Carcinosoma (meaning "crab body") is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Carcinosoma are restricted to deposits of late Silurian (Late Llandovery to Early Pridoli) age. Classified as part of the family Carcinosomatidae, which the genus lends its name to, Carcinosoma contains seven species from North America and Great Britain.
Carcinosomatid eurypterids had unusual proportions and features compared to other eurypterids, with a broad abdomen, thin and long tail and spined and forward-facing walking appendages. They were not as streamlined as other groups but had considerably more robust and well developed walking appendages. In Carcinosoma, these spined walking appendages are thought to have been used to create a trap to capture prey in. The telson (the posteriormost division of the body) of Carcinosoma appears to have possessed distinct segmentation, Carcinosoma is the only known eurypterid to possess this feature.
At 2.2 meters (7.2 ft) in length, the species C. punctatum is the largest carcinosomatoid eurypterid by far and is among the largest eurypterids overall, rivalling the large pterygotid eurypterids (such as Jaekelopterus) in size. Other species of the genus were considerably smaller, with most ranging from 70 centimeters (2.3 ft) to 100 centimeters (3.3 ft) in length.
Carcinosoma was among the largest eurypterids, with isolated fossil remains consisting of a 12.7 centimeters (5.0 in) long metastoma (a plate overlaying the coxae of the first six appendages) of the species C. punctatum indicating a full length of 2.2 meters (7.2 ft). Fossil prosomal appendages (appendages attached to the head) referred to the species could possibly increase this estimate to an overall length of around 2.5 meters (8.2 ft).[1] This massive size makes C. punctatum the largest of all known carcinosomatoid eurypterids and it rivals the largest pterygotid eurypterids, such as the 2.5-meter (8.2 ft) long Jaekelopterus, in size.[1][2] Other species of Carcinosoma were smaller, most being in the range of 70 centimeters (2.3 ft) to 100 centimeters (3.3 ft) in length.[2]
Perhaps the most recognizable features of Carcinosoma are its spined appendages and its broad and flattened mesosoma (the first six segments of its abdomen). Carcinosomatid eurypterids such as Carcinosoma had less streamlined bodies than those of some other groups, notably the highly streamlined pterygotid eurypterids. In contrast, the walking legs of the carcinosomatids were in general more robust and better developed.[3][4] Indeed, the walking legs (the second to fifth pair of appendages) were stout and strong and increased in size anteriorly, from the fifth to third pair of appendages, though the first pair of appendages were much shorter than the following pairs. As such, the second pair of walking legs were the longest. Each walking appendage possessed long and curved spines, often two such spines occurring per joint.[4] These spines are one of the defining features of the carcinosomatid family, along with that the swimming legs have slightly elongated and expanded seventh and eighth podomeres (leg segments).[3]
The body of Carcinosoma was somewhat oddly proportioned in comparison to other eurypterids, though similar to that of related carcinosomatids (particularly Eusarcana). The preabdomen (frontal part of the body) was broad and ovally shaped whilst the postabdomen (the posterior part of the body) was narrow and cylindrical. The prosoma (head) was subtriangular in shape with the small compound eyes placed at the front. The metasoma of Carcinosoma was covered in fine and elongated scales and was quite flat, a feature which separates the genus from Eusarcana where the metasoma was almost cylindrical.[4]