Atractaspis: a burrowing snake not related to other burrowing snakes

Snake origins have traditionally perplexed paleontologists,
but not the large reptile tree (LRT, 1745+ taxa; Fig. 2). In the LRT snakes arise from tiny Tetrapodophis and Barlochersaurus (both with tiny vestigial limbs) following a series of aquatic taxa and a basal split from geckos. At their LRT genesis, snakes split into terrestrial and fossorial (burrowing) taxa.

Then along comes the burrowing viper, Atractaspis
(Fig. 1) studied in µCT scans by Strong, Palci and Caldwell 2020. “The genus Atractaspis, known commonly as the burrowing asp, is a fossorial lineage within the Colubroidea (= cobras, vipers and rattlesnakes), the most deeply nested clade of extant snakes.”

Figure 1. Atractaspis skull µCT scan from Strong, Palci and Caldwell 2020.

Figure 1. Atractaspis skull µCT scan from Strong, Palci and Caldwell 2020. Note the immature fangs ready to take the place of the main fangs and otherwise toothless jaws.

While sharing many traits with traditional burrowing snakes,
Atractaspis nests with the rattlesnake, Croatlus in the LRT. So, no big surprises here. The LRT confirms traditional snake topology, that is, once they lose their limbs and become snakes. Prior to that, taxon exclusion has made snake origins the enigma it remains in the traditional academic literature.

From the Strong, Palci and Caldwell abstract:
“Comparative osteological analyses of extant organisms provide key insight into major evolutionary transitions and phylogenetic hypotheses. This is especially true for snakes, given their unique morphology relative to other squamates and the persistent controversy regarding their evolutionary origins.”

re: Persistent controversy? Click here.

“However, the osteology of several major snake groups remains undescribed, thus hindering efforts to accurately reconstruct the phylogeny of snakes. One such group is the Atractaspididae, a family of fossorial colubroids. We herein present the first detailed description of the atractaspidid skull, based on fully segmented micro‐computed tomography (micro‐CT) scans of Atractaspis irregulars.”

See figure 1.

“The skull of Atractaspis presents a highly unique morphology influenced by both fossoriality and paedomorphosis. This paedomorphosis is especially evident in the jaws, palate, and suspensorium, the major elements associated with macrostomy (large‐gaped feeding in snakes).”

Figure 3. Subset of the LRT focusing on geckos and their sister snake ancestors.

Figure 2. Subset of the LRT focusing on geckos and their sister snake ancestors. Atractaspis (not listed here) nests with Croatlus among the terrestrial snakes.

Continuing from Strong, Palci and Caldwell:
“Comparison to scolecophidians—a group of blind, fossorial, miniaturized snakes—in turn sheds light on current hypotheses of snake phylogeny. Features of both the naso‐frontal joint and the morphofunctional system related to macrostomy refute the traditional notion that scolecophidians are fundamentally different from alethinophidians (all other extant snakes). Instead, these features support the controversial hypothesis of scolecophidians as “regressed alethinophidians,” in contrast to their traditional placement as the earliest‐diverging snake lineage.”

In the LRT, fossorial (burrowing) snakes follow tradition and diverge from all other snakes at the genesis of snakes. Therefore, Atractaspis is convergent with fossorial snakes, but keeps that wide-gape elongate quadrate (red in figure 1) from its viper ancestors.

The origin of snakes alongside the origin of mosasaurs and the origin of Lanthanotus, all in phylogenetic order. Not to scale. Note the branching off of burrowing snakes.

Figure 3, From 2015, prior to the insertion of Tetrapodophis: The origin of snakes alongside the origin of mosasaurs and the origin of Lanthanotus, all in phylogenetic order. Not to scale. Note the branching off of burrowing snakes at the origin of snakes near Dinilysia and Pachyrhachis.

Continuing from Strong, Palci and Caldwell:
“We propose that Atractaspis and scolecophidians fall along a morphological continuum, characterized by differing degrees of paedomorphosis. Altogether, a combination of heterochrony and miniaturization provides a mechanism for the derivation of the scolecophidian skull from an ancestral fossorial alethinophidian morphotype, exemplified by the nonminiaturized and less extreme paedomorph Atractaspis.”

“The phylogeny of scolecophidians is uncertain, due in large part to disagreement between morphological and molecular data.”

Suggestion based on evidence: Ignore molecular data.

Continuing from Strong, Palci and Caldwell:
“Interestingly, a recent morphological phylogeny (Garberoglio et al., 2019) focussing on extinct snakes recovered scolecophidians as nested within Alethinophidia in stark contrast to the more orthodox placement of this group as basally divergent among Serpentes. These results strongly highlight the importance of continued morphological and phylogenetic analyses of this group, including a renewed examination of potential alethinophidian affinities of scolecophidians.”

A valid set of outgroup taxa (Fig. 2) should settle this issue. Just add taxa to find out for yourself.


References
Strong CRC, Palci A and Caldwell MW 2020. Insights into skull evolution in fossorial snakes, as revealed by the cranial morphology of Atractaspis irregularis (Serpentes: Colubroidea). Journal of Anatomy. https://doi.org/10.1111/joa.13295

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