The Aïstopods may be splitting apart

Those long, limbless amphibians,
the Aïstopoda, were once (in the 1920s) the oldest known tetrapods, known from Westphalian (310 mya) strata. Of course, since the publication of Ichthyostega (1932) and the rest of the Devonian tetrapods, that’s old news. Baird 1964 wrote: “The remarkable specialization [in aîstopods] already achieved by the early Mississippian implies an origin well back in Devonian time; a tetrapod ancestry rather than direct derivation from the crossopterygians fishes is indicated. Relationships of the order are obscure.” 

Interesting, that 1964 comment,
as some of the aïstopods continue to nest traditionally with lepospondyls, but others now nest with paratetrapods, closer to crossopterygian fishes.

Aistopods are traditionally considered lepospondyls
because the three parts of each vertebrae are fused to become one. Today three aîstopods were added to the large reptile tree (LRT, 967 taxa). Of those three, two did not nest within the Lepospondyli, or within the Tetrapoda.

Ophiderpeton and Oestocephalus
(Fig. 1) nests with Acherontiscus, within the Lepospondyli and within the Tetrapoda in the LRT. The orbits were far forward and the temples were fenestrated, narrowing the parietal. The supratemporal, tabular, jugal and squamosal and quadratojugal are all reduced, but still present.

Phlegethontia
Phlegethontia longissima (above; CGH 129)  Phlegethontia linnearis (below; Cope 1871, Anderson 2002, Fritsch 1875; Huxley and Wright 1867) was considered a aîstopod, but it does not nest with Ophiderpeton, despite the complete fusion of each vertebrae by convergence. Here Phlegethontia nests as a basal pro-tetrapod with Pholidogaster and Colosteus (Fig. 5). P. longissima CGH 129 (below) has not yet developed a temporal fenestra.

Unlike most other paratetrapods,
the premaxilla of P. longissima was drawn out to a very long tip and the premaxillary teeth were the largest. The large lateral naris became elongate over the maxilla and prefrontal, perhaps contacting the postorbital and indicating this was a full time air-breather. Not sure what is happening with the supratemporals, which appear to extend laterally. The naris is elongate atop the maxilla.

The vertebrae
of outgroup taxa, like Pholidogaster, are still tripartite. The vertebrae in the transitional taxon, Colosteus, are largely hidden by osteoderms. In Colosteus the forelimbs are vestiges compared to those in Pholidogaster. They disappear in Phlegethontia. Evidently snake-like taxa fuse the neural spine, intercentrum and pleurocentrum as they switch from limb locomotion to vertebral undulation.

Like Ophiderpeton
AMNH 6966 (below) has a large temporal fenestra and the parietal is reduced to the portion anterior to the pineal foramen and fused to the fused frontals. The postorbital, suprateomporal and tabular are replaced by a larger occiput (braincase). The rostrum is also shorter.

Distinct from other paratetrapods,
the dorsal ostederms were absent. The ventral osteoderms had become elongate gastralia, convergent with tetrapods. Having just acquired limbs and girdles, this clade promptly got rid of them and emphasized cerebral undulation. The tiny’ gill bones’ illustrated by Fritsch 1875 (erased here, Fig. 4) are actually displaced gastralia, according to Baird 1964. The naris is larger in Phlegethontia compared to outgroup taxa with legs. So it was likely breathing air, rather than using gills.

Despite the many convergent traits
the LRT is able to separate Ophiderpeton from Phlegethontia and from all other long bodied, limbless tetrapods.

The traditional basal taxon
for the Aïstopoda is Lethiscus (Viséan. 340 mya). Hopefully data will come in soon on that taxon so it can be added to the LRT. Wikipedia reports, “The skull is specialised and light, very like that of Ophiderpeton, with the orbits, far forward, and the cheek region unossified (lacking bone). There are approximately 30 closely spaced teeth on the maxilla and dentary, and a sutural pattern of the skull closely resembles that of the Late Carboniferous aïstopod Oestocephalus.”

References
Anderson JS 2002. Revision of the aïstopod genus Phlegethontia (Tetrapoda: Lepospondyli). Journal of Paleontology. 76 (6):1029–1046. Online here.
Baird D 1964. The aïstopod amphibians surveyed. Breviora 206:1-17.
Cope ED 1871. Stated Meeting, Nov. 3d, 1871. Proceedings of the American Philosophical Society 12:176-177
Fritsch A 1875. Über die Fauna der Gaskohle des Pilsner und Rakonitzer Beckens. Sitzungsberichtde er Böhemischen Gesellschaft der Wissenschaften. Prague: 70–79.
Huxley TH 1862. On new labyrinthodonts from the Edinburgh Coal-field. Quarterly Journal of the Geological Society London18:291-296.
Panchen AL 1975. A New Genus and Species of Anthracosaur Amphibian from the Lower Carboniferous of Scotland and the Status of Pholidogaster pisciformis Huxley. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 269(900):581-637.

wiki/Pholidogaster
/wiki/Phlegethontia
wiki/Oestocephalus
wiki/Ophiderpeton