Slimonia

Slimonia is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Slimonia have been discovered in deposits of Silurian age in South America and Europe. Classified as part of the family Slimonidae alongside the related Salteropterus, the genus contains three valid species, S. acuminata from Lesmahagow, Scotland, S. boliviana from Cochabamba, Bolivia and S. dubia from the Pentland Hills of Scotland and one dubious species, S. stylops, from Herefordshire, England. The generic name is derived from and honors Robert Slimon, a fossil collector and surgeon from Lesmahagow. Out of the four described species of Slimonia, three measured below or up to 20 centimetres (7.9 in) in length. Only S. acuminata was larger, with the largest specimens measuring 100 centimetres (39 in) in length. Though this is large for a predatory arthropod, Slimonia would be exceeded in length by later and more derived (more 'advanced') members of the closely related pterygotid family of eurypterids, which would become the largest known arthropods to ever live. The discovery of a specimen of S. acuminata with a fully articulated series of telsonal and postabdominal (tail) segments in a tight curve suggests that the tail of Slimonia, and potentially that of other eurypterids, was considerably more flexible than previously thought. Studies on the anatomy and hydrodynamics of eurypterid postabdomens, particularly those of the closely related pterygotid eurypterids, have shown that the postabdomen would not have been used for locomotion. Instead, it is possible that the tail and spiked telson (the most posterior segment) would have been used as a weapon for predatory attacks and self-defense. Slimonia is in many ways similar to the more derived (more 'advanced') eurypterids of its superfamily, the Pterygotioidea. In particular, the expanded and flattened telson (the most posterior segment of the body) of Slimonia is similar to that of the pterygotid eurypterids and is a feature that Slimonia and the pterygotids only share with some derived hibbertopterid eurypterids (where the feature convergently evolved). The pterygotid telson was in general slightly larger than that of Slimonia and was more slender. The telson spike of Slimonia was much longer than any seen in the Pterygotidae (constituting just over half of the total telson length) however, serrated and ending in a fine point. The largest species of Slimonia, S. acuminata, reached a maximum length of 100 cm (39 in) whilst the smallest, S. dubia, grew to 12 cm (5 in) in length. Though 100 cm is large for a predatory arthropod, Slimonia would be exceeded in length by later and more derived (more 'advanced') members of the closely related pterygotid family of eurypterids, which would become the largest known arthropods to ever live. Slimonia can be distinguished from other members of its family, the Slimonidae, by a variety of characteristics. The prosoma (head) is quadrate (square-shaped) in shape and had small compound eyes on the frontal corners. The bodies were large and cordate (heart-shaped), with a narrow postabdomen and a telson with a strongly expanded anterior half. The chelicerae (frontal appendages) were small in comparison to those of the pterygotids and the walking legs had denticles, but no spines. Genital appendages were long and narrow in both males and females. The type species of Slimonia, S. acuminata, was first described as a species of Pterygotus, 'Pterygotus acuminata' (acuminata being Latin for 'sharp' or 'tapering'), by John William Salter in 1856, based on fossils recovered from deposits of Llandovery-Wenlock (Early to Middle Silurian) age in Lesmahagow, Scotland. That same year David Page erected a new genus to contain the species, as several distinctive characteristics made the species considerably different from other known species of Pterygotus, among them the shape of the carapace and S. acuminata lacking the large cheliceral claws known from Pterygotus. The generic name is derived from and honors Robert Slimon, a fossil collector and surgeon from Lesmahagow. Slimon was the first to discover eurypterid fossils in Lesmahagow, bringing them to the attention of Roderick Murchison in 1851. S. acuminata remains the largest known species, with the largest specimens measuring up to 100 cm (39 in) in length. In 1899, an additional species, S. dubia, would be referred to the genus. This species was recovered from slightly earlier deposits (Llandovery age) in the Pentland Hills of Scotland and could be distinguished from S. acuminata by the more elongated telson (also not as broad in the parts furthest back), thinner telson spike and a slightly different, tapering, body shape that tapers evenly the whole way instead of suddenly narrowing near the seventh segment as in S. acuminata. The type specimen of S. dubia is a badly preserved carapace, with fragments of various degrees of completion of the first eleven segments found associated. Despite its fragmentary nature, the quadrangular (square) shape of the carapace and the eyes placed at its corners allowed zoologist and paleontologist Malcolm Laurie to place it within Slimonia when describing it in 1899. The size of the carapace suggests that the species would have grown to 12 cm (5 in) in length.