselopterus elongatus Onychopterella augusti Onychopterella kokomoensis Tylopterella boylei Dolichopterus macrocheirus Strobilopterus princetonii Eurypterus remipes Erieopterus microphthalmus Hughmilleria socialis Megalograptus ohioensis Mixopterus kiaeri Only the alimentary canals of a few species of eurypterids, such as Carcinosoma newlini, Acutiramus cummingsi and Eurypterus lacustris, have been described.

These valves have developed independently in certain groups of fishes in response to the need to increase the absorptive area of the gut, so Braddy, Aldridge and Theron considered the case was the same for the eurypterids.

The appendages of H. wittebergensis suggest it was a sediment feeder, using its valve to extract food from the soil and increase the absorptive capacity of the gut.

It is possible that the spiral structure of O. augusti had a similar function, acting as a valve for the mesenteron (midgut, formed by the stomach and intestines), thus increasing the absorptive area of this region.

Apart from this spiral structure, the pretelson of the paratype preserves a faint impression that expanded posteriorly to a depression in the medial area, representing an anal opening.

Given the relative lengths of the second to sixth pair of appendages, it is feasible that it could have had a stance in which it used eight limbs (an octopodous position) spaced far apart, ensuring little interference.

This suggests the legs were maintained in a posterolateral position to walk in order, thus allowing sufficient stability at the animal's center of gravity,[2] which was probably located near the second tergite (upper half of the segment).

[16] The presence of proximal podomeres and a terminal spine in the swimming leg indicates their primitive condition as natatory organs and their functional adaptation to walk.

[7] In 1999, Braddy, Aldridge, Theron and Sarah E. Gabbott, a geologist, described a new specimen of O. augusti from the Soom Shale (GSSA C1179, housed at the Geological Survey of South Africa) which preserves four pairs of vertical lamellate (composed of thin plates or scales) book gills (external gills arranged like the pages of a book), part of the respiratory system.

O. augusti would have been able to make incursions to the terrestrial surface, but it would have been uncomfortable for it, performing an undulatory gait and keeping its telson in regular contact with the ground, hence the median line.

[2] The less developed and narrow swimming legs, as well as the short and stout postabdomen, show that the genus was not a very good nor active swimmer, possibly using its terminal spine to walk.

[7][18] The type species, O. kokomoensis, inhabited a peritidal environment (in a part of the shore) along with other eurypterids such as Drepanopterus longicaudatus, Carcinosoma newlini and Erieopterus limuloides, as well as with the ostracod Leperditia ohioensis and the brachiopods Pentamerus divergens and Schuchertella interstriata.

Onychopterella does not represent the only occurrence of basal genera in Gondwana; Paraeurypterus, a genus known from deposits of the Şort Tepe Formation in southeastern Turkey, probably arrived there by the same method as O.