Adelophthalmoidea

''. The size of the adelophthalmoid eurypterids ranged from 4 centimetres (1.6 inch) to 32 cm (12.6 in), the smallest species being Nanahughmilleria clarkei and the largest one being Adelophthalmus khakassicus. The adelophthalmoids were relatively small compared to their relatives, such as the gigantic pterygotid Jaekelopterus rhenaniae, which easily exceeded 2 metres (6.5 feet), and was the largest arthropod ever discovered. The adelophthalmoids were small swimming eurypterids with a parabolic (approximately U-shaped) carapace (the dorsal plate of the head, Unionopterus possibly representing an exception) and with intramarginal (occurring within the margin) eyes. The swimming leg (sixth limb) was of Adelophthalmus-type, that is, with a seventh podomere (segments of the limbs) long and an eighth podomere coarsely serrated. The body had a midsection second order differentiation present (that is, with lateral "extensions" in the seventh body segment) and with a lanceolate (lance-shaped) telson (the posteriormost division of the body). The morphology of the group varies depending on how derived (evolutionary advanced) the genus was. In fact, from the basal ("primitive") Nanahughmilleria to the more derived Adelophthalmus, a gradual decrease in the spinosity (possessing spines) of the appendages (limbs) and an increase in the size of the genital spatulae (a long, flat piece in the genital area) occurred. The even more primitive Pittsfordipterus probably did not even possess the latter. ==History of research==

of Adelophthalmus granosus, the first adelophthalmid ever described. The first adelophthalmoid fossils to be uncovered were those of the type genus, Adelophthalmus. The German paleontologist Hermann Jordan collected the first specimen of the species A. granosus at Jägersfreude in Saarland, Germany. The specimen would be described three years later by Jordan and Hermann von Meyer, who immediately recognized the eurypterid nature of the fossils by the great resemblance of the overall shape and form of the carapace and appendages with that of Eurypterus. One of the main differences that Jordan and von Meyer noticed was the apparent lack of eyes, which gives name to Adelophthalmus (meaning "no obvious eyes") and the entire superfamily. This feature is now assumed to be due to a preservational artifact and that was not present in Adelophthalmus, nor in any other adelophthalmoid. Since then, a total of 33 species have been described, some of which have been historically classified within other genera (Anthraconectes, Glyptoscorpius, Lepidoderma and Polyzosternites, all now synonymous with Adelophthalmus), It is possible that the large amount of species in Adelophthalmus will eventually provoke its separation into two or three separate genera. It was named Unionopterus, and its classification is controversial due to the poor illustrations given by Chernyshev and the fact that the only known fossil is presumed to be lost. The genus has been treated as an indeterminate eurypterid, an adelophthalmoid or a member of the genus Adelophthalmus, as well as completely ignored. The spinosity of its appendages may suggest the second option, but this is not entirely certain and the phylogenetic classification of Unionopterus may never be resolved. In 1964, Kjellesvig-Waering described the genital operculum of the species H. (N.) phelpsae and suggested that this part of the body could have great phylogenetic importance in the future. Two years later, together with the American paleontologist Kenneth Edward Caster, he raised H. (N.) phelpsae to the generic level under the name Pittsfordipterus. They also described a new genus and species, Bassipterus virginicus. It differs essentially from other adelophthalmoids by the specialization of its genital appendage, with characteristics reminiscent of Eurypterus. In 1989, Victor P. Tollerton Jr. described the family Adelophthalmidae along with many others, including Adelophthalmus, Parahughmilleria, Bassipterus and Unionopterus. This clade was based on the presence of spines in the second to fifth pair of appendages, a swimming leg of Adelophthalmus-type and the presence of epimera (lateral "extensions" of the segment) in the seventh segment. Tollerton commented that some species of Adelophthalmus that did not have spines in the appendages may be better placed in a new genus in the family Slimonidae (he mentioned the now invalid Slimonioidea). Although a new genus for spineless species could be phylogenetically supported, moving it to Slimonidae based on the loss of a feature which seems to have been lost separately in the two groups is not in line with common practice. This superfamily would be formally described two years later by Tetlie and Peter Van Roy. In a 2025 monograph, James Lamsdell divided the Adelophthalmoidea into four families to reflect the phylogenetic relationships inside the group: Adelophthalmidae (Adelophthalmus, Selkiepterella, Unionopterus, and Wiedopterus), Nanahughmilleriidae (Nanahughmilleria), Parahughmilleriidae (Parahughmilleria and Pruemopterus), and Pittsfordipteridae (Archopterus, Bassipterus, and Pittsfordipterus). Lamsdell transferred Eysyslopterus to the Waeringopteridae.'''''' ==Evolutionary history==

