Jaekelopterus rhenaniae

Jaekelopterus is a genus of predatory eurypterid, a group of extinct aquatic arthropods. Fossils of Jaekelopterus have been discovered in deposits of Early Devonian age, from the Pragian and Emsian stages. There are two known species: the type species J. rhenaniae from brackish to fresh water strata in the Rhineland, and J. howelli from estuarine strata in Wyoming. The generic name combines the name of German paleontologist Otto Jaekel, who described the type species, and the Greek word πτερόν (pteron) meaning "wing".
Based on the isolated fossil remains of a large chelicera (claw) from the Klerf Formation
 of Germany, J. rhenaniae has been estimated to have reached a size of
around 2.3–2.6 metres (7.5–8.5 ft), making it the largest arthropod ever
 discovered, surpassing other large arthropods such as fellow
eurypterids Acutiramus and Pterygotus and the millipede Arthropleura. J. howelli was much smaller, reaching 80 centimetres (2.6 ft) in length.
In overall appearance, Jaekelopterus is similar to other pterygotid eurypterids, possessing a large, expanded telson
 (the hindmost segment of the body) and enlarged pincers and forelimbs.
Both species of Jaekelopterus were first described as species of the
closely related Pterygotus but were raised as a separate genus based on
an observed difference in the genital appendage. Though this feature has
 since proved to be a misidentification, other features distinguishing
the genus from its relatives have been identified, including a telson
with a triangular shape and a different inclination of the denticles of
the claws.
The chelicerae and compound eyes of Jaekelopterus indicate it was active and powerful with high visual acuity, most likely an apex predator in the ecosystems of Early Devonian Euramerica. Although eurypterids...

Jaekelopterus is the largest known eurypterid and the largest known arthropod to have ever existed. This was determined based on a chelicera (claw) from the Emsian Klerf Formation of Willwerath, Germany,
 that measures 36.4 centimetres (14.3 in) long, but is missing a quarter
 of its length, suggesting that the full chelicera would have been 45.5
centimetres (17.9 in) long. If the ratio of body length to chelicera
length matches those of other giant pterygotids, such as Acutiramus and Pterygotus,
 where the ratio between claw size and body length is relatively
consistent, the organism that possessed the chelicera would have
measured between 233 and 259 centimetres (7.64 and 8.50 ft) in length.
With the chelicerae extended, another metre would be added to this
length. This estimate exceeds the maximum body size of all other known
giant arthropods by almost half a metre even if the extended chelicerae
are not included.
Jaekelopterus is similar to other pterygotid eurypterids in its overall morphology, distinguished by its triangular telson
 (the hindmost segment of its body) and inclined principal denticles on
its cheliceral rami (the moving part of the claws). The pterygotids, a
group of highly derived
 ("advanced") eurypterids, differ from other groups in several features,
 especially in the chelicerae and the telson. The chelicerae of the
Pterygotidae are enlarged and robust, clearly adapted for active prey
capture, with chelae (pincers) more similar to the claws of some modern crustaceans,
 with well-developed teeth on the claws, relative to the chelicerae of
other eurypterid groups. Another feature distinguishing the group from
other eurypterid groups is their flattened and expanded telsons, likely
used as rudders when swimming.
J. howelli, known from over 30
specimens, has an almost identical pattern of denticulation on the
chelicerae as J. rhenaniae and also preserves a flattened posterior
margin of the telson, which results in a triangular shape, as in J.
rhenaniae. Its serrated telson margin and the massive elongation of the
second…

Jaekelopterus was originally described as a species of Pterygotus, P. rhenaniae, in 1914 by German palaeontologist Otto Jaekel based on an isolated fossil pretelson (the segment directly preceding the telson) he received that had been discovered in Lower Devonian deposits of the Rhineland
 in Germany. Jaekel considered the pretelson to be characteristic of
Pterygotus, other discovered elements differing little from previously
known species of that genus, such as P. buffaloensis, and he estimated
the length of the animal in life to be about 1 metre (1.5 metres if the
chelicerae are included, 3.3 and 4.9 ft).
Based on more comprehensive material, including genital appendages, chelicerae and fragments of the metastoma (a large plate that is part of the abdomen) and telson discovered by German palaeontologist Walter R. Gross near Overath,
 Germany, Norwegian palaeontologist Leif Størmer provided a more
comprehensive and detailed description of the species in 1936. Størmer
interpreted the genital appendages as being segmented, distinct from
other species of Pterygotus.
British palaeontologist Charles D.
Waterston erected the genus Jaekelopterus in 1964 to accommodate
Pterygotus rhenaniae, which he considered sufficiently distinct from
other species of Pterygotus to warrant its own genus, primarily due to
the abdominal appendages of Jaekelopterus being segmented as opposed to
those of Pterygotus. Waterston diagnosed Jaekelopterus as a pterygotid
with segmented genital appendages, a trapezoid prosoma,
 narrow and long chelicerae with terminal teeth almost at right angles
to the rami and the primary teeth slightly angled anteriorly and with a
telson with an expanded terminal spine and dorsal keel. The generic
name honours Otto Jaekel, the Greek word πτερόν (pteron) meaning "wing", is a common epithet in eurypterid names.

Jaekelopterus is classified within the family Pterygotidae in the superfamily Pterygotioidea.
 Jaekelopterus is similar to Pterygotus, virtually only distinct in
features of its genital appendage and potentially its telson. The close
similarities between the two genera have prompted some researchers to
question if the pterygotids are oversplit
 on the generic level. Based on some similarities in the genital
appendage, American palaeontologists James C. Lamsdell and David A. Legg
 suggested in 2010 that Jaekelopterus, Pterygotus and even Acutiramus
could be synonyms of each other. Though differences have been noted in
chelicerae, these structures were questioned as the basis of generic
distinctions in eurypterids by Charles D. Waterston in 1964 since their
morphology is dependent on lifestyle and varies throughout ontogeny
 (the development of the organism following its birth). Whilst telson
morphology can be used to distinguish genera in eurypterids, Lamsdell
and Legg noted that the triangular telson of Jaekelopterus might still
fall within the morphological range of the paddle-shaped telsons present
 in Pterygotus and Acutiramus. Genital appendages can vary even within
genera; for instance, the genital appendage of Acutiramus changes from
species to species, being spoon-shaped in earlier species and then
becoming bilobed and eventually beginning to look similar to the
appendage of Jaekelopterus. Lamsdell and Legg concluded that an
inclusive phylogenetic analysis with multiple species of Acutiramus,
Pterygotus and Jaekelopterus is required to resolve whether the genera
are synonyms of each other.
The cladogram below is based on the nine best-known pterygotid species and two outgroup taxa (Slimonia acuminata and Hughmilleria socialis).