Adding Anilius, a Basal Snake, to the Large Reptile Tree

A recent paper on the phylogeny and prehistory of blind snakes (Vidal et al. 2010) listed Anilius (Fig. 1) as an outgroup taxon along with Tropidophis (dwarf boa), Elapidae (venomous snakes like the cobra) and Boa (the famous constrictor). Anilius is primitive enough to retain a vestigial pelvic girdle, visible as a pair of cloacal spurs, according to Wiki. Wiki also reports that Anilius “is considered to be the snake that most resembles the original and ancestral snake condition, such as a lizardlike skull.” Boa also retains a rudimentary pelvis and hind legs that externally appear as minor spurs.

Anilius skull

Figure 1. The skull of Anilius in dorsal and lateral views, courtesy of Digimorph.org. Elements here colorized and labeled. Click to enlarge. The post frontal is fused to the frontal. The jugal is fused to the maxilla. The supratemporal is tiny.

Anilius is medium-sized, reaching 27 inches (70 cm) in length. The eyes are covered beneath a clear scale.  It is a burrowing snake feeding on small reps, amphibians and even other blind snakes.

I wondered if this basal snake would change or enhance the current diphyletic nesting of snakes in the large reptile tree. After testing it nested between Lanthanotus (with legs) and Cylindrophis (without legs), nowhere near Boa, which nests with other larger non-burrowing snakes, like Pachyrhachis. The Vidal et al. (2010) nesting of Anilius at the base of the tree is due to the lack of more closely related more primitive taxa, a situation remedied by employing the large reptile tree.

The Vidal et al. (2010) study was quite remarkable and complete. However, deeper outgroup taxa were not employed and for good reason. The blind snakes are widely considered monophyletic.

Blind snake origins are distinct from that of other snakes when deeper outgroups are employed and Anilius appears to be closer to them than to non-burrowing snakes.

The Vidal et al. (2010) study postulated that the split between non-burrowing snakes (Alethinophidia, including Anilius) and the blind snakes (Scolecophidia) occurred sometime in the Jurassic, with taxa like Anilius likely little changed since then. This coincides with the breakup of Pangaea, which makes sense if burrowing blind snakes are going to spread world wide prior to the continental breakup. They prefer warm climes.

Bone Fusion
It’s important to recognize when bones fuse (Fig. 1) so we don’t consider certain bones “absent” in phylogenetic analysis. It’s also important to include lots of prehistoric taxa to discover where blind snakes do nest in the grand scheme of things.

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.

References
Vidal N, Marin J, Morini M, Donnelllan S, Branch WR, Thomas R, Vences M, Wynn A, Cruaud C and Hedges SB 2010. Blindsnake evolutionary tree reveals long history on Gondwana. Biology Letters 2010 6, 558-561.

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