Rhinocarcinosoma
Rhinocarcinosoma is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Rhinocarcinosoma have been discovered in deposits ranging of Late Silurian age in the United States, Canada and Vietnam. The genus contains three species, the American R. cicerops and R. vaningeni and the Vietnamese R. dosonensis. The generic name is derived from the related genus Carcinosoma, and the Greek ῥινός (rhinós, "nose"), referring to the unusual shovel-shaped protrusion on the front of the carapace (head plate) of Rhinocarcinosoma, its most distinctive feature.
Other than the protrusion, Rhinocarcinosoma was anatomically very similar to its close relative, Eusarcana, though it lacked the scorpion-like telson (the posteriormost division of the body) of that genus. Further distinguishing features include more slender appendages and slightly different ornamentation of scales. In terms of size, Rhinocarcinosoma was a medium-sized carcinosomatid eurypterid, with the largest species, R. vaningeni, reaching lengths of 39 centimetres (15.4 in). In contrast to other carcinosomatids, Rhinocarcinosoma is not known only from marine settings, but also from deposits that were once lakes or rivers. It was adapted to a bottom-dwelling lifestyle, as either a burrowing or digging scavenger or top predator, feeding on other invertebrates and small fish.
Rhinocarcinosoma was a medium-sized carcinosomatid eurypterid, with the largest species, R. vaningeni, reaching lengths of 39 centimetres (15.4 in) and the second largest, R. dosonensis, reaching lengths of 22 centimetres (8.7 in).[1] In terms of the outline of the body, with a broad abdomen and a nearly tubular postabdomen (tail), Rhinocarcinosoma was similar to the related genus Eusarcana.[2][3] Its appendages, large and with spines, are also similar to those of Eusarcana,[3] though the more anterior (forwards) appendages of Rhinocarcinosoma were more slender than those of Eusarcana.[4] Although historically assumed to have had a scorpion-like telson (the posteriormost division of the body), like Eusarcana,[3][2] more complete fossils of R. dosonensis revealed that this was not the case, although the telson did slightly curve upwards.[4] Like Eusarcana, Rhinocarcinosoma possessed an ornamentation of scales on its carapace (head plate), though the scales of Rhinocarcinosoma were smaller and more closely arranged.[3] The type A genital appendage (female reproductive organ) of Rhinocarcinosoma was broader and more rounded than that of Eusarcana.[4]
The most distinctive feature of Rhinocarcinosoma was the shovel-shaped protrusion developed from the most forward-facing portion of the carapace.[5] It is possible that the snout was used for digging purposes.[3] As of yet, this feature is only confidently known from R. vaningeni, given that the relevant portion of the carapace has not been preserved in adult fossils of R. dosonensis[4] and R. cicerops (which lacks known adult fossils altogether).[3]
All species of Rhinocarcinosoma also share certain other features of the carapace, including the carapace being more or less subtriangular in shape, with a width to length ratio of about 3:2, and the ocellar mound (the raised surface where the ocelli, smaller eyes, were located) being placed centrally and being the highest point of the carapace.[3][4] The species R. cicerops shares the forward and prominent position of the ocelli with R. vaningeni, but only has a slight development of the snout compared to the others.[3] Its fossils are also the smallest of any species, at only 4 centimetres (1.6 in) in length,[1] but all known fossil specimens of R. cicerops are of immature individuals, meaning that it is possible that the adults were even more similar to R. vaningeni.[3] Although formal synonymisation has never been conducted, it is possible that R. cicerops were actually juvenile R. vaningeni.[3][4]
