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.

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.

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.

[6] 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 fossil was immediately recognized by Jordan as that of a eurypterid, with both the overall shape and form and the individual parts (particularly the head and the appendages) being very similar to those of Eurypterus which had been described in the United States in 1825, 29 years earlier.

Among the differences noted between the specimens were the smaller size and later age of the Saarbrücken fossil and what Jordan and von Meyer perceived to be a complete lack of eyes.

[45] Though modern researchers tend to treat the assumed eyelessness as a preservational artifact and not a feature that A. granosus would have had in life, this issue was not resolved immediately which made the naming of subsequently discovered species confusing and problematic.

The descriptor, Austrian paleontologist August Emanuel von Reuss, noted that Lepidoderma likely was synonymous with Adelophthalmus, but ignored the rules of taxonomical priority and used his younger name due to it being based on material that he considered to be better preserved.

[14] 1924 saw the description of the species Anthraconectes sellardsi by American paleontologist Carl Owen Dunbar based on two incomplete fossils and few other small fragments from Elmo in Kansas.

889, was collected in the Czech Republic in 1930 or 1931 and first mentioned in a manuscript by French Carboniferous worker Pierre Pruvost, who dubbed it "Eurypterus (Anthraconectes) Zadrai", but he did not formally describe the specimen or taxon.

[10] All synonymous genera; Anthraconectes, Glyptoscorpius, Lepidoderma and Polyzosternites, were subsumed into Adelophthalmus in studies during the middle twentieth century, notably that of Belgian paleontologist Fredrik Herman van Oyen (1956).

[10] The species A. luceroensis was described by American paleontologists Barry S. Kues and Kenneth K. Kietzke in 1981 based on 150 fossil specimens recovered from the Madera Formation of New Mexico.

Poschmann also referred the species Rhenopterus waterstoni (described earlier in 2004 based on the singular specimen BMNH In 60174 from the Late Devonian of Australia) to Adelophthalmus.

The fossils, from the Tournaisian Solomennyi Stan Formation, could confidently be assigned to Adelophthalmus based on their scalelike ornamentation, the position of their eyes and the shape of the carapace shortly after their excavation.

[59] In 2020, Lamsdell, Victoria E. McCoy, Opal A. Perron-Felle and Melanie J. Hopkins described a new species of Adelophthalmus from the Tournaisian stage of (most likely) the Lydiennes Formation, in France.

The good preservation of A. pyrrhae allowed researchers to examine parts of its respiratory system, and after their study it was confirmed that even if they had a mostly aquatic lifestyle, the eurypterids could venture on to land for long periods.

Baltica would later collide with the continents Avalonia and Laurentia and form the landmass Euramerica, where most of basal adelophthalmid evolution took place in the Early Devonian.

[61] The earliest known species of Adelophthalmus is A. sievertsi from Early Devonian (Emsian) deposits of the Klerf Formation in Wilwerath (in the Rhineland-Palatinate), Germany, then part of Avalonia within Euramerica.

[57][62] By the Late Devonian, Adelophthalmus had already become widespread, with the species A. waterstoni having been recovered from deposits of Frasnian (~382.2 to 372.2 million years old) age in the Gogo Formation of Western Australia, the only eurypterid with the exception of Acutiramus and Pterygotus known from the continent.

[6] The abundance of the bivalve Anthracomya suggests strong evidence of freshwater deposition in the habitat of A. bradorensis, a Radstockian (Upper Westphalian) species from Canada.

The first stage of the period, the Asselian (from 298.9 to 295 million years ago), saw the appearance of A. luceroensis in New Mexico, USA, A. douvillei in Bussaco, Portugal and the continued survival of the Carboniferous Chinese species A. chinensis.

Climate conditions favorable for the spread and maintenance of such environments were optimal during the Late Carboniferous and Early Permian, with Adelopthalmus being widespread and numerous in these times.

A changing climate during the Permian altered depositional and vegetation patterns across the northern hemisphere, which drastically affected previously widespread environments such as the signature Carboniferous coal forests as well as brackish and fresh water habitats.

Whilst some stylonurine eurypterids (Hastimima and Campylocephalus) that occupied niches outside of these habitats continued to survive for a time, Adelophthalmus, restricted to a rapidly disappearing type of environment, became extinct.

[34] The results of the analysis showed that all the genera featured (including Adelophthalmus), with the exception of Nanahughmilleria, where the basal species N. patteni was assigned to the new genus Eysyslopterus, were (or had the potential to be) monophyletic.

In particular, A. imhofi was suggested by Fredrik Herman van Oyen in 1956 to possibly represent a senior synonym of many species, including A. zadrai, A. corneti, A. cambieri, A. pruvosti, A. bradorensis, A. kidstoni, A. wilsoni and A. sellardsi.

[63] In general, post-Devonian eurypterids are rare and occur in habitats of brackish or fresh water, having migrated from the marginal marine environments inhabited during the Silurian.

Indeed, these coal swamp Adelophthalmus seem to form a minority, with most species being confined to paralic or lowland basins in depositional environments with close connections to marginally marine habitats.

Instead, the brackish of fresh water environments typically inhabited by Adelophthalmus, such as the Early Permian Madera Formation in New Mexico (where fossils of A. luceroensis have been recovered) preserve other organisms, such as insects, branchiopods, ostracods, millipedes and spirorbid worms.

The lack of large coal beds suggests that the fossil localities which have yielded Adelophthalmus was a moderately elevated region with less dense vegetation and better drainage than the swamplands that occupied much of the rest of the United States.