Adelospondyli is an order of elongated, presumably aquatic, Carboniferous amphibians (sensu lato). They have a robust skull roofed with solid bone, and orbits located towards the front of the skull. The limbs were almost certainly absent,[1] although some historical sources reported them to be present. Despite the likely absence of limbs, adelospondyls retained a large part of the bony shoulder girdle. Adelospondyls have been assigned to a variety of groups in the past. They have traditionally been seen as members of the subclass Lepospondyli, related to other unusual early tetrapods such as "microsaurs", "nectrideans", and aïstopods.[1][2][3] Analyses such as Ruta & Coates (2007) have offered an alternate classification scheme, arguing that adelospondyls were actually far removed from other lepospondyls, instead being stem-tetrapod stegocephalians closely related to the family Colosteidae.[4]
Most adelospondyls belong to the family Adelogyrinidae, and prior to 2003 the order and family were considered synonymous. In 2003, Ruta et al. assigned Acherontiscus to the order as the only known non-adelogyrinid member.[2] Members of this group are very rare; only six known specimens can be assigned to the five known genera with absolute confidence. These specimens are known from Mississippian (Serpukhovian Age) geological deposits in Scotland, and they were among the oldest "lepospondyls" known from fossils.[2]
Adelospondyls share a variety of traits with other lepospondyls, although whether these traits are an example of convergent evolution is a controversial topic. Like the aïstopods and lysorophian "microsaurs", they had very elongated bodies similar to that of snakes and eels. In addition, they lacked limbs (similarly to the aïstopods), although forelimbs were supposedly found in various adelogyrinids in the late 1960s. Andrews & Carroll (1991) found that all cases of forelimb bones in adelogyrinids were actually misinterpretations. For example, putative forelimbs discovered in Adelogyrinus and Palaeomolgophis by Brough & Brough (1967) were re-identified as hyoid bones and ribs, respectively. Carroll (1967) also claimed that forelimbs were present in Adelospondylus, but these later considered to be hyoids as in Adelogyrinus.[1]
Adelospondyls had somewhat long and low skulls, with large orbits (eye sockets) shifted towards the front of the skull. As a result, the proportions of the skull bones were affected. For example, the nasal bones along the midline of the skull in front of the eyes were much shorter than the frontal bones directly behind them, which extended forwards past the level of the eyes. The frontal bones were omitted from the edge of the orbits (eye holes) due to a contact between the pre- and post-frontals which typically lie in front of and behind the orbits, respectively. Unlike lysorophians and aïstopods, which had snake-like skulls with large openings and reduced bone material, adelospondyl skulls were strongly built and covered with ridges, pits, and grooves, including lateral line sulci. They typically possessed many teeth, although such teeth differ in structure between families. Adelogyrinids, for example, had many numerous "chisel-shaped" teeth,[1] while Acherontiscus had blunt teeth at the back of the mouth and sharp, thin teeth at the front.[5] As is the case in other lepospondyls, the teeth of adelospondyls did not have a maze-like internal structure like those of "labyrinthodonts", nor did adelospondyls possess enlarged fang-like teeth on the roof of the mouth.[1]