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.

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 Greekword πτερόν (pteron) …

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.

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 …

Gigantism The pterygotid eurypterids include many of the largest known eurypterids, such as Pterygotus and Acutiramus. Several factors have been suggested that might have contributed to the unprecedented large size of Jaekelopterus, its relatives and other large Paleozoic invertebrates, such as predation, courtship behaviour, competition and environmental resources.

Factors such as respiration, the energy costs of moulting, locomotion and the actual properties of the exoskeleton restrict the size of arthropods. Other than the robust and heavily sclerotised claws, most of the preserved large body segments of the pterygotids are thin and unmineralised. Even tergites and sternites (the plates that form the surfaces of the abdominal segments) are generally preserved as paper-thin compressions, suggesting that pterygotids were very lightweight in construction. Similar lightweight adaptations can be observed in other Paleozoic giant arthropods, such as the giant millipede-like Arthropleura, and it has been suggested to be vital for the evolution of giant arthropod sizes. A lightweight build decreases the influence of factors that restrict body size.

Despite being the largest arthropods, the lightweight build of Jaekelopterus and other giant pterygotid eurypterids meant that they likely were not the heaviest. Other giant eurypterids, particularly the deep-bodied walking forms in the Hibbertopteridae, such as the almost 2-metre-long Hibbertopterus, may have rivalled the pterygotids and other giant arthropods in weight, if not surpassed them.

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 Jaekleopterus 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. 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 cheliceral morphology and visual acuity of the pterygotid eurypterids separates them into distinct ecological groups. The primary method for determining visual acuity in arthropods is by determining the number of lenses in their compound eyes and the interommatidial angle (IOA), which is the angle between the optical axes of adjacent lenses. The IOA is especially important as it can be used to distinguish different ecological roles in arthropods, being low in modern active arthropod predators.

Both Jaekelopterus rhenaniae and Pterygotus anglicus had high visual acuity, as indicated by the low IOA and many lenses in their compound eyes. 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 scorpionsand crustaceans. Some puncture wounds on fossil…