[3] Phylogenetic analyses created by Marcello Ruta and other paleontologists in the 2000s indicate that Acherontiscus is part of Adelospondyli, closely related to other snake-like animals such as Adelogyrinus and Dolichopareias.

However, the rock slab in which it was preserved is a type known as coal shale, similar to that of early Carboniferous (Mississippian)-era ironstone from Burghlee in Midlothian.

[1] The pollen spores correspond to species of plants which lived approximately between the late Viséan and middle Namurian (Serpukhovian) ages of the Carboniferous.

By dissolving away remaining fragments with hydrochloric acid casting these impressions in silicone rubber, the preparators of the specimen were able to more easily describe it.

[2] In order to accommodate for the small size of the nasals, the frontals and adjacent prefrontal bones are elongated, occupying the length of almost the entire upper side of the snout.

[1][8] The part of the skull behind the eyes is composed of several bones which were difficult to interpret due to crushing and differing hypotheses about their arrangement and naming.

Prior to the 1970s, most studies of microsaurs often considered the single bone they retain to be the supratemporal (including in the original 1969 description of Acherontiscus).

[3] Although the original description of Acherontiscus depicted the squamosal as separate from the tabular, Ruta et al. (2003) interpreted the skull differently.

[4] A 2010 dissertation by David Marjanović claimed that the squamosotabular of adelospondyls was simply a typical squamosal bone, with the tabular being completely lost.

However, Acherontiscus' teeth lacked the maze-like internal folding of enamel which gave "labyrinthodonts" (including colosteids) their name.

However, a well-developed dermal shoulder girdle (consisting of clavicles and an interclavicle) is preserved, indicating that the ancestors of Acherontiscus possessed forelimbs.

[8] The dermal shoulder girdle is represented by a plate-like interclavicle (which would have been at the center of a living animal's chest) as well as two clavicle bones branching up from it.

As a whole, the lack of known limb and endochondral material yet the retention of a dermal shoulder girdle is a condition similar to that of the other family of adelospondyls, the adelogyrinids.

[1] The classification of Acherontiscus has gone through much revision in the past, as it shows a mixture of characteristics from various groups of non-amniote tetrapods (amphibians in the broad definition of the term).

Labyrinthodonts can be characterized by their maze-like tooth enamel, the presence of multiple bones forming each vertebral segment, and a generally crocodile-like appearance.

Modern analyses have concluded that the order "Labyrinthodontia" is composed of various groups of non-amniote stegocephalians scattered within and near the tetrapod family tree.

For example, its eel-like and potentially legless body was similar to members of the groups Lysorophia, Aïstopoda, and Adelospondyli, while its skull and teeth closely resembled those of several microsaurs such as Microbrachis.

However, the presence of multiple bones of near-equal size composing each vertebra was more similar to the condition in labyrinthodonts, specifically members of the group Embolomeri.

The authors of this study re-examined the specimen, providing new interpretations of the crushed skull bones, such as the presence of a squamosotabular and a postorbital which did not contact the orbit.

The snake-like body, highly reduced or absent limbs, and large pectoral girdle were also general features supporting the placement of Acherontiscus among the adelospondyls.

Vallin & Laurin (2004) considered adelospondyls to be closely related to aïstopods due to their similar body types and the fact that they are among the oldest known lepospondyls.

[13] The following cladogram is a portion of the strict consensus tree of Ruta et al. (2003), showing Lepospondyli as monophyletic and Acherontiscus as a member of Adelospondyli.

An MPT is a family tree with the fewest possible number of evolutionary 'steps' where sampled anatomical traits are acquired, lost, and reacquired.

A strict consensus tree is a compromise between MPTs which retains the broad shape of every MPT while not being specific enough to closely resemble a single one.

[4] Batropetes Tuditanus Pantylus Stegotretus Asaphestera Saxonerpeton Hapsidopareion Micraroter Pelodosotis Rhynchonkos Cardiocephalus Euryodus Microbrachis Hyloplesion Odonterpeton Brachydectes Acherontiscus Adelospondylus Adelogyrinus Dolichopareias Scincosauridae Diplocaulidae Urocordylidae Aistopoda If Lepospondyli is indeed polyphyletic, then its constituents must find new locations on the family tree of stegocephalians.

Traits which are prone to being convergently evolved are emphasized less during "steps" in the family trees, while traits that are unique to only a few groups are emphasized more:[5] Ventastega Acanthostega Ichthyostega Ossinodus Tulerpeton Whatcheeriidae Crassigyrinus Colosteus Greererpeton Acherontiscus Adelogyrinus Adelospondylus Dolichopareias Baphetoidea Temnospondyli (including Lissamphibia) Caerorhachis Silvanerpeton Embolomeri Gephyrostegus Bruktererpeton Utegenia Discosauriscus Ariekanerpeton Leptoropha Microphon Seymouria Kotlassia Solenodonsaurus Diadectomorpha (including Amniota) Westlothiana Lepospondyli

This study, Bernardi et al. (2016), noted that specializations in advanced amphibians may pollute the relations of more basal taxa when all of them are placed in a single phylogenetic analysis.

In order to test this possibility, the study separated prehistoric stegocephalians into five groups depending on their age: Devonian (D), Mississippian (M), Pennsylvanian (P), Permian (R), and Mesozoic (Z).

[13] Noting the large, blunt, and ridged teeth of the lower jaw, Clack et al. (2019) argued that Acherontiscus was durophagous, meaning that it fed on hard-shelled invertebrates.

Some of its prey items could have included aquatic molluscs and crustaceans, especially ostracods, which were common in the rock slab which preserved the specimen.