Vidal writes:
"The monophyly of the xenodermatines,[3] a small group of very poorly known Asiatic snakes, appears questionable as no unambiguous morphological synapomorphy (a shared derived character state) is identified.[26]"
The monophyly of xenodermatids have been confirmed by Dowling and Pinou (2003, Herpetol. Rev. 34:20-23), who list several synapomorphies for this clade.
Vidal continues
"Second, the presumed most basal lineage of colubroids, the xenodermatines, [25,27] lacks a differentiated maxillary dentition but has alternating cords of mucous and serous cells along the supralabial region.[81] So, the presence of serous cells, even if they do not form a Duvernoy’s gland, is contemporary with the origin of colubroids."
Vidal's tree is contradicted by immunological data. According to Dowling and Pinou (2003, Herpetol. Rev. 34:20-23) Cadle (1994) found that none of the colubrids were closer than 70 AID from Mehelya, meaning that they last shared an ancestor no earlier than the Eocene. Considering the fact that the AID between Thamnophis sirtalis and Elaphe obsoleta is 74 and the AID between Diadophis punctatus and Elaphe obsoleta is 102, Mehelya cannot be excluded from the Colubridae on the basis of immunological distance. Rather than a sister taxon to the Colubroidea, the xenodermatids are more likely a distinctive lineage within the Colubridae.
Seen in this light, the fact that xenodermtids do not have a Duvernoy's gland is strong evidence against venom or the Duvernoy's gland having evolved in the ancestor of the Colubroidea. There is no study to show that all Duvernoy's glands are homologous, even though the supporters of the early evolution of venom assume that they are.
Reference:
Vidal, Nicolas 2002. Colubroid Systematics: Evidence for an Early Appearance of the Venom Apparatus Followed by Extensive Evolutionary Tinkering. J. Toxicol.—Toxin Reviews, 21(1&2), 21–41
