The Solomon Islands skink (genus Corucia) enters the LRT

Today the extant Solomon Islands skink
(Corucia zebrata, Gray 1855; Figs. 1, 2) enters the large reptile tree (LRT, 1714+ taxa). It nests basal to Gymnophthlamus + Vanzosaura and between Chalcides and Sirenoscincus.

Figure 1. The Solomon Islands skink (Corucia zebrata) is the largest skink on the planet, gives birth with a placenta and lives in communities.

Figure 1. The Solomon Islands skink (Corucia zebrata) is the largest skink on the planet, gives birth with a placenta and lives in communities.

This nesting comes as no surprise.
After all, skeletally Corucia is just another widely recognized skink, albeit with some unique reproductive and social qualities (see below).

Figure 2. The skink, Corucia zebrata with DGS colors added.

Figure 2. The skink, Corucia zebrata with DGS colors added.

Do not confuse Corucia with Carusia
(Fig. 3). The two are not the same, nor are they closely related.

Figure 1. Carusia intermedia, a basal lepidosaur close to Meyasaurus now, but looks a lot like Scandensia. Note the primitive choanae and broad palatal elements. None of the data I have shows the caudoventral process of the jugal, so I added it here from the description. Same with the epipterygoid.

Figure 3. Carusia intermedia, a basal lepidosaur close to Meyasaurus now, but looks a lot like Scandensia. Note the primitive choanae and broad palatal elements. None of the data I have shows the caudoventral process of the jugal, so I added it here from the description. Same with the epipterygoid.

Corucia zebrata
(Gray 1855, Figs. 1, 2) is the extant Soloman Islands skink, the largest known extant species of skink. Long chisel teeth distinguish this herbivorous genus. The tail is prehensile. This is one of the few species of reptile to live in communal groups. Rather than laying eggs, relatively large young are born after developing within a placenta. Single babies are typical. Twins are rare according to Wikipedia.

Removing all Carusia sister taxa in the LRT
fails to shift Carusia from its traditionally overlooked node basal to squamates.

The Wikipedia entry
on the ‘clade’ Carusioidea excludes great swathes of taxa relative to the LRT, so it mistakenly suggests that extinct Carusia is a member of the Squamata. Adding pertinent