Updated July 24, 2015 with the hypothesis that snakes are monophyletic, arrived at earlier when taxa were included to link burrowers with non-burrowers.
A recent paper by Longrich et al. (2012) reports on a “missing link” snake from the Late Cretaceous. The paper argues that all snakes descended from a burrowing terrestrial ancestor rather than a swimming marine ancestor. Coniophis precedens (Marsh 1892) is a tiny snake, previously only known from short-spined vertebrae. Now that the jaws are known, more can be said of this species. “It moves like a snake,” said Longrich, “but it doesn’t feed like a snake.”
From the Longrich et al. 2012 abstract, “Coniophis occurs in a continental floodplain environment, consistent with a terrestrial rather than a marine origin; furthermore, its small size and reduced neural spines indicate fossorial habits, suggesting that snakes evolved from burrowing lizards. The skull is intermediate between that of lizards and snakes. the maxilla is firmly united with the skull, indicating an akinetic rostrum. Coniophis therefore represents a transitional snake, combining a snake-like body and a lizard-like head.”
In contrast to modern snakes, the jaws remained fixed, limiting the size of its prey, which were swallowed whole. Coniophis’s status as the most primitive snake does not make it the oldest known snake. Others, like Pachyrhachis, extend back another 35 million years. Rather, according to Longrich, “It appears to have been ‘a living fossil’ in its own time, co-existing with more advanced snakes, as chimpanzees and humans still do.”
“It’s not the direct ancestor of modern snakes, but it tells us what the ancestor looks like,” Longrich said.
Adaptations permitting cranial kinesis followed the divergence of Coniophis. Coniophis exhibits an intramandibular joint (1). More derived Serpentes exhibit a maxilla–premaxilla joint (2), loss of maxilla–vomer contact (3), a nasofrontal joint (4), a maxilla–prefrontal joint (5), a mobile dentary symphysis (6) and an articular saddle joint (7). Alethinophidia is characterized by a reduced postorbital bar (8) and a palatine–pterygoid hinge (9). Macrostomata is characterized by a hinged supratemporal (10).
The Longrich et al. (2012) tree nests the new taxon at the very base (as the outgroup) of the rest of the snakes. This is a problem because nearly every other taxon ever known would also nest as the outgroup (elephants, flies, pterosaurs). To make a better case several other outgroups would have to be shown with Coniophis representing the transition between legged and legless taxa, or any other transitional grade that would be more pertinent.
There are too few traits shown in Coniophis to nest it in the large reptile tree, but we’ll take the Longrich examination as valid. The dorsally concave dentary appears to bridge the gap between Heloderma and Cylindrophis (Fig. 1). Pachyrhachis and the larger non-burrowing snakes do not share this trait.
The burrowing snakes are all tiny and so is Coniophis.
Many lizard genera are long-lived taxa. Ghost lineages abound in this clade. Bahndwivici, ancestral to both Varanus and Heloderma, is only known from the Eocene, but evidently extended back to the middle Jurassic. Varanus and Heloderma are known today, but likewise must extend back to the Jurassic.
Longrich NR, Bullar B-A S and Gauthier JA 2012. A transitional snake from the Late Cretaceous period of North America. Nature 488, 205-208.
Marsh OC 1892. Notice of new reptiles from the Laramie Formation. American Journal of Science 43:449-453.