Aïstopoda

Aïstopoda are an order of highly specialised snake-like amphibians known from the Carboniferous and Early Permian of Europe and North America, ranging from tiny forms only 5 cm to nearly a meter in length. The first appear in the fossil record in the Mississippian period and continue through to the Early Permian.

Skull
The skull is small but very specialised, with large orbits, and large fenestrae. The bones at the back of the skull reduced or absent. The primitive form Ophiderpeton has a pattern of dermal bones in the skull similar in respects to the temnospondyls. But in the advanced genus Phlegethontia the skull is very light and open, reduced to a series of struts supporting the braincase against the lower jaw, just as in snakes, and it is possible that the Aïstopods filled the same ecological niches in the Paleozoic that snakes do today.

Body
The body is extremely elongate, with up to 230 vertebrae. The vertebrae are holospondylous, having only a single ossification per segment. They lack intercentra, even in the tail, and there are no free haemal arches. The neural arch is low and is fused to the centrum. In structure they are very similar to those of the Nectridea, both representing the typical lepospndylous condition.

The ribs are slender, either single or double-headed (the head shaped like a K). There is no trace of limbs or even limb girdles in any known species, and the tail is short and primitive.

Evolutionary relationships with other early tetrapods remain controversial, as even the earliest Aïstopod, the Viséan species Lethiscus stocki, was already highly specialised. Aïstopods have been variously grouped with other Lepospondyls, or placed at or prior to the Batrachomorph-Reptilomorph divide. The group was quite diverse during the Late Carboniferous, with a few forms continuing through to the Permian.