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. Adelopththalmus 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 and 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 2020, Adelophthalmus is the most taxonomically diverse of all eurypterid genera, containing 33 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.
Description
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.
Table of species The status of the 35 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. SpeciesAuthorYearStatusLengthTemporal rangeNotes & descriptionAdelophthalmus approximatusHall & Clarke1888Valid7 cmFamennian (Devonian)Originally described as a species of Eurypterus. A. approximatus is similar to A. mansfieldi, the spikes running alongside the abdomen being very similar. A. approximatus can be differentiated from A. mansfieldi by the lack of differentiation between the first three pairs and the last pair of the endognathites being less distinct.Adelophthalmus asturicaMelendez1971Valid? cmMoscovian (Carbonferous)Originally described as a species of the synonymized genus Lepidoderma. The posterior swimming paddles of A. asturica were particularly large.Adelophthalmus bradorensisBell1922Valid Possible synonym of A. imhofi? cmKasimovian (Carboniferous)Originally described as a species of Anthraconectes. A. bradorensis is most similar to A. kidstoni, differing only in being proportionally shorter and not possessing indentation around the edges of the exoskeleton as A. kidstoni does.Adelophthalmus caledonicusPeach1882Synonym of A. perornatus––Originally described as a species of the synonymized genus Glyptoscorpius. Designated as a synonym of A. perornatus, though was noted as being significantly different in it possessing "comb organs" by British paleontologist Charles D. Waterston in 1958.Adelophthalmus cambieriPruvost1930Valid Possible synonym of A. imhofi? cmBashkirian (Carboniferous)Originally described as a species of the synonymized genus Anthraconectes.Adelophthalmus carbonariusChernyshev1933Uncertain? cmBashkirian – Kasimovian (Carboniferous)Originally described as a species of Eurypterus. A. carbonarius differed from A. luceroensis in the proportions of the body. Its carapace had a length/width ratio similar to the average value of A. luceroensis specimens of the same size class. The same ratio of the length of the metasoma to length of the mesosoma was somewhat larger than in A. luceroensis, while the ratio of the length of the prosoma to length of the mesosoma was smaller than in the latter.Adelophthalmus chinensisGrabau1920Valid? cmLatest Carboniferous – Asselian (Permian)Originally described as a species of the synonymized genus Anthraconectes.Adelophthalmus cornetiPruvost1939Valid Possible synonym of A. imhofi? cmBashkirian (Carboniferous)Originally described as a species of the synonymized genus Anthraconectes.Adelophthalmus douvilleide Lima1890Valid4 cmAsselian – Sakmarian (Permian)Originally described as a species of Eurypterus.Adelophthalmus dubiusShpinev2012Valid Possible synonym of A. kamyshtensis18 cmMiddle DevonianA slightly larger than medium-sized species, A. dubius is incredibly poorly known, the only known specimen lacking the eyes, the appendages and even the abdominal spines otherwise always present in Adelophthalmus. The species is most similar to A. mazonensis (differing in size and having a relatively longer prosoma), A. moyseyi (differing in size and having a narrower prosoma), A. nebraskensis (differing in size and having a wider prosoma), A. wilsoni (differing in a smaller and narrower prosoma) and A. zadrai (differing in size). A. dubius, A. khakassicus and A. kamyshtensis might represent synonyms.Adelophthalmus dumontiStainier1915Valid6 cmMoscovian (Carboniferous)Originally described as a species of Eurypterus. Very similar to A. mansfieldi, similar in overall shape and proportions and in the pattern of the ornamentation. They differ in A. dumonti having a broader carapace, larger eyes, a more slender thorax and the characteristic spikes running along its abdomen pointing backwards (in A. mansfieldi they point backwards and outwards).Adelophthalmus granosusJordan1854Valid, type species15 cmMoscovian (Carboniferous)A. granosus had relatively broad proportions. With the only known specimen lacking eyes and appendages, its status as diagnostic is somewhat questionable. It is possible that the large abdominal spikes of A. granosus are on the sternites (a feature shared only by A. nebraskensis) and not on the tergites as in other species, but this feature may also simply be due to deformation.Adelophthalmus imhofiReuss1855Valid? cmMoscovian (Carboniferous)Originally described as the type species of the synonymized genus Lepidoderma.Adelophthalmus irinaeShpinev2006Valid13 cmTournaisian (Carboniferous)Adelophthalmus khakassicusShpinev & Filimonov2018Valid Possible synonym of A. kamyshtensis32 cmGivetian (Devonian)A. khakassicus is similar to A. mazonensis, A. moysei and A. sellardsi. It differs from all these species in a narrower mesosoma and in a wider metasoma. It had undeveloped epimera in the seventh, eleventh and twelfth segments (being in these last two segments flattened and leaf-like). A. khakassicus, A. kamyshtensis and A. dubius might represent synonyms.Adelophthalmus kamyshtensisShpinev2012Valid15 cmMiddle DevonianA medium-sized and poorly known species, A. kamyshtensis can be distinguished from most other species by its first segment being narrower relative to the other segments of the mesosoma. The characteristic abdominal spikes were present in the last segment of the mesosoma and all the metasomal segments. Compared to other species, A. kamyshtensis is most similar to A. luceroensis, A. sellardsi, A. imhofi, A. granosus, A. mazonensis, A. wilsoni and A. sievertsi. A. kamyshtensis, A. khakassicus and A. dubius might represent synonyms.Adelophthalmus kidstoniPeach1888Valid Possible synonym of A. imhofi? cmMoscovian (Carboniferous)Originally described as a species of the synonymized genus Glyptoscorpius. A. kidstoni is similar to A. bradorensis but possessed indentations around the edges of its exoskeleton, a feature that separates it from all other known species of Adelophthalmus. A. kidstoni was considerably different from A. wilsoni, a contemporary species from the same location.Adelophthalmus lohestiDewalque1889Uncertain