Carcinosomatidae (the name deriving from the type genus Carcinosoma, meaning "crab body")[1][2] is a family of eurypterids, an extinct group of aquatic arthropods.

Fossils of carcinosomatids have been found in North America, Europe and Asia, the family possibly having achieved a worldwide distribution, and range in age from the Late Ordovician to the Early Devonian.

They were swimming eurypterids (belonging to the suborder Eurypterina), with large swimming paddles, a set of powerful and spiny forelimbs, a broad and rounded preabdomen (central body) and a slender,[4] tubular abdomen,[9] which ended in a telson (the posteriormost division of the body) of variable morphology,[10][11] often curved.

[10] The telson varied considerably between genera: in Rhinocarcinosoma it was robust and flattened, curving slightly upwards,[10] in Eusarcana it was cylindrical and fashioned into a sharp, scorpion-like tail spike[6] and in Carcinosoma it was flattened, ending in an expanded and segmented structure unseen in other eurypterids.

[11] The earliest carcinosomatid species to be described was Carcinosoma punctatum, first described under the name Pterygotus punctatus by John William Salter in 1859.

[15] The earliest genus later seen as a carcinosomatid to be described was Eusarcus (and its type species E. scorpionis), described by August R. Grote and William Henry Pitt in 1875 based on fossils recovered from the Pridoli-age Buffalo Waterlime of New York State.

[4] Also described in the late 19th century was the genus Eurysoma, named alongside its type species, E. newlini, by Edward Waller Claypole in 1890.

Størmer also introduced the family Carcinosomatidae, initially under the name 'Carcinosomidae', in 1934, to contain the four genera Carcinosoma, Mixopterus, Echinognathus and Megalograptus.

In 1942, Embrik Strand proposed another replacement name for Eusarcus, Eusarcana, despite the matter having been dealt with by Størmer eight years prior.

[10] When revising the carcinosomatids in 1964, Kenneth Edward Caster and Erik N. Kjellesvig-Waering recognised Eusarcus and Carcinosoma to be distinct genera, determining the 1912 synonymisation to have been erroneous.

[20] The known geographical range of the carcinosomatids was considerably extended with the discovery of Rhinocarcinosoma fossils in Vietnam in the late 20th century,[21] named as the species R. dosonensis in 2002.

[4] A 2015 phylogenetic analysis by Lamsdell and colleagues recovered Holmipterus, a problematic eurypterid genus of uncertain affinities, as a basal carcinosomatid.

[15] This lifestyle is especially exemplified in Rhinocarcinosoma, where the shovel-like protrusion at the front of its carapace may have been used for digging, or "mud-grubbing", and the swimming paddles were reduced in size compared to those of other carcinosomatids.