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

Cynodontipus – a “hairy paw” from the Middle Triassic?

Dr. Paul Ellenberger will go down in history for his work on Cosesaurus, but his passion was fossil footprints. One impression he considered was this purported “hairy paw” from the Middle Triassic of France. Ellenberger (1976) named it Cynodontipus (Fig. 1). Each “toe” was about 2 inches (5 cm) in width.

Cynodontipus in situ, a partial hairy paw print.

Figure 1. Cynodontipus in situ, a partial purported hairy paw print. This is an excellent print, but difficult to interpret. The hair seems misplaced and anachronistic. The wide toes are odd. So is the lack of ungual imprints.

Ellenberger (1976) interpreted his fossil this way (Fig. 2).

Ellenberger's interpretation of Cynodontipus.

Figure 2. Ellenberger’s interpretation of Cynodontipus.

Hairy pads are not known even in modern arctic mammals, so why should we expect hairy pads in Triassic cynodonts? Especially when one can’t differentiate the pads?? There’s no match for this foot among known Triassic taxa. Perhaps there is another explanation for this enigma.

 

Figure 3. Cynodontipus burrows, likely from a procolophonid. Each color represents a new burrow direction from a central origin.

Figure 3. Cynodontipus burrows, likely from a procolophonid. Each color represents a new burrow direction from a central origin.

So what is it?
When we consider the “hair,” we are drawn to the therapsids as possible candidates, as Ellenberger surmised. But this fossil demonstrates way more hair than can be expected at such an early date. Even in mammals the hands and feet are the last parts to get hairy, and usually pads are plain to see, so this fossil just doesn’t fit several typical ichnite patterns.

Luckily there’s Olsen’s 2012 take on it.
Olsen 2012 wrote: In addition to its type locality in the Middle Triassic of France, Cynodontipus has been identified from the Middle Triassic of Germany, the Middle and Late Triassic of Morocco, the Late Triassic of Nova Scotia, Canada, and the Late Triassic of Connecticut, USA. This last occurrence consists of unlabeled part and counterpart slabs discovered in the Hitchcock collection at the Beneski Museum of Natural History at Amherst College. These specimens show that Cynodontipus is a vertebrate burrow that terminates at a recalcitrant subsurface bedding interface and is not a footprint. The simplest hypothesis of the trace maker of Cynodontipus is that it was a produced by burrowing procolophonids, which are know from the same deposits, are the right size, and are known to have burrowed.

Thus the lines that Ellenberger considered hairs must be tunnel scratch marks instead. Doesn’t that make more sense?

The key take away on this
Even experts can have different opinions on the same fossil. More data appears to clarify enigmas. That’s the progress of Science and that’s what makes this paleo study so fascinating. No one need vilify Ellenberger for his misinterpretation. Likewise, no one need denigrate the results published in reptileevolution.com or here at pterosaurheresies, even if and when results are shown to be in error. Errors need to be corrected, but never by blackwashing an entire output. DN and MW, I hope you’re listening. Olsen (2012) handled Ellenberger’s error very well indeed. We should all take note. Be specific and back up your corrections with evidence.

References
Ellenberger P 1976. Une piste avec traces de soies épaisses dans le Trias inférieur a moyen de Lodéve (Hérault, France): Cynodontipus poythrix nov. gen. nov. sp. les cynodontes in France. Géobios 9(6)769-787.
Olsen PE. 2012.
Cynodontipus: A procolophonid burrow – not a hairy cynodont track (Middle-Late Triassic: Europe, Morocco, Eastern North America. Geological Society of America Abstracts with Programs, Vol. 44, No. 2, p. 92.
Olsen PE, Et-Touhami, M, Whiteside, JH, 2013 (in prep). Cynodontipus Ellenberger is a vertebrate burrow, not a hairy synapsid track. for Journal of Vertebrate Paleontology.