Blind-snake evolution in dorsal, lateral and palatal views

Today’s blogpost was sparked by
Fachini et al. 2020, who described a new, rather large (~1m) Cretaceous blind-snake lacking a skull, Boipeba.

Unfortunately,
this is a taxon I cannot add to the large reptile tree (LRT, 1765+ taxa; subset Fig. 1) because all included blind-snakes are scored on skull-only traits.

The Fachini et al. cladogram of blind-snakes
differs markedly from the LRT (Fig. 1) in that Fachini et al. nests the most derived taxa in the LRT at basal nodes and vice versa. They also nest extant burrowing snakes basal to extant terrestrial snakes instead of splitting the two at the genesis of snakes.

Fachini et al. report,
“Blindsnakes (Scolecophidia) are minute cryptic snakes that diverged at the base of the evolutionary radiation of modern snakes.”

By contrast the LRT recovers all terrestrial snakes (extant AND extinct) diverging from burrowing snakes at the origin of snakes (Fig. 1). Fachini et al. report both adding and deleting Tetrapodophis from their analysis resulting in no topological changes.

In doing so, Fachini et al. cited an abstract we looked at earlier (Caldwell et al. 2016), proving workers sometimes do cite abstracts.

On this disagreement, we agree:
“there is disagreement between morphological and molecular phylogenetic analyses with regard to their phylogenetic position and monophyly.” 

As often reported here genomic testing too often leads to false positives as compared to phenomic (trait-based) testing in deep time paleo studies. Seemingly the paleo community has not yet realized this, or taken evasive action on this, despite often writing about it (e.g. Fachini et al.).

On this statement we disagree:
Fachini et al. report, “The origin of blindsnakes is unclear.”

In the LRT blind-snakes (burrowing snakes) clearly arise from Tetrapodophis (snake outgroup), Najash and Loxocemus (Figs. 2–4). We looked a phylogenetic problems in snake origins earlier here, here and here in 2013.

Figure 4. Subset of the LRT focusing on snakes. Compare to figure 3.

Figure 4. Subset of the LRT focusing on snakes. Compare to figure 3.

The most derived blind-snake taxa
in the LRT are instead basal taxa in the cladogram of Fachini et al.

Figure 2. Evolution of blind-snake skulls in dorsal view.

Figure 2. Evolution of blind-snake skulls in dorsal view. The white and black backgrounds are not significant, but represent the original background for the photos and drawings.

How was it possible that Fachini et al.
inverted the order of blind-snakes given Najash as a common outgroup taxon? I don’t know. It does not make sense. Given the clear similarity of python-like Loxocemus to Najash (Figs. 2–4) how is it possible that nearly blind Anomochilus through Typhlops (Figs. 2–4) nested closer to Najash? They were following a long tradition.

Figure 3. Evolution of blind-snake skulls in lateral view.

Figure 3. Evolution of blind-snake skulls in lateral view.

When you examine these three illustrations
of blind-snake skull evolution in dorsal, lateral and palatal views (Figs. 2–4, primitive at the top of each), remember, Fachini inverts the order (i.e. primitive at the bottom). In this way Fachini et al. separate python-like Najash and Pachyrhachis from extant pythons, like Boa, with burrowing snakes separating them. They may need to rethink that hypothesis of interrelationships.

Figure 4. Evolution of blind-snake skulls in palatal view.

Figure 4. Evolution of blind-snake skulls in palatal view.

Trends in blind-snake evolution recovered by the LRT:

  1. naris shortens and migrates from anterodorsal to anteroventral
  2. sharp snout becomes blunt, then round
  3. vomernasal fenestra shrink and disappear
  4. premaxilla rotates to the palate
  5. maxilla shrinks and fuses to premaxilla
  6. dentary shortens
  7. all marginal teeth reduce and disappear
  8. frontal enlarges vs parietal
  9. parietal retreats away from orbit
  10. circumorbital bones disappear (then reappear only in Liotyphlops)
  11. parietal loses sagittal crest
  12. palatine becomes mobile with transverse axial rotation
  13. ectopterygoid shrinks
  14. pterygoid loses teeth and becomes gracile loose strut
  15. basisphenoid and basioccipital become bulbous
  16. squamosal and supratemporal shrink and disappear
  17. quadrate stretches and leans posteriorly
  18. coronoid enlarges

Backstory
Blind snakes were first added to the LRT years ago. Many were not colorized as they are now (Figs. 2–4). Some new insights were gained by reviewing the taxa after coloring the drawings and recoloring the photos in a consistent manner.

Most snake workers label the squamosal the supratemporal. This topic was examined earlier here in 2018. The dorsal view image documents the reduction and disappearance of the supratemporal (bright green tiny bony near occiput) prior to the reduction and disappearance of the squamosal (magenta larger bone near quadrate)

The quadrate of Typhlops and Liotyphlops (Fig. 3) informed the previously misunderstood fusion of the quadrate with the stem-like articular in Leptotyphlops (Fig 3).

The realization that the unique appearance of circumorbital bones in Liotyphlops (Fig. 3) was a reappearance and a reversal solved an identity problem first posed by Rieppel, Kley and Maisano 2009.

Skull photos above are from Digimorph.org
and used with permission.


Bonus unrelated paleo news item for your enjoyment:
Some recent ‘stunning fossil finds” are listed and shown here:
https://www.boredpanda.com


References
Fachini TS et al. (5 co-authors) 2020. Cretaceous blind snake from Brazil fills major gap in snake evolution. iScience ISCI 101834. https://doi.org/10.1016/j.isci.2020.101834
Rieppel 0, Kley NJ and Maisano JA 2009. Morphology of the skull of the white-nosed blindsnake, Liotyphlops albirostris (Scolecophidia: Anomalepididae. Journal of Morphology, 270, 536-557.

digimorph/Leptotyphlops
wiki/Leptotyphlops
digimorph/Liotyphlops
wiki/Liotyphlops
digimorph/Typhlops
wiki/Typhlops

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