|Artist's reconstruction of Pachyrhachis problematicus.|
The authors ran three phylogenetic analyses using alternative out groups (varanoids; iguanians; and skinks+ amphisbaenids, + dibamids) to polarize the character transformations. The in-group consisted of all well-preserved fossil snakes from the Cretaceous, the madtsoiids (mostly Gondwanan snakes with a fossil record extending from the Upper Cretaceous to late Pleistocene from South America, Africa, India, Australia and Southern Europe), and taxa that are representative of all major groups of living snakes. The analyses suggested Pachyrhachis, Eupodophis, and Haasiophis are either a series of stem taxa at the base of the radiation of snakes, or they are members of a clade of fossil snakes that are the sister group to all living alethinopidians (all living snakes minus the scolecophidians).
The authors also found free intercentra located at the base of the anterior pre-cloacal vertebrae of Haasiophis terrasanctus. If these intercentra are homologous with the cervical intercentra of limbed squamates, this would suggest snakes experienced a considerable amount of axial elongation that involved not only the dorsal but also the cervical region, a point that is supported by the posterior extension of some cervical muscles in snakes. Of interest, a similar pattern of axial elongation as been observed in dolichosaurs, adriosaurs, and pontosaurs—a group of lizards that have been considered close relatives of snakes since the 19th century. The authors comment that they do not consider these lizards ancestral to snakes, but they may have a close phylogenetic relationship with them.
Alessandro Palci , Michael W. Caldwell and Randall L. Nydam (2013) Reevaluation of the anatomy of the Cenomanian (Upper Cretaceous) hind-limbed marine fossil snakes Pachyrhachis, Haasiophis, and Eupodophis. Journal of Vertebrate Paleontology, 33:6, 1328-1342.