Squamate tooth complexity: Lafuma et al. 2020

Updated July 7, 2020
the LRT moves Meyasaurus, Indrasaurus and Hoyalacerta to the base of the Yabeinosaurus + Sakurasaurus clade within the Scleroglossa and Squamata.

This blogpost builds slowly. 
If you are short of time, drag down to the final paragraphs.

Lafuma et al. 2020 report,
“Complexity increase through cusp addition has dominated the diversification of many mammal groups.”

Be careful with blanket statements like that. What they wrote may be true of pre-mammal cynodonts (adding cusps), but teeth decrease in complexity in the lineage of pangolins, edentates, odontocetes and mysticetes. Carnivores have fewer teeth. So do elephants and manatees.

“However, studies of Mammalia alone don’t allow identification of patterns of tooth complexity conserved throughout vertebrate evolution.”

That sentence needs a re-write. It does not make sense.

“Here, we use morphometric and phylogenetic comparative methods across fossil and extant squamates (“lizards” and snakes) to show they also repeatedly evolved
increasingly complex teeth, but with more flexibility than mammals.”

Starting to sound iffy here knowing that Iguana (Fig. 1) is a basal squamate in the large reptile tree (LRT, 1669+ taxa, subset Fig. 2) and it has complex multi-cusp teeth. In the LRT varanids, sea-going mosasaurs and all legless lizards (including snakes) are all highly derived — and they have simple cones for teeth.

Pet Peeve: The authors don’t discuss lepidosaur pterosaurs that likewise had multi-cusp teeth in the Triassic, and only one cusp or no teeth in derived taxa.

Figure 2. The basalmost tested iguanid, Iguana. Note the resemblance to basalmost scleroglossans.

Figure 2. The basalmost tested iguanid, Iguana, one of the basalmost squamates in the LRT, contra Lafuma et al. who omitted so many outgroup taxa that their cladogram was upside-down.

Lafuma et al. 2020 continue,
“Since the Late Jurassic, six major squamate groups independently evolved multiple-cusped teeth from a single-cusped common ancestor.”

And those six in their phylogenetic order are:

  1. Scincoidea
  2. Polyglyphanodontia
  3. Lacertoidea
  4. Mosasauria
  5. Anguimorpha
  6. Iguania

Sophineta and three members of the Rhynchocephalia are outgroups to Squamata in the Lafuma et al. cladogram. That’s reasonable, but far from complete, and with disastrous consequences (see below).

“Unlike mammals reversals to lower cusp numbers were frequent in squamates, with varied multiple-cusped morphologies in several groups resulting in heterogenous evolutionary rates.”

See above.

The Lafuma et al. 2020 cladogram
lists the following clades of Squamates in this order (LRT order in parentheses).

  1. Gekkota (4th in the LRT and they share an ancestry with Serpentes in the LRT)
  2. Dibamia (last in the LRT, within skinks)
  3. Scincoidea (last in the LRT)
  4. Polyglyphanodontia (third in the LRT)
  5. Lacertoidea (second in the LRT)
  6. Mosasauria (fourth to last in the LRT)
  7. Serpentes (4th and they share an ancestry with Gekkota in the LRT)
  8. Anguimorpha (second to last in the LRT)
  9. Iguania (first in the LRT)

Due to taxon exclusion
the Lafuma et al. 2020 cladogram is inverted (upside-down) compared to the LRT (Fig. 2). As a result, so is their conclusion.

But let’s dig deeper trying to figure out how
this inversion happened. The authors report, “For topology we followed the total evidence phylogeny of Simões et al. – the first work to find agreement between morphological and molecular evidence regarding early squamate evolution.” Take a second look, dear readers. Borrowing a cladogram, taxon exclusion and genomics has given these workers an upside-down topology.

Figure 1.  Subset of the LRT focusing on lepidosaurs and snakes are among the squamates.

Figure 2.  Subset of the LRT from 2019 focusing on lepidosaurs including squamates.

Lafuma et al. 2020 list several hundred more squamate taxa
than the LRT includes, but this is where outgroups become important. Here is a list of missing Protosquamata taxa from the Lafuma et al. taxon list. Adding these taxa would bring much needed polarity to the Lafuma et al. cladogram:

  1. Lacertulus
  2. Schoenesmahl
  3. Fraxinisaura
  4. Hoyalacerta
  5. Indrasaurus
  6. Homoeosaurus
  7. Dalinghosaurus
  8. MFSN 19235
  9. Scandensia
  10. Calanguban
  11. Liushusaurus
  12. Purbicella
  13. Hongshanxi
  14. Euposaurus

But that’s not all… add to that list:
Tetraphodophis, Jucaraseps and Ardeosaurus. These three taxa link Norellius, Eichstaettisaurus and geckos to basal snakes. In the Lafuma cladogram Norellius, Eichstaettisaurus and geckos nest apart from Pontosaurus + Adriosaurus. For some reason, the basalmost gekko in the LRT, Tchingisaurus, nests with the basal amphisbaenan, Sineoamphisbaena in the Lafuma et al. tree. A sister, Sineoscincus, is omitted from the Lafuma et al. tree. Bahndwivici and Yabeinosaurus, nest basal to varanids and mosasaurs in the LRT, but are not listed by Lafuma et al.

If you’re going to report on the order of acquisition of traits,
you have to have your phylogenetic order established correctly. To do that you have to include more outgroup taxa, something that was not done in the Lafuma et al. study. By contrast, the LRT includes outgroup taxa back to Cambrian headless chordates, just to be sure all the bases are covered.


References
Lafuma F, Corfe IJ, Clavel J and Di-Pol N 2020. Multiple evolutionary origins and losses of tooth complexity in squamates. biRxiv preprint: https://doi.org/10.1101/2020.04.15.042796

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