The eurypterids as a group peaked in diversity during the Silurian; of the approximately 250 valid species accounted for as of , around 139 (≈56 %) were from the Silurian alone. The eurypterids were one of the groups most heavily affected by the Late Devonian extinction event, following a major decline in diversity during the Early Devonian, eurypterids were rare in marine environments by the Late Devonian. Of the 16 eurypterid families that had been alive at the beginning of the Devonian, only three persisted into the Carboniferous, all of them non-marine groups. The suborder Eurypterina was rendered almost completely extinct, only surviving Adelophthalmoidea (represented by Adelophthalmus). '', included in the chart, was previously considered an adelophthalmoid but has since been reclassified as a waeringopteroid.|left Adelophthalmus would rapidly diversify, already being present in Siberia and Gondwana (Australia) since the Devonian. In the Carboniferous, the distribution of Adelophthalmus became approximately circumequatorial (around the Equator). and becoming the most common of all eurypterids of the Late Paleozoic. A. sellardsi from the Artinskian (around 290–284 mya, Early Permian) epoch of Kansas, United States, was the last species of Adelophthalmus and therefore of all the suborder Eurypterina. ==Classification==

External phylogeny of Pittsfordipterus phelpsae. The adelophthamids are classified within the infraorder Diploperculata, in the Eurypterina suborder of eurypterids. The infraorder Diploperculata contains the four most derived superfamilies of eurypterine eurypterids; Carcinosomatoidea, Adelophthalmoidea, Pterygotioidea and the waeringopteroids, united by the shared feature that the genital operculum (the structure that contains the genital appendage) is made up of two fused segments. Adelophthalmoidea was one of the most diverse taxonomically eurypterid clades, with about 40 species described. The sister group of Adelophthalmoidea, Pterygotioidea, surpassed this amount with around 50 described species, becoming the most diverse superfamily to date. These sister-clades are the most derived in the Eurypterina and make up more than a third of all the species of eurypterids, with almost 100 species between both of them. The cladogram below is simplified from 2007 study by O. Erik Tetlie, showcasing the position of Adelophthalmoidea within the suborder Eurypterina. Placement of Diploperculata follows Lamsdell et al. 2013. }} }} }} }} }} }} }} }} }}|label1=Eurypterida}} Internal phylogeny All adelophthalmoids have a series of shared characteristics that make them different from the rest of eurypterids. However, some genera developed different features within Adelophthalmoidea that divide the superfamily into several smaller clades and groupings. The genera Parahughmilleria and Adelophthalmus form a derived clade based on the presence of enlarged spines on at least one podomere in the appendage V (fifth limb), the presence of epimera in the postabdomen (body segments 8 to 12) and the large spatulae that has been associated with the genital operculum. Nanahughmilleria is placed as the sister taxon of this clade but more basal due to the increased spinosity of its appendage V and in the small size of the genital spatulae. Bassipterus and Pittsfordipterus are positioned as relatively more basal to this clade and form a group supported by two synapomorphies (shared characteristics different from that of their latest common ancestor); long narrow eyes and a complex termination of the genital appendage. The cladogram below follows Lamsdell (2025). ==Paleoecology==

, Frankfurt. The adelophthalmoids as a whole inhabited environments situated near the coastal realm, with preferences in lagoons, estuaries or deltas, which have reduced salinity. A marine influence is commonly found in horizons (sections of the fossil site) that comprise eurypterids, but marine index fossils (fossils that indicate a marine environment and ecosystem) in many cases are not associated with eurypterids. In some cases, where the adelophthalmoids are very rare, the fossils could have been deposited in a different place from the one they originated, The first adelophthalmoids, for example the Scottish Parahughmilleria hefteri, have been conserved in non-marine brackish-estuarine habitats, with possible tidal influence, although basal forms that inhabited completely marine deposits are known. This preference remained constant in Adelophthalmoidea until Adelophthalmus appeared. In deposits of the Early Devonian of Germany, where Parahughmilleria hefteri and Adelophthalmus sievertsi coexisted, a more obvious marine influence is observed in the sections dominated by Parahughmilleria than in those dominated by Adelophthalmus. This suggests that Adelophthalmus had a preference for habitats dominated by freshwater, unlike other more basal forms. This preference for freshwater habitats is stronger during the Bashkirian and Moscovian times (in the Late Carboniferous), times in which fossils of Adelophthalmus have been associated with freshwater bivalves and terrestrial organisms in coal-bearing strata (indicating a coal swamp environment). However, this could be related to the diversification of Adelophthalmus in those epochs more than an ecological change in the whole genus. Most of the Adelophthalmus species were confined to paralic (in shallow water near the coast) or lowland basins, in depositional environments that had a close connection with marginally marine habitats. For example, during the Moscovian, the Saar-Nahe Basin (where fossils of A. granosus have been found), was connected or even part of a vast western subsiding area (a sunken zone) in whose drainage went towards the Paleo-Tethys Ocean, some 1,500 kilometres (930 miles) towards the south. During the Late Pennsylvanian and Early Permian, due to the southern uplift, the drainage began to go north towards the Panthalassa Ocean, increasing the distance to the ocean (about 2,400 km, 1490 mi). Adelophthalmus is not found in these deposits, while others indicative of a connection with a marginal marine habitat, such as the belinurid xiphosurans, persisted. Some of the last occurrences of the adelophthalmoids (end of the Carboniferous and Early Permian) seem to have had a kind of reversion in ecological preferences, inhabiting environments with a strong marine influence again, the especially tidally estuarine environments. Adelophthalmoidea is considered as a euryhaline (able to adapt in different salinities) clade that inhabited stressful habitats with regard to paleosalinity and possibly oxygen whose conquest of freshwater habitats was not as successful as in other animals such as the aforementioned bellinurids. ==See also==