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Jaekelopterus (Otto Jaekel's wing) is an extinct genus of sea scorpion. Jaekelopterus lived approximately 390 million years ago. At an estimated length of 2.5 metres (8.2 feet), it is the largest known arthropod ever discovered, surpassing the Pterygotus and even the millipede-like Arthropleura. There are two species, one being the type species, J. rhenaniae, from freshwater strata in the Rhineland, and J. howelli from estuarine strata in Wyoming.


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.

History Of research[]

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 at Alken 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.

In 1974, Størmer erected a new family to house the genus, Jaekelopteridae, due to the supposed considerable differences between the genital appendage of Jaekelopterus and other pterygotids. This diverging feature has since been proven to simply represent a misinterpretation by Størmer in 1936, the genital appendage of Jaekelopterus in fact being unsegmented like that of Pterygotus. As such, the family Jaekelopteridae has subsequently been rejected and treated as synonymous with the family Pterygotidae.

Another species of Pterygotus, P. howelli, was named by American palaeontologist Erik Kjellesvig-Waering and Størmer in 1952 based on a fossil telson and tergite (the dorsal part of a body segment) from Lower Devonian deposits of the Beartooth Butte Formation in Wyoming. The species name howelli honours Dr. Benjamin Howell of Princeton University, who loaned the fossil specimens examined in the description to Kjellesvig-Waering and Størmer. This species was assigned to Jaekelopterus as Jaekelopterus howelli by Norwegian palaeontologist O. Erik Tetlie in 2007.


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). Jaekelopterus had previously been classified as a basal sister taxon to the rest of the Pterygotidae since its description as a separate genus by Waterston in 1964 due to its supposedly segmented genital appendages (fused and undivided in other pterygotids), but restudy of the specimens in question revealed that the genital appendage of Jaekelopterus also was undivided. The material examined and phylogenetic analysis conducted by British palaeontologist Simon J. Braddy, German palaeontologist Markus Poschmann and O. Erik Tetlie in 2007 revealed that Jaekelopterus was not a basal pterygotid, but one of the most derived taxa in the group. The cladogram also contains the maximum sizes reached by the species in question, which was suggested to possibly have been an evolutionary trait of the group per Cope's rule ("phyletic gigantism") by Braddy, Poschmann and Tetlie.


The morphology and body construction of Jaekelopterus and other eurypterids in the Pterygotidae suggests they were adapted to a completely aquatic lifestyle. Braddy, Poschmann and Tetlie considered in a 2007 study that it was highly unlikely that an arthropod with the size and build of Jaekelopterus would be able to walk on land. Eurypterids such as Jaekelopterus are often popularly referred to as "sea scorpions", but the deposits from which Jaekelopterus fossils have been discovered suggest that it lived in non-marine aquatic environments. The Beartooth Butte Formation in Wyoming, where J. howelli fossils have been discovered, has been interpreted as a quiet, shallow estuarine environment. This species has been found together with two other eurypterid species: Dorfopterus angusticollis and Strobilopterus princetonii. The fossil sites yielding J. rhenaniae in the Rhineland have also been interpreted as having been part of a shallow aquatic environment with brackish to fresh water.

The chelicerae of Jaekelopterus are enlarged, robust and have a curved free ramus and denticles of different lengths and sizes, all adaptations that correspond to strong puncturing and grasping abilities in extant scorpions and crustaceans. Some puncture wounds on fossils of the poraspid agnathan fish Lechriaspis patula from the Devonian of Utah were likely caused by Jaekelopterus howelli. The latest research indicates that Jaekelopterus was an active and visual predator. Fully grown Jaekelopterus would have been apex predators in their environments and likely preyed upon smaller arthropods (including resorting to cannibalism) and early vertebrates.

A powerful and active predator, Jaekelopterus was likely highly agile and possessed high maneuverability. The hydromechanics of the swimming paddles and telsons of Jaekelopterus and other pterygotids suggest that all members of the group were capable of hovering, forward locomotion and quick turns. Though they were not necessarily rapidly swimming animals, they were likely able to give chase to prey in habitats such as lagoons and estuaries.