Tetrapodophis is not a snake? Lesson for today: Don’t omit pertinent taxa

Snake experts Caldwell et al. (6-co-authors) 2021
reported “Tetrapodophis [Fig. 1] is not a snake. We also present the results of a series
of phylogenetic analyses of squamates focused on the sister-group relationships of Tetrapodophis, finding it to be related to dolichosaur mosasaurians within Pythonomorpha (Mosasauria + snakes).”

In November 2016 we got a sneak-preview of this paper
in an SVP abstract. Click here to see the same problems and solutions.

Co-authors Caldwell and Lee
have been attempting to resurrect the invalid clade Pythonomorpha (Cope 1869, Fig. 2) = snakes + mosasaurs since 1997. The large reptile tree (LRT, 1999 taxa, subset Fig. 3) shows these two are not related. Wikipedia reports, “If Pythonomorpha is valid, it contains not only mosasauroids but the Ophidiomorpha, which was defined as a node-based clade containing the most recent common ancestor of dolichosaurs, adriosaurs, Aphanizocnemus, and fossil and extant Ophidia and all of its descendants.”

Caldwell et al. are confusing these two water-loving squamate clades.
Dolichosaurs + adriosaurs + Aphanizocnemus, are closer to Tetrapodophidis and snakes in the LRT. They are not related to mosasaurs (Fig. 3), which are closer to varanid lizards in the LRT, but not in Caldwell et al. (Fig. 2). Not sure how these snake experts didn’t see this obvious red flag. Varanids and mosasaurs should always nest together. We’ve known this for years.

It’s a good thing
that paleontologists and systematists are checking each others’ work in phylogenetic analysis. The authors said they checked and rescored many traits.

Unfortunately…
It’s not a good thing when PhDs borrow cladograms from others, and omit pertinent taxa.

Unfortunately no one checked the logic of the Caldwell et al. 2021 results (Fig. 2). Something is wrong when burrowing blind snakes with odd jaws are basal to fully sighted terrestrial snakes with normal jaws. Something is wrong when varanids are not close to mosasaurs while plant-munching taxa, like Iguana, separate these predator taxa.

Unfortunately taxon exclusion mars the Caldwell et al. results. These are the stem snakes (Fig. 3) not mentioned in the Caldwell et al. text: Barlochersaurus, Primitivus, Eichstaettisaurus, Ardeosaurus, Norellius, Tchingisaurus. The authors do mention Hoyalacerta and Jucaraseps, but only in regards to their small size, comparable to Tetrapodophis (Fig. 1). All these taxa lead pre-snake origins back to gecko origins, something we’ve known since 2015.

Unfortunately
Caldwell et al. nests tiny Tetrapodophis within the Mosasauria, a clade known for giant ocean-going taxa not related to pre-snakes in the LRT.

Let’s start with a baseline.
In LRT, Fig. 3) tiny Early Cretaceous Tetrapodophis nests with even tinier Barlochersaurus (Fig. 1) basal to the basal snake dichotomy: terrestrial snakes on one branch, burrowing snakes on the other with blind burrowing snakes with oddly working jaws at the most derived nodes. That makes sense. Adding taxa further cements these relationships.

Caldwell et al. label Tetrapodophis a dolichosaurid lizard,
as are snakes in the LRT. That doesn’t mean they are related to mosasaurs, the pet hypothesis of the co-authors, Caldwell and Lee.

It doesn’t matter how closely you study specimens,
or who you borrow cladograms from, or how long you’ve been studying snakes… if you don’t include pertinent taxa, you’re not going to have an accurate or valid cladogram. The LRT minimizes taxon exclusion by including so many taxa. That’s why it’s here (subset Fig. 2), online, free to everyone on the planet.

Caldwell et al. report,
“The tree topology from the combined evidence Bayesian analysis (Figs 19, Supplemental material, Fig. S4) supports all major clades recovered in the past decade by molecular data and by most combined evidence data (see Simões & Pyron 2021 for a recent review). There is an early diversification of gekkotans and dibamids,followed by scincoids, lacertoids and toxicoferans. Within toxicoferans, the first evolutionary split is between anguimorphans and a clade composed of the iguanians, borioteiioids and pythonomorphs, i.e. mosasaurians and snakes (see also Reeder et al. 2015; Pyron 2017).”

None of this is supported by the LRT.

Caldwell et al. report,
“In an effort to assess the placement of Tetrapodophis using a similar dataset to that of Martill et al. (2015), we utilized a modified version of the morphological dataset of Gauthier et al. (2012). To further investigate the placement of Tetrapodophis, we included the taxon in the dataset of Simões et al. (2018) as later expanded by Garberoglio et al. (2019). Scorings were confirmed and no further modifications were performed to that dataset.”

Don’t borrow cladograms. Build your own. And for the sake of science, add taxa.

Back to the original question: Is Tetrapodophis a snake?
Based on the last common ancestor definition, Tetrapodophis and Barchelosaurus are last common ancestors of all tested snakes in the LRT. Someday someone will find a taxon that nests between Tetrapodophis and extant snakes. But for now, Tetrapodophis is still a snake.

Many of the publicity headlines for this paper used the word, “debunk”.
So here’s yet another lesson. Sometimes the debunker is correct. Other times, not so much. Watch out for ‘personal agendas’. We’ve seen this before with professor Michael Benton and his then student David Hone. We’ve seen this before with other pterosaur origin workers. We’ve seen this before with whale origin workers. We’ve seen this before with turtle origin workers. If professors are willing to omit taxa to support their pet hypotheses, watch out and be wary when you sign up for their classes or buy their textbooks.

References
Caldwell MW and Lee MSY 1997. A snake with legs from the marine Cretaceous of the Middle East. Nature 386:705-709.
Caldwell MW, Reisz RR, Nydam RL, Palci A and Simoes TR 2016. Tetrapodophis amplectus (Crato Formation, Lower Cretaceous, Brazil) is not a snake. Abstract from the 2016 meeting of the Society of Vertebrate Paleontology.
Caldwell et al. (6-co-authors) 2021. Tetrapodophis is not a snake: reassessment
of the osteology, phylogeny and functional morphology of an Early Cretaceous
dolichosaurid lizard. Journal of Systematic Palaeontology,
DOI:10.1080/14772019.2021.1983044
Garberoglio FF et al. 2019. New skulls and skeletons of the Cretaceous legged snake Najash, and the evolution of the modern snake body plan. Science Advances, 5, eaax5833. doi:10.1126/sciadv.aax5833
Gauthier JA et al. 2012. Assembling the squamate tree of life: perspectives from the phenotype and the fossil record. Bulletin of the Peabody Museum of Natural History, 53, 3–308. doi:10.3374/014.053.0101
Martill DM, Tischlinger H and Longrich NR 2015. A four-legged snake from the Early Cretaceous of Gondwana. Science, 349, 416–419. doi:10.1126/science.aaa9208
Simões TR, et al. 2018. The origin of squamates revealed by a Middle Triassic lizard from the Italian Alps. Nature, 557, 706–709. doi:10.1038/s41586-018-0093-3

https://pterosaurheresies.wordpress.com/2016/11/03/tetrapodophis-is-not-a-snake-svp-abstracts-2016/

https://pterosaurheresies.wordpress.com/2018/06/22/primitivus-a-new-marine-pre-snake-dolichosaur/

https://arstechnica.com/science/2021/11/new-study-debunks-controversial-2015-fossil-find-its-not-a-four-limbed-snake-after-all/?comments=1