Jaekelopterus rhenaniae ("Otto Jaekel's wing from the Rhineland") was a species of the extinct arthropod group Eurypterida. At an estimated length of 2.5 metres (8 ft 2 in), it is one of the two largest arthropods ever discovered (the other is a giant millipede-like animal, Arthropleura, although which animal was larger is unclear). The second largest eurypterid known is a species of Pterygotus. Jaekelopterus lived approximately 390 million years ago.
Although the fossil has been called a "sea scorpion", the strata in which it was found suggest that the living animal dwelled in fresh-water rivers and lakes, rather than in saltwater seas. The animal was described in November 2007 by Simon Braddy and Markus Poschmann of the University of Bristol in the journal Biology Letters: they found a 46-centimetre (18 in) chelicera (claw-like mouth part), and estimated the total size of the animal based on the proportions of this claw.[1][2][Note 1] When extended, the chelicerae would have added 1 metre (3 ft 3 in) to its length.[1]
The animal's fossilised remains were discovered in the Lower Devonian (Emsian) Klerf Formation Lagerstätte of Willwerath near Prüm, Germany.