Jaekelopterus

Jaekelopterus is a genus of predatory eurypterid, a group of 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 meters (7.5–8.5 ft), which would make 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 centimeters (2.6 ft) in length.

In overall appearance, Jaekelopterus is very similar to other pterygotid eurypterids, possessing a large, expanded telson (the posteriormost segment of the body) and enlarged pincers and forelimbs. Both species of Jaekelopteruswere first described as species of the closely related Pterygotus but were raised as a separate genus based on observed differences 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 an active, powerful predator with high visual acuity, and it likely represented an apex predator in the ecosystems of Early Devonian Euramerica. Although eurypterids such as Jaekelopterus are often called "sea scorpions", the strata in which Jaekelopterus fossils have been found suggest that it did not dwell in marine environments proper.