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. They were members of the superfamily Carcinosomatoidea, also named after Carcinosoma. 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 among the most marine eurypterids, known almost entirely from marine environments.
Carcinosomatids varied considerably in size, from species only a few centimetres in length to some of the largest known arthropods. The largest carcinosomatid species, Carcinosoma punctatum, reached lengths of at least 2.2 metres (7.2 ft) and rivalled the largest eurypterid of all, Jaekelopterus, in size. Morphologically, carcinosomatids were highly distinct from other eurypterids, known for their powerful and spiny set of forelimbs, a broad and rounded central body and a slender and tubular tail ending in a telson (the posteriormost division of the body) that was typically curved in some way. With these adaptations, the carcinosomatids were quite similar to scorpions, and the group may have helped contribute to the common name of eurypterids becoming 'sea scorpions'. The family contains four, possibly five, genera: Carcinosoma, Eocarcinosoma, Eusarcana, Rhinocarcinosoma and possibly the problematic genus Holmipterus.
It is unlikely that the carcinosomatids were strong and active swimmers, given their non-streamlined shape. It is more probable that they were nektobenthic (swimming near the bottom), possibly being top predators (given their size) or scavengers, digging for food or perhaps even burrowing and lying in wait as ambush predators.
Carcinosomatid eurypterids differed considerably in size depending on the genus and species,[3] though most species were quite large.[4] The largest species was Carcinosoma punctatum at 2.2 metres (7.2 ft), one of the largest eurypterids of all, with some specimens suggesting that it may even have reached lengths of 2.5 metres (8.2 ft), rivalling Jaekelopterus, the largest eurypterid, in size.[3][5] The smallest carcinosomatid species was Eusarcana obesus, at 4 centimetres (1.5 in) in length.[3][a]
Morphologically, the carcinosomatids were highly distinct among the eurypterids. 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.[4][10] In a sense, the carcinosomatids were rather scorpion-like in appearance,[4][12] and may have contributed to the common name of eurypterids having become 'sea scorpions'.[4]
There was considerable variety in morphology within the group. The carapace was triangular to subtriangular in shape in all members of the group, through the exact shape could vary.[7] In Rhinocarcinosoma, there was a distinctive, shovel-shaped protrusion at the front of the carapace.[13] The preabdomen was wide in all species, but the width also differed from species to species. The widest species, relatively speaking, was Eusarcana obesus, in which the fourth segment was as wide as the first eight segments combined were long.[14] The spinosity (how many spines) and size of the forelimbs also varied from genus to genus, with the forelimbs of Eusarcana for instance being more powerful than those of Rhinocarcinosoma.[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]