Adelophthalmus

Adelophthalmus is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Adelophthalmus have been discovered in deposits ranging in age from the Early Devonian to the Early Permian, which makes it the longest lived of all known eurypterid genera, with a total temporal range of over 120 million years. Adelopthtalmus was the final genus of the Eurypterina suborder of eurypterids and consisted the only known genus of swimming eurypterids from the Middle Devonian until its extinction during the Permian, after which the few surviving eurypterids were all walking forms of the suborder Stylonurina. Fossils of Adelophthalmus have been described from four continents; North America, Europe, Asia, Australia, which indicates that Adelophthalmus might have had a nearly cosmopolitan (worldwide) distribution, one of few eurypterid genera to achieve one besides potentially Pterygotus. The territorial expansion of Adelophthalmus had begun early, with representatives found in both Siberia and Australia during the Devonian, but it first gained its almost cosmopolitan distribution following the amalgamation of the supercontinent Pangaea during the Carboniferous and Permian. The generic name Adelophthalmus means 'no obvious eye', referencing that the holotype fossil seemingly represented an eyeless eurypterid, with a carapace (head plate) completely lacking any indication of eyes. Though this has caused much subsequent confusion, including the naming of several junior synonyms, the apparent eyelessness of the type specimen is treated by modern researchers as a preservational artifact, and not a feature that any species of Adelophthalmus would have possessed in life. Adelophthalmus was a genus of comparatively small eurypterids, with species ranging in size from 4 cm (1.6 in, A. douvillei) to 32 cm (12.6 in, A. khakassicus). As of 2018, Adelophthalmus is the most taxonomically diverse of all eurypterid genera, containing 32 species considered valid. This large amount of species, many named long ago, have prompted some researchers to designate Adelophthalmus as a 'wastebasket taxon' with poorly known internal relationships and phylogeny. The genus as it is currently seen may form a monophyletic group (a group sharing a common ancestor) but might most appropriately be split into different genera along distinct clades formed within the current confines of the genus. Adelophthalmid eurypterids such as Adelophthalmus were small and streamlined nektonic (actively swimming) eurypterids with prominent cuticle sculptures (ornamentation consisting of small, minute, scales across their backs). These scales are perhaps the most distinguishing feature of the group, though similar scales have been reported in other eurypterid groups, most notably the pterygotids, as well. Though the largest adelophthalmid, Adelophthalmus was, in comparison to larger apex predatory members of the group (such as Jaekelopterus), a genus of relatively small eurypterids. The largest species of Adelophthalmus known, A. khakassicus, reached a maximum length of approximately 32 cm (12.6 in). Many species were smaller, the smallest being the Permian A. douvillei at just 4 cm (1.6 in) in length. The genus as a whole does not appear to have fluctuated much in size over the course of its long evolutionary history, with 'large' species occurring in the Devonian (A. sievertsi at 18 cm, 7 in, and A. waterstoni at 15 cm, 6 in), the Carboniferous (the aforementioned A. mazonensis, A. wilsoni at 20 cm, 7.9 in, and both A. granosus and A. zadrai at 15 cm, 6 in) and during the Permian (A. luceroensis at 18 cm, 7 in). Most of the smaller species are known from the Carboniferous, when Adelophthalmus was the most abundant, including the 'medium-sized' A. irinae (13 cm, 5.1 in) and A. moyseyi (12 cm, 4.7 in) and the smaller A. mansfieldi, A. pennsylvanicus (both at 8 cm, 3.1 in), A. approximatus (7 cm, 2.8 in) and A. dumonti (6 cm, 2.4 in). Like most eurypterids (with some exceptions, such as Slimonia and Rhinocarcinosoma), the carapace (the segment covering the prosoma, the 'head') of Adelophthalmus was parabolic in shape, with a narrow marginal rim (edge). The carapace was held in place with the aid of a small and hinged triangular 'locking' mechanism placed anteriorly. The eyes were reniform (bean-shaped) and the small ocelli were located between, or slightly behind (depending on the species), the larger eyes. The metastoma (a large plate part of the abdomen) of Adelophthalmus was oval in shape, with the first opisthosomal (the opisthosoma refers to all segments after the carapace, essentially the abdomen) having a reduced length and being tapered laterally. The body of Adelophthalmus ended with a long and sharp styliform telson (the posteriormost segment, here in the shape of a spike). The feature that primarily distinguishes Adelophthalmus from other adelophthalmid eurypterids is its elongated body and the spurs present on its abdominal segments. The status of the 34 names (out of which two are synonyms) listed below follow a 2018 survey by German paleontologists Jason A. Dunlop and Denise Jekel and British paleontologist David Penney and size- and temporal ranges follow a 2009 study by American paleontologists James Lamsdell and Simon J. Braddy unless otherwise noted.