One of the mysteries of herpetology and paleontology is just where do the cervicals end and the dorsals begin on a snake. It’s impossible to say in modern taxa (Fig.1). The cervicals blend imperceptibly with the dorsals. The question is complicated by the presence of two distinct clades of “snakes” in the large reptile tree.
The location of the thyroid and lung some 20+ cervicals back of the head appear to delineate where the dorsals begin in most snakes. I haven’t found much information on the post-crania or anatomy of the so-called “burrowing” snakes like Cylindrophis and Leptotyphlops. [help!!]
One clade of snakes arose from a sister to long-necked Adriosaurus (14 cervicals) that leads to Pachyrhachis (22 cervicals, Fig. 2) and Boa (unknown number, but likely about 22 based on the location of the thyroid and lungs, Fig. 1).
Lee and Caldwell (1998) report, “In the case of Pachyrhachis, this also means the putative cervicals are diferentiated from the putative dorsals by a marked diverence in size, the shape of the neural spine, and the length, robustness and ossication of the associated ribs. The anterior region is well-preserved in both specimens and there is no sign of a pectoral girdle or forelimb. These elements were therefore absent: however, the cervical-dorsal boundary (vertebrae 20-22) presumably represented the approximate original position of the shoulder girdle and forelimb.”
Another clade arising from a sister to Heloderma (6 cervicals) and Lanthanotus (apparently more than 6 cervicals, but I have no data) that includes Cylindrophis through Leptotyphlops. (Any post-cranial data on these taxa would be gratefully appreciated). Dr. Nate Kley sent two references (Bergman 1953) that I’d like to see on Cylindrophis.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Bergman RAM 1953. The anatomy of Cylindrophis rufus (Laur.). I. Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen, Series C 56:650–656.
Bergman RAM 1953. The anatomy of Cylindrophis rufus (Laur.). II. Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen, Series C 56:657–666.
Lee MSY and Caldwell MW 1998. Anatomy and relationships of Pachyrhachis problematicus, a primitive snake with hindlimbs. Philosophical Transactions of the Royal Society London B 353:1521-1552.