Acherontiscus

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.

== Discovery == 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. left|thumb|328x328px|Skeletal diagram showing preserved bones of the Acherontiscus holotype. Limbs are not depicted in this interpretation. 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.