Adelophthalmidae (the name deriving from the type genus Adelophthalmus, meaning "no obvious eyes") is a family of eurypterids, an extinct group of aquatic arthropods.

The size of the adelophthalmid eurypterids ranged from 4 centimetres (1.6 inch) to 32 cm (12.6 in),[1] the smallest species being Nanahughmilleria clarkei and the largest one being Adelophthalmus khakassicus.

[2] The adelophthalmids 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 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.

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).

[17] 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.

Adelophthalmoidea was diagnosed as eurypterids with parabolic carapaces, small reniform eyes, appendages of variable spinosity and a lanceolate telson, among others.

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.

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).

The unequivocally oldest representative was P. hefteri, with fossils found in the Kip Burn Formation, Lesmahagow, Scotland, from the beginning of the Wenlockian epoch (around 433–427 mya).

[11] In the Emsian (around 393–408 mya, in Early Devonian), the earliest species of Adelophthalmus appeared, A. sievertsi, presenting basal features such as the wide swimming leg (as in Nanahughmilleria and Parahughmilleria).

[3] During the Late Carboniferous and Early Permian, Adelophthalmus lived in brackish and freshwater environments adjacent to coastal plains, a type of common and stable habitat at the time.

They began to disappear due to a climatic change that caused alterations of depositional and vegetational patterns across the world, provoking a decrease in number of the genus.

[25] 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.

The Russian hibbertopterid species Campylocephalus permianus persisted until the Changhsingian (around 254–252 mya, Late Permian) stage, being the last known eurypterid.

[27] Stylonurina Megalograptoidea Eurypteroidea Carcinosomatoidea Waeringopteroidea Adelophthalmoidea Hughmilleria Herefordopterus Slimonia Pterygotidae All adelophthalmids have a series of shared characteristics that make them different from the rest of eurypterids.

Nanahughmilleria is placed as the sister taxon of this clade but more basal due to the increased spinosity of its appendage V[11] and in the small size of the genital spatulae.

[3] 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.

[11] Orcanopterus Waeringopterus Grossopterus Eysyslopterus Bassipterus Pittsfordipterus Nanahughmilleria Parahughmilleria Adelophthalmus Hughmilleria Herefordopterus Slimonia Erettopterus Pterygotus Acutiramus Jaekelopterus The adelophthalmids as a whole inhabited environments situated near the coastal realm, with preferences in lagoons, estuaries or deltas, which have reduced salinity.

In some cases, where the adelophthalmids are very rare, the fossils could have been deposited in a different place from the one they originated,[11] exemplified by A. waterstoni, which is known from a single specimen that is in turn the only eurypterid of the zone, the Gogo Formation of Australia, where more than 2,000 crustaceans have been found.

[28] The first adelophthalmids, 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.

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.