| Onychopterella Temporal range: Late Hirnantian-Early Pridoli,
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| Bottom view of the holotype specimen of O. kokomoensis recovered at Kokomo, in the United States | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Chelicerata |
| Order: | †Eurypterida |
| Superfamily: | †Onychopterelloidea |
| Family: | †Onychopterellidae |
| Genus: | †Onychopterella Størmer, 1951 |
| Type species | |
| †Onychopterella kokomoensis Miller & Gurley, 1896
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| Species | |
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| Synonyms | |
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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. Fossils of Onychopterella have been discovered in deposits from the Late Ordovician to the Late Silurian. The genus contains three species: O. kokomoensis, the type species, from the Early Pridoli epoch of Indiana; O. pumilus, from the Early Llandovery epoch of Illinois, both from the United States; and O. augusti, from the Late Hirnantian to Early Rhuddanian stages of South Africa.
Its prosoma (head) could be subquadrate (almost square) or subrectangular (almost rectangular), with reniform (kidney-shaped) eyes. The appendages (limbs) were generally long, narrow and pointed at their tip. The abdomen and telson (the posteriormost division of the body) had different shapes and sizes depending on the species. The ornamentation of the body consisted of small, pointed scales. The largest species of the genus was O. kokomoensis with a total length of 16 centimetres (6.3 inches) long, followed by O. augusti (14.3 cm, 5.6 in) and O. pumilus (4 cm, 1.6 in).
The first Onychopterella fossils, belonging to O. kokomoensis, were discovered in 1896 at the Waterlime Group of Kokomo, Indiana. The species has received attention from eurypterid researchers for its terminal claw in the sixth pair of appendages or swimming legs. 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.
The exceptional preservation of the fossils of O. augusti has permitted scientists to describe part of the alimentary canal that has only been observed in a few species of eurypterids, as well as the internal muscular structure of its limbs and even part of the external branchial respiratory system. This turned out to be highly similar to that of the scorpions of today, supporting a eurypterid-scorpion relationship. Onychopterella was a genus that was able to swim. Most of the time it was likely in the stratum, probably using its spines to walk and its head to dig in the ground.