Phlegethontia longissima (above; CGH 129)
Phlegethontia linnearis (below; Cope 1871, Anderson 2002, Fritsch 1875; Huxley and Wright 1867; AMNH 6966; Westphalian, Late Carboniferous; 1m in length) was considered a aîstopod, but it does not nest with another lepospondyl aïstopod, Ophiderpeton, despite the complete fusion of each vertebrae by convergence. Here Phlegethontia nests as a basal pro-tetrapod withPholidogaster and Colosteus. CGH 129 (above) is just beginning to develop a temporal fenestra. 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.

Unlike most other basal tetrapods, 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 tiny' gill bones' illustrated by Fritsch 1875 are actually displaced gastralia. Distinct from other paratetrapods, the dorsal ostedoerms were absent. The ventral ones had become gastralia, convergent with tetrapods.