Carcinosomatidae
Introduction '''Carcinosomatidae''' (the name deriving from the type genus ''Carcinosoma'', meaning "crab body") 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  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.
Description Reconstruction of ''Carcinosoma''Carcinosomatid eurypterids differed considerably in size depending on the genus and species, though most species were quite large. The largest species was ''Carcinosoma punctatum'' at , one of the largest eurypterids of all, with some specimens suggesting that it may even have reached lengths of , rivalling ''Jaekelopterus'', the largest eurypterid, in size. The smallest carcinosomatid species was ''Eusarcana obesus'', at 4 centimetres (1.5 in) in length.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, tubular abdomen, which ended in a telson (the posteriormost division of the body) of variable morphology, often curved. In a sense, the carcinosomatids were rather scorpion-like in appearance, and may have contributed to the common name of eurypterids having become 'sea scorpions'.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. In ''Rhinocarcinosoma'', there was a distinctive, shovel-shaped protrusion at the front of the carapace. 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. 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''. The telson varied considerably between genera: in ''Rhinocarcinosoma'' it was robust and flattened, curving slightly upwards, in ''Eusarcana'' it was cylindrical and fashioned into a sharp, scorpion-like tail spike and in ''Carcinosoma'' it was flattened, ending in an expanded and segmented structure unseen in other eurypterids.
History of research Carapaces of ''Rhinocarcinosoma'', as figured by John Mason Clarke and Rudolf Ruedemann in 1912The earliest carcinosomatid species to be described was ''Carcinosoma punctatum'', first described under the name ''Pterygotus punctatus'' by John William Salter in 1859.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. The description of the genus was lacking and seemingly based only on the outline and shape of the fossil, which led Henry Woodward to refer ''E. scorpionis'' to ''Eurypterus'' on the grounds that it was similar in shape to ''Eurypterus punctatus'' (''Pterygotus punctatus'' having been reclassified as a species of ''Eurypterus''). Unbeknownst to Grote and Pitt, ''Eusarcus'' had already been named as a genus of extant (currently living) laniatorid harvestmen of the family Gonyleptidae, in 1833 and as such constituted a preoccupied name. The name being preoccupied went unnoticed until the 1930s. Also described in the late 19th century was the genus ''Eurysoma'', named alongside its type species, ''E. newlini'', by Edward Waller Claypole in 1890.When Claypole discovered later in 1890 that the name was preoccupied by a genus of modern beetles, he replaced the name ''Eurysoma'' with the name ''Carcinosoma''.In 1912, John Mason Clarke and Rudolf Ruedemann declared that the differences between ''Eusarcus'' and all related forms of eurypterids were so great that it was "entirely evident" that ''Eusarcus'' was distinct from other eurypterids. Clarke and Ruedemann referred several new species to ''Eusarcus'', including new species that would later be seen as species of the genus ''Rhinocarcinosoma'', and also concluded that ''Eusarcus'' was sufficiently similar to ''Carcinosoma'' to be synonymised. Because ''Eusarcus'' had been named earlier than ''Carcinosoma'', the taxonomical laws of priority dictated that ''Eusarcus'' would be the name of the taxon. ''Eusarcus'' was finally recognised as a preoccupied name by Leif Størmer in 1934.Størmer substituted the name for the next oldest available non-preoccupied synonym, ''Carcinosoma''. Størmer also introduced the family Carcinosomatidae, initially under the name 'Carcinosomidae', in 1934, to contain the four genera ''Carcinosoma'', ''Mixopterus'', ''Echinognathus'' and ''Megalograptus''. The family was amended by Erik N. Kjellesvig-Waering in Størmer's 1955 ''Treatise on Invertebrate Paleontology'', with the name changed to the correct Carcinosomatidae and the genera other than ''Carcinosoma'' transferred to their own families (''Mixopterus'' to the Mixopteridae and ''Megalograptus'' and ''Echinognathus'' to the Megalograptidae). In 1942, Embrik Strand proposed another replacement name for ''Eusarcus'', ''Eusarcana'', despite the matter having been dealt with by Størmer eight years prior. ''Rhinocarcinosoma'' was split off from ''Carcinosoma'' in 1962 by Nestor Ivanovich Novozhilov, based on its carapace being different from that of other ''Carcinosoma''.Comparison of the sizes of the three largest species of the genera ''Carcinosoma'', ''Eusarcana'' and ''Rhinocarcinosoma'' and a humanWhen 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. Since ''Eusarcus'' was preoccupied, Caster and Kjellesvig-Waering, likely unaware of Strand's ''Eusarcana'', coined the replacement name ''Paracarcinosoma'' for the species previously referred to ''Eusarcus''. Also in 1964, Caster and Kjellesvig-Waering named the new genus ''Eocarcinosoma'' to account for Ordovician specimens of ''Eusarcus''/''Paracarcinosoma''. Though most of those specimens have since been identified as pseudofossils, the type specimen of ''Eocarcinosoma'' is an authentic fossil and the earliest record of the family. The known geographical range of the carcinosomatids was considerably extended with the discovery of ''Rhinocarcinosoma'' fossils in Vietnam in the late 20th century, named as the species ''R. dosonensis'' in 2002.Though ''Paracarcinosoma'' was frequently used by later researchers, ''Eusarcana'', named earlier, was recognised by Jason A. Dunlop and James Lamsdell in 2012 as the valid replacement name of ''Eusarcus'', transferring the species assigned to ''Paracarcinosoma'' to that genus and designating ''Paracarcinosoma'' as a junior synonym. A 2015 phylogenetic analysis by Lamsdell and colleagues recovered ''Holmipterus'', a problematic eurypterid genus of uncertain affinities, as a basal carcinosomatid. The position of ''Holmipterus'', on account of incomplete fossil material and an apparent combination of traits from different families, is far from certain within the eurypterid family tree and its fossils may even represent two different genera, mistakenly grouped together.
Classification Mixopterids, such as ''Mixopterus'' (pictured), were the closest relatives of the carcinosomatidsThe carcinosomatids are classified as part of the superfamily Carcinosomatoidea, within the infraorder Diploperculata. The Carcinosomatoidea also contains the families Mixopteridae and Megalograptidae. Carcinosomatidae was previously, from 1989 to the early 2000s, grouped with the family Hughmilleriidae in the superfamily 'Hughmillerioidea', on account of the spined limbs and all limbs, with the exception of the swimming paddles, being of a consistent type. The Hughmilleriidae is today regarded as basal members of the superfamily Pterygotioidea.The internal phylogeny of the Carcinosomatoidea is poorly resolved (unclear). The first cladogram below follows a 2007 study by eurypterid researcher O. Erik Tetlie, which was in turn based on results from various phylogenetic analyses on eurypterids conducted between 2004 and 2007, whereas the second cladogram follows a 2015 study by James Lamsdell and colleagues. Both cladograms have been simplified to only display the Carcinosomatoidea. Tetlie (2007) recovered the Carcinosomatidae as a paraphyletic grouping, accounting for basal members of the Carcinosomatoidea, whereas Lamsdell ''et al''. (2015) recovered the carcinosomatids as a monophyletic group. '''Tetlie (2007)''''''Lamsdell ''et al.'' (2015)'''
Palaeoecology Model of a ''Eusarcana'' on a Silurian seafloorCarcinosomatid eurypterids were among the most marine eurypterids, known from deposits that were once reefs, some in lagoonal settings, and deeper waters. This is in sharp contrast to their closest relatives, the mixopterids, which are not known from deeper waters. The only other eurypterid family known from deeper waters are the pterygotids, which had a similar distribution to the carcinosomatids, albeit more successful. Based on the distribution of the pterygotids, it is possible that carcinosomatids ranged worldwide. They are, alongside the pterygotids, the only eurypterid family known from the southern continent of Gondwana in the Silurian and Devonian. The only carcinosomatid genus known from non-marine deposits is ''Rhinocarcinosoma'' (though it is also known marine deposits), which has been found in fluvial (river) and lacustrine (lake) settings as well.Because of their bodies not being as streamlined as those of many other swimming eurypterids, and on account of the unique morphologies of their telsons, it is considered likely that the carcinosomatids were not very active swimmers, probably adopting a more nektobenthic (swimming near the bottom) lifestyle. 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. Given their size, carcinosomatids may have been top predators or scavengers, digging for food or perhaps even burrowing and lying in wait as ambush predators. They may have fed on worms, other arthropods, brachiopods and fish, using their forelimbs to push food into their mouths.
See also * List of eurypterid genera* Timeline of eurypterid research* Megalograptidae* Mixopteridae
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