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 (29.5 in). It was characterised by a robust exoskeleton with scattered tubercles or semicircular scales. The prosoma (head) was subquadrate, protruding antemedially. The chelicerae (claws in front of the mouth) were small. The two first pairs of legs of Mixopterus (appendages II and III) were highly specialized and not used for walking, instead being highly developed with long paired spines. The two following pairs (appendages IV and V) were moderately sized and typically spiniferous walking legs. The last legs, (VI), form a pair of swimming legs characteristic of the Eurypterida. The preabdomen, the front portion of the body, was narrow with axial furrows, while the postabdomen was narrow. The telson was a curved spine. By thrusting and pushing the tail, the telson spine would be capable of stinging prey in between the front legs. The structure of the tail of indicates that Mixopterus would be able to move on land, but such movement would likely be difficult because of its weight. Respiration would not be an issue, since moist gills would be well concealed above the ventral plates. Mixopterus might have buried itself in the sand to wait for prey, as the swimming legs were very well-adapted for digging. The frontal legs and foremost portion of the prosoma, including the eyes, would have been kept above the substrate. When a prey was sufficiently close, the frontal appendages would be clasped together, almost forming a cage.