The evolution of the basicranium in the Henophidia (Reptilia: Serpentes)

In lizards, a short Vidian canal pierces the base of the basipterygoid process. In snakes, the anterior opening of the primitively short Vidian canal lies on the dorsal surface of the basisphenoid, trapped in an intracranial position by downgrowths of the frontal and parietal which meet the lateral edges of the basisphenoid-parasphenoid complex. This condition is observed in anilioid snakes which retain other primitive features in the braincase: the paroccipital process and the spheno-occipital tubercle (Aniliidae only) and participation of the basioccipital in the apertura lateralis of the recessus scalae tympani. Subsequent evolution within booid snakes shows a shift of the anterior opening of the Vidian canal around the anterior edge of the basisphenoid, thus acquiring a secondary extracranial position. This occurs in parallel within boine and pythonine snakes. Dinilysia shows a parallel development of die condition observed in advanced booid snakes. Pseudoboa, with its short Vidian canal opening in-tracranially, demonstrates that caenophidians originated from a basal henophidian or pre-henophidian stock. The Acrochordidae show a basicranium that can be interpreted as either primitive henophidian or primitive caenophidian.