Burrowing squamates: Ebel et al. 2020 examines bone micro-anatomy

From the Ebel et al 2020 abstract:
“We reconstructed the acquisition of a fossorial lifestyle in 2813 lepidosaurs and assessed the skull roof compactness from microCT cross-sections in a representative subset (n = 99). We tested this and five macroscopic morphological traits for their convergent evolution. We found that fossoriality evolved independently in 54 lepidosaur lineages. Furthermore, a highly compact skull roof, small skull diameter, elongate cranium, and low length ratio of frontal and parietal were repeatedly acquired in concert with a fossorial lifestyle.”

Unfortunately the Ebel team relied on a genomic cladogram. By contrast the large reptile tree (LRT, 1772+ taxa, subset Fig. 2) found only 6 lepidosaur fossorial clades. Perhaps this is so because the LRT is a phenomic (trait-based) cladogram in which fossorial and legless Dibamus is a highly derived skink, alongside Bipes and Amphisbaena, not a basalmost squamate nesting alone. In the LRT, basalmost squamates have legs, fingers and toes, traits many derived squamates retain then lose.

Figure 1. Cladogram of burrowing (fossorial) squamates from Ebel et al. 2020. Compare to figure 2 from the LRT.

Figure 1. Cladogram of burrowing (fossorial) squamates from Ebel et al. 2020. Compare to figure 2 from the LRT.

Ebel et al 2020 abstract continues:
“We report a novel case of convergence that concerns lepidosaur diversity as a whole. Our findings further indicate an early evolution of fossorial modifications in the amphisbaenian ‘worm-lizards’ and support a fossorial origin for snakes.”

By contrast the LRT employs a wider gamut of taxa, including fossils, and finds a basal dichotomy at the genesis of snakes. One branch remained terrestrial while the other branch became fossorial. The morphologically weirdest burrowing snakes are the most derived. That’s how evolution works: from simple and plesiomorphic to bizarre and derived.

The latest LRT addition to snakes, Atractaspis, is a burrowing venomous snake arising from terrestrial snakes. That taxon would have been overlooked prior to addition to the LRT, so there may be other squamate burrowers not yet tested by the LRT.

Figure 2. Subset of the LRT focusing on Squamata. Compare to Ebel et al. 2020 in figure 1.

Figure 2. Subset of the LRT focusing on Squamata. Compare to Ebel et al. 2020 in figure 1.

The LRT finds only six clades
of burrowing squamates (Fig. 2) and an entirely different tree topology that includes fossils and protosquamates, and, need I say it, tritosaurs.


References
Ebel R, Müller J, Ramm T, Hipsley C and Amson E 2020. First evidence of convergent lifestyle signal in reptile skull roof micro anatomy. BMC Biology 18…185. https://doi.org/10.1186/s12915-020-00908-y

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.