Mixopterus kiaeri

Mixopterus is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Mixopterus have been discovered in deposits from Late Silurian age, and have been referred to several different species. Fossils have been recovered from two continents; Europe and North America.

Mixopterus was a medium-sized predatory eurypterid. The largest species, M. simonsoni, reached lengths of 75 cm. It was characterised by a robust exoskeleton with scattered tubercles or semicircular scales. The prosoma was subquadrate, protruding antemedially. The chelicerae were small. The two first pairs of legs of Mixopterus were highly specialized and not used for walking, instead being highly developed with long paired spines.

Locomotion Eurypterids with swimming legs were capable of both walking on the bottom of bodies of water or swimming through them. Unlike some highly derived eurypterines, Mixopterus is not thought to have been a good swimmer and it likely kept near the bottom. During swimming, the prosomal

Reproduction Mixopterus preserves a long ventro-medially placed genital appendage. Mating in Mixopterus would likely be similar to that in horseshoe crabs. There is a presumed clasping organ on appendage II shaped as a flat and round lobee with a blade-shaped flat spine behind, overall similar.

Tracks attributed to Mixopterus have been discovered in fossil deposits in Ringerike, Norway. The tracks, referred to the ichnogenus Merostomichnites, were made by an arthropod in which only three pairs of legs took part in the gait, the last pair being swimming legs. Swimming legs that constitute the last limbs of the prosoma are present in eurypterine eurypterids, out of which the only reasonably large taxa present in the silurian deposits of Ringerike are Mixopterus kiaeri and Pterygotus holmi.

Mixopterus contains three valid species, with some others that historically have been assigned to it being recovered as outside of the genus. The species currently seen as valid species of the genus are