The Facial Carotid Artery in Snakes (Reptilia, Serpentes): Variations and Possible Cladistic Significance

It is reported that in henophidian snakes examined, except Acrochordus, the facial carotid artery passes forward ventral to the mandibular branch of the trigeminal nerve, and is typically of lesser diameter than the cerebral carotid. In contrast, in caenophidian snakes except Viperinae (not including Causus) and in Acrochordus, the facial carotid passes dorsal to the mandibular branch of the trigeminal, and is not of lesser diameter than the cerebral carotid. Viperinae except Causus show the ventral course. Because it also occurs in Scolecophidia, and is correlated with primitive states of many other characters, the ventral course seen in Henophidia except Acrochordus is interpreted as the primitive condition in snakes. The dorsal course seen in Caenophidia and Acrochordus is interpreted as a derived condition indicating that these two taxa form a monophyletic group. This is corroborated by presence of several other shared derived features in the group. The ventral course in Viperinae except Causus is interpreted as a derived reversal indicating the monophyly of 'true' Viperinae (ie. excluding Causus).