Adelophthalmoidea (the name deriving from the type genus Adelophthalmus, meaning "no obvious eyes") is a superfamily of eurypterids, an extinct group of aquatic arthropods. Adelophthalmoidea is classified within the infraorder Diploperculata, in the suborder Eurypterina. The superfamily contains four families: Adelophthalmidae, Nanahughmilleriidae, Parahughmilleriidae, and Pittsfordipteridae.
Adelophthalmoid eurypterids were small and swimming eurypterids that appeared in the Silurian period. With the earliest known members of the group, Nanahughmilleria prominens and Parahughmilleria maria, being known from deposits of Early Silurian (possibly the Llandovery epoch) age and the last members, belonging to the long-lasting and widespread genus Adelophthalmus, going extinct in the Early Permian, the Adelophthalmidae is the longest lasting single family of eurypterids. The survival of the group, and of swimming eurypterids (the suborder Eurypterina) beyond the Late Devonian is entirely due to the survival, and subsequent success, of Adelophthalmus throughout the Devonian and Carboniferous. Adelophthalmus (and possibly Unionopterus) represents the only known genus of swimming eurypterids beyond the extinction of the rest of the group in the Late Devonian, extending the temporal range of the group by over a hundred million years.
Though the last swimming eurypterids and the final members of the traditionally more successful and numerous suborder Eurypterina, the adelophthalmoids were not the last eurypterids. The stylonurines or the "walking eurypterids" were the last ones, surviving in the family Hibbertopteridae until the Permian–Triassic extinction event or shortly before a few million years after the extinction of the adelophthalmoids in the Early Permian.
Description
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
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.Jordan, H. & von Meyer, H. 1854. "Ueber die Crustaceen der Steinkohlenformation von Saarbrücken". Palaeontographica 4: 1–15. 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), making Adelophthalmus the most diverse eurypterid genus to date. However, many of these species are fragmentary and could represent synonyms of other species within Adelophthalmus or even species of other genera. It is possible that the large amount of species in Adelophthalmus will eventually provoke its separation into two or three separate genera.
In 1948, the Ukrainian paleontologist and geologist Boris Isidorovich Chernyshev described a new genus known by one single specimen from Kazakhstan (at that time part of the Soviet Union) and tentatively classified it as part of the Pterygotidae family. 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 1961, the American paleontologist Erik Norman Kjellesvig-Waering considered several species of the genus Hughmilleria sufficiently different from the type species to be separated into a new subgenus, which he named Nanahughmilleria. These species shared a key characteristic, small intramarginal eyes of reniform form (bean-shaped), in contrast to the large ovoid eyes placed in the margin of the carapace present in the genotype and its allied forms. Kjellesvig-Waering designated H. (Nanahughmilleria) norvegica as the type species of the new subgenus. At the same time, he erected a new genus, Parahughmilleria, with P. salteri as the type species. Kjellesvig-Waering based its new clade mainly on the presence of supplementary lobes (distal lobes in a plate-like segment which contains the genital aperture called genital operculum), as well as in the intramarginal eyes.
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.
Odd Erik Tetlie in an unpublished thesis of 2004 erected the superfamily Adelophthalmoidea and the family Nanahughmilleridae. Adelophthalmoidea was diagnosed as eurypterids with parabolic carapaces, small reniform eyes, appendages of variable spinosity and a lanceolate telson, among others. This superfamily would be formally described two years later by Tetlie and Peter Van Roy. On the other hand, Nanahughmilleridae was described to contain the adelophthalmoids with no or reduced genital spatulae and the second to fifth pair of prosomal (of the prosoma, "head") appendages of Hughmilleria-type. This family contained Nanahughmileria, Pittsfordipterus and perhaps Parahughmilleria. However, the clade was almost never used in subsequent studies and lists of eurypterids, and instead, they classified the nanahughmillerids as part of Adelophthalmidae.
In 2008, Nanahughmilleria patteni was recognized as a different and much more basal species, and therefore a new genus, Eysyslopterus, was named by Tetlie and Markus Poschmann. The carapace of Eysyslopterus and other basal members of the closely related Pterygotioidea (Herefordopterus) and the waeringopteroids (Orcanopterus) has been shown as almost identical, only differing between them by the position of the eyes. Due to the intramarginal position of the eyes, Eysyslopterus has been classified within Adelophthalmidae, but it has also been suggested that it is the sister taxon (closest relative) of a clade formed by Adelophthalmoidea and Pterygotioidea. However, this can not be demonstrated until more fossil material is found.
In 2023, Archopterus anjiensis, a new genus and species, was described based on a single specimen found in the Ordovician-aged Wenchang Formation of Zhejiang, China, representing the oldest known record of the Adelophthalmidae, extending the stratigraphic record of the family some 10 million years older from the early Silurian into the late Ordovician.
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. Many eurypterid groups are first recorded from the Silurian, such as Pterygotioidea, Mycteropoidea, Stylonuroidea and Adelophthalmoidea itself. The most primitive members of Adelophthalmoidea evolved in Laurussia (an ancient supercontinent, also known as Euramerica). In fact, the most basal species of the clade so far (Eysyslopterus patteni) has been recovered from Ludlovian (around 427–423 mya) deposits of the paleocontinent Baltica (Scandinavia and Eastern Europe, precisely Estonia). However, it is not possible to determine where the clade originated, probably in Baltica or Laurentia (most of eastern continental North America). Although most of the representatives of Adelophthalmoidea have been found in Laurentia, Avalonia (Germany, Great Britain, parts of eastern North America) and Baltica (that is, Laurussia), the nearly cosmopolitan (worldwide) genus Adelophthalmus was also present in the Rheno-Hercynian Terrane (western and central Europe), Siberia and in the Gondwana part of the current Australia.