With regard to legless lizards, including snakes, blogger and paleontologist Mickey Mortimer mentioned in a recent comment here, “Vidal and David (2004) found that Uropeltis groups with Boa instead of Leptotyphlops by two nodes supported with 100% Bayesian posterior probability based on two nuclear genes.”
Curious about this, I added Boa constrictor to the large reptile tree (I’ll add it to the graphic tree when I get a few more taxa). To no surprise, Boa nested with Pachyrhachis, “the snake with (hind) legs.” Uropeltis did not group with Boa, but stayed with Leptotyphlops. Once again, DNA results did not match morphological results.
Morphology divides the Squamata into Iguania and all remaining squamates into a group named Scleroglossa. In the latter, the tongue is at least partly keratinized and flattened relative to iguanians (Estes et al., 1988). Members of the Iguania are mainly ambush predators, using visual prey discrimination and capture their prey by their sticky tongue., Members of the Scleroglossa use their tongue to seek out prey, but rely on their jaws to apprehend prey. The scleroglossan skull is less rigid (Vitt et al., 2003). This reaches an acme with snakes.
According to Rieppel (1988) there have been four main hypotheses as to the affinities of snakes:
1. With varanoids/mosasauroids (Cope, 1869; Nopcsa, 1923).
2. Independent of other squamates (Underwood, 1970).
3. With legless burrowing scincomorphs and dibamids (Senn and Northcutt, 1973).
4. With legless amphisbaenians (Rage, 1982).
Since 1988, Pachyrhachis, the snake with legs, was described. A fifth hypothesis (Palci A and Caldwell MW 2007) based on a derivation from the aquatic varanoid, Adriosaurus, appeared. Conrad (2008) sided with (3 and 4) in linking snakes to amphisbaenids, burrowing scincomorphs and dibamids.
The large reptile tree represents a sixth hypothesis in which most snakes were derived from a sister to Pachyrhachis, Adriosaurus and Ardeosaurus in that order. Other, less common, burrowing snakes (Cylindrophis, Uropeltis, Anomochilus and Leptotyphlops were derived from a sister to Lanthanotus, Heloderma and Cryptolacerta in that order.
Odd little Dibamus was tested and it nested between Spathorhynchus and Tamaulipasaurus two sisters of Bipes, Amphisbaena and other skinks. Not sure if fossil taxa have been included with other Dibamus studies, but I think not. Let me know if otherwise.
I have to admit, one of the reasons why my snakes don’t nest with my legless skinks is because virtually no post-cranial traits are used because I don’t have good data on post-crania. If leglessness is convergent, this works. If homoplastic, which appears doubtful, then it doesn’t.
DNA studies cannot pull data from fossils, so left with living taxa, some with a legacy going back to the Permian and beyond (before their eventual splits), we have to rely on morphological studies. We have to find a gradual accumulation of traits, something that DNA doesn’t give us, in reptiles at least. Mammals seem to fare better. Not sure why.
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.
Caldwell MW and Palci A 2010. A new species of marine ophidiomorph lizard, Adriosaurus skrbinensis, from the Upper Cretaceous of Slovenia. Journal of Vertebrate Paleontology 30(3): 747-755. doi:10.1080/02724631003762963.
Conrad JL 2008. Phylogeny and systematics of squamata (Reptilia) based on morphology. Bulletin of the American Museum of Natural History 310, 182 pp.
Estes R, de Queiroz K and Gauthier JA 1988. in Phylogenetic Relationships of the Lizard Families (eds Estes, R. & Pregill, G. K.) 119–281 (Stanford Univ. Press).
Palci A and Caldwell MW 2007. Vestigial forelimbs and axial elongation in a 95 million-year-old non-snake squamate. Journal of Vertebrate Paleontology 27 (1): 1-7. doi:10.1671/0272-4634(2007)27[1:VFAAEI]2.0.CO;2.
Seeley HG 1881. On remains of a small lizard from Neocomian rocks of Comen, near Trieste, preserved in the Geological Museum of the University of Vienna. Quarterly Journal of the Geological Society of London 37: 52-56.
Vidal N and David P 2004. New insights into the early history of snakes inferred from two nuclear genes. Mol. Phyl. Evol. 31:783-787.
Vidal N and Hedges SB 2002. Higher-level relationships of snakes inferred from four nuclear and mitochondrial genes. C. R. Biologies 325 (2002) 977–985.
Vidal N and Hedges SB 2004. Molecular evidence for a terrestrial origin of snakes. Biol. Lett. 2004;271:226–229.doi:10.1098/rsbl.2003.0151