Acherontiscus is an extinct genus of stegocephalians that lived in the Early Carboniferous (Mississippian era) of Scotland. The type and only species is Acherontiscus caledoniae, named by paleontologist Robert Carroll in 1969. Members of this genus have an unusual combination of features which makes their placement within amphibian-grade tetrapods uncertain. They possess multi-bone vertebrae similar to those of embolomeres, but also a skull similar to lepospondyls. The only known specimen of Acherontiscus possessed an elongated body similar to that of a snake or eel. No limbs were preserved, and evidence for their presence in close relatives of Acherontiscus is dubious at best. 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. Adelospondyls are traditionally placed within the group Lepospondyli due to their fused vertebrae (although Acherontiscus is an exception among adelospondyls). Some analyses published since 2007 have argued that adelospondyls such as Acherontiscus may not actually be lepospondyls, instead being close relatives or members of the family Colosteidae. This would indicate that they evolved prior to the split between the tetrapod lineage that leads to reptiles (Reptiliomorpha) and the one that leads to modern amphibians (Batrachomorpha). Members of this genus were probably aquatic animals that were able to swim using snake-like movements.

Acherontiscus is known only from a single skeleton, RSM 1967/13/1, which is housed at the Royal Scottish Museum in Edinburgh. Although it is known that this specimen was discovered in 1964, additional information on the location of its discovery is not known. 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. The slab also includes remains of tiny crustaceans known as ostracods, as well as pollen spores. The ostracods were identified as the late Paleozoic genus Carbonita, although different scientists studying the slabs disagree on the precise species of Carbonita. The pollen spores correspond to species of plants which lived approximately between the late Viséan and middle Namurian (Serpukhovian) ages of the Carboniferous.

This holotype skeleton was mostly complete, but poorly preserved. The skull was flattened and some of the surface was eroded, while the vertebrae were mostly missing, with only detailed impressions remaining. 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. The specimen received a formal description and name as the species Acherontiscus caledoniae courtesy of Robert Carroll in 1969. The generic name Acherontiscus is a reference to Acheron, a river which in Greek mythology flowed into the underworld as a tributary of the river Styx. This naming convention is an homage to Edward Drinker Cope's affection for naming snake-like lepospondyls after infernal rivers, such as Phlegethontia (named after Phlegethon) and Cocytinus (named after Cocytus). The specific name, caledoniae, references Caledonia, the Latin name for Scotland.

Acherontiscus was serpentine in general body shape, with an elongated body and relatively small head in comparison. It may have been completely legless due to a lack of preserved limb bones. Although this hypothesis seems probable, Acherontiscus probably had limbed ancestors considering that it possessed a well-developed dermal shoulder girdle. It was small in size; the only known specimen was about 14 centimeters (5.5 inches) long. However, it may have been slightly longer considering that part of the tail is believed to be missing.

The skull is robust, with small orbits (eye holes) set towards the front of the head. Although erosion and overlap makes it difficult to distinguish individual bones of the skull, certain ones can be identified. The tip of the snout contained tiny external nares (nostril holes) preceded by premaxillary bones and followed by unusually small lacrimal and nasal bones. 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.

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. Perhaps the least controversial were the jugal and postorbital bones, which were found by most paleontologists who studied the specimen. Some authors, such as Carroll (1969), Carroll & Kuhn (1998), and Ruta et al. (2003) identified a postfrontal bone in front of the postorbital, but CT data from Clack et al. (2019) argued that this bone was actually the postorbital, which was followed by a supratemporal bone. Above those bones were a pair of parietals which overlooked much longer bones that reached the rear face of the skull. These bones included the squamosal and quadratojugal bones (near the jaw), and possibly one or more additional bones. The rear edge of the skull was smoothly convex, with no unusual embayments such as the otic notch of temnospondyls and other "labyrinthodonts". The bones at the rear part of the head are covered with shallow pits, while bones located further forward are smoother. Certain bones near the orbits possessed sensory grooves known as lateral lines.

The skull of Acherontiscus was generally similar to that of microsaurs such as Microbrachis and Cardiocephalus. Some have argued that this similarity is enhanced further due to how possibly only a single bone formed the temporal region of the skull (between the bones of the rear skull roof and those of the jaw area). Most other early tetrapods have two to three distinct bones in that area: the supratemporals, tabular bones, and sometimes the intertemporals. 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). However, many studies since then identified it as the tabular bone. According to Andrews & Carroll (1991), adelospondyls possess an even more extreme reduction in the number of skull bones in this area. Their tabular bones fuse to the squamosal bones under them, creating a bone termed a 'squamosotabular'. Although the original description of Acherontiscus depicted the squamosal as separate from the tabular, Ruta et al. (2003) interpreted the skull differently. According to their interpretation, Acherontiscus also possessed the fused squamosotabular of adelogyrinids, supporting its status as an adelospondyl. A 2010 dissertation by David Marjanović claimed that the squamosotabular of adelospondyls was simply a typical squamosal bone, with the tabular being completely lost. On the other hand, Clack et al. (2019) once again found multiple bones above the squamosal in Acherontiscus, namely a large postparietal (or a pair of postparietals) edged by small tabulars.

The lower jaw was deep yet tapered towards the front, and contained at least 18 preserved teeth. Four teeth near the back of the jaw were large, blunt, and ridged, while most of the rest were much smaller. The upper jaw's teeth were smaller and more numerous, with at least 32 shared between the maxilla and premaxilla. Some teeth near the rear of the maxilla were somewhat enlarged. The differing numbers of upper and lower jaw teeth is also observed in colosteids. However, Acherontiscus' teeth lacked the maze-like internal folding of enamel which gave "labyrinthodonts" (including colosteids) their name. Bones of the hyoid apparatus have also been found near the skull.