An order of extremely elongate, limbless fossil amphibians in the subclass Lepospondyli from Permo-Carboniferous rocks of North America and the British Isles. The order includes three families: Lethiscidae (Lethiscus), Ophiderpetontidae (Coloraderpeton Ophiderpeton), and Phlegethontiidae (Aornerpeton, Phlegethontia, Sillerpeton). All genera are monotypic with the exception of Ophiderpeton (six species) and Phlegethontia (possibly three species). In seeming contrast to their derived state, aistopods are among the very oldest of all known fossil tetrapods.