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 and narrow with a spine on 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 on the second to fourth pair of appendages and a lack of them on the fifth and sixth pair of appendages (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.
Description[]
Like the other onychopterellids, Onychopterella was a small eurypterid. The smallest species, O. pumilus, measured only 4 centimetres (1.6 inches). The size of the largest one, O. kokomoensis, is estimated at 16 cm (6.3 in), representing the biggest species of the family Onychopterellidae. O. augusti had a similar size, with the largest specimen reaching 14.3 cm (5.6 in).
The prosoma (head) had a subquadrate (almost square) shape in O. kokomoensis and O. augusti, while in O. pumilus it was subrectangular (almost rectangular). The outline of the carapace (the exoskeleton part covering the prosoma) varied according to the species. It was anteriorly rounded in the corners in O. kokomoensis, with almost straight lateral margins in O. pumilus, and resembled a horseshoe in O. augusti. In O. kokomoensis and O. pumilus, the eyes were reniform (kidney-shaped), standing out for their size and prominence in the latter species. The eyes of O. augusti are unknown. The metastoma (a large plate that is part of the abdomen) of O. kokomoensis was small, oval and slightly narrower anteriorly than posteriorly, while that of O. augusti was subcordate (almost heart-shaped) anteriorly and rounded posteriorly. The metastoma of 'O'. pumilus is not known in its entirety. Only the posterior part, which was rounded, is known.
In O. kokomoensis, the preabdomen (body segments 1 to 6) was as long as it was wide, while the postabdomen (segments 7 to 12) was short and compact and gradually increased in length posteriorly. It was the only species with considerably large epimera (lateral "extensions" of the segment) in the pretelson (segment that preceded the tail). In O. pumilus, the preabdomen was wider than long, the fourth segment being the widest. The postabodmen was longer, decreasing in width more rapidly in the eighth and ninth segments than in the rest. The length of the segments gradually increased towards the telson (the posteriormost division of the body). The opisthosoma (abdomen) of O. augusti is known in more detail; the preabdomen was a little wider than long, with raised areas representing branchial (of the gills) chambers or respiratory tissue of the branchial tract. The postabdomen was short, and its segments gradually narrowed towards the telson. Each of the postabdominal segments had small epimera. The shape of the telson in O. kokomoensis was clavate (resembling a club), in O. pumilus it was styliform (pen-shaped) and in O. augusti it was lanceolate (lance-shaped).
The walking legs (second to fifth pair of appendages or limbs) were generally undifferentiated, long, narrow and without spines except in the distal end. The swimming legs (sixth pair of appendages) were inconspicuous, very narrow and ending in a very long spike-like ninth podomere (leg segment). The swimming legs of O. augusti were easily recognizable from the other species by the prolongation of the eighth podomere into two lateral projections (parts of the body that protrude) and by the curved shape and great length of the spike-like podomere. The body ornamentation of Onychopterella, composed of small pointed scales, was the same in all species.
History of research[]
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, collected at the Waterlime Group at Kokomo, Indiana, in the United States. They noticed differences between the new species and E. remipes, the type species of Eurypterus, such as the proportions of the carapace, the shorter telson and the size and general form of the body. Only two pairs of appendages were described, suggesting the rest broke away during preservation, although the outline of a pair of unusually large swimming legs was reported. Miller and Gurley considered the preserved parts sufficiently similar to assign the species to Eurypterus. FMNH UC6638, an almost complete specimen, was designated as the type specimen.
In 1912, paleontologists John Mason Clarke and Rudolf Ruedemann considered E. kokomoensis different enough from other Eurypterus species and raised it to the subgeneric level under the new name Onychopterus on the basis that E. kokomoensis was the development path of other eurypterids such as Dolichopterus, Drepanopterus and Stylonurus from Eurypterus. They also noted the swimming legs were better preserved than stated in the original description, noticing four pairs of appendages and not two. The name Onychopterus is composed by the Ancient Greek words ὄνῠξ (ónyx, "claw"), and πτερόν (pteron, "wing"), therefore translated as "claw wing". They also described E. ranilarva as a close relative of O. (Eurypterus) kokomoensis, differing from the latter mainly in the greater width of the carapace. 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. He also synonymized E. ranilarva with O. kokomoensis as he considered that the greater width was due only to differences of the preservation. 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. Størmer substituted the old name with a new one, Onychopterella.
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 from the Edgewood Limestone near Essex, Illinois, in the United States. E. pumilus was placed within Onychopterella in 1948 by Kjellesvig-Waering, who indicated that by its slender appendages without spines and the form of the telson, it clearly belonged to this genus. 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. The only known specimen of O. pumilus is deposited at the University of Illinois, but its accession number is unknown. It has been debated whether O. pumilus really belongs to this genus. In 1999, professor and paleobiologist Roy E. Plotnick considered that the species represented a form of Drepanopterus, while geologist and paleobiologist James C. Lamsdell suggested in 2012 that it belonged to Stoermeropterus because of the similarities of the metastoma, genital appendage and telson. A 1995 study led by paleontologist Simon J. Braddy proposed that because of its small size, O. pumilus could simply represent an ontogenetic stage (a different developmental stage of the same animal throughout its life) of O. kokomoensis. Due to the lack of a known accession number for the fossil, its re-examination is impossible.
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. It is based on two specimens of which one preserves soft tissues. 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. It differed from the rest of the species by the lack of large epimera in the pretelson, wider body proportions, the short length of the postabdomen and telson, the lanceolate form of the latter, the two projections of the eighth podomere and in a distal spine longer than in the rest of the species. The paratype, GSSA C427, is the largest known specimen. O. augusti was also compared to the enigmatic Silurian eurypterid Marsupipterus sculpturatus, concluding that the differences between the telson (the only known part of Marsupipterus) of both species are probably preservational.
Classification[]
Onychopterella is classified as part of its own family, Onychopterellidae, the only clade (taxonomic "group") within the monotypic superfamily Onychopterelloidea. It was originally described as a subgenus of Eurypterus, but it was recognized as a distinct genus in 1948.
It has been suggested that Onychopterella could represent a paraphyletic genus, that is, a grouping sharing a last common ancestor but not including all descendants of this ancestor, and therefore an invalid genus. 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. They also considered Onychopterella the most basal (primitive) member of the suborder Eurypterina (in which the sixth appendage is modified in a swimming leg), mainly because of the narrowness of the swimming leg and its distal spine, resulting in a better model for a primitive swimming leg than other derived members of Eurypterina such as Dolichopterus, ending in a large plate. The species O. pumilus was not included in the analysis because of its possible affiliation with Drepanopterus.
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. He also erected the new superfamily Onychopterelloidea and family Onychopterellidae, placing within the latter the genera Onychopterella and Tylopterella. This family was characterized by the presence of spines in the second to fourth pairs of appendages, lack of spines in the fifth and sixth (except occasionally a distal spine in the last podomere of the sixth appendage), the shape of the carapace with lateral eyes and a lanceolate or styliform telson, among other features. 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. 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.
The cladogram below is based on a larger study (simplified to only show eurypterids) in a 2011 phylogenetic analysis carried out by Lamsdell, showcasing the basal members of the Eurypterina suborder of eurypterids with other derived groups.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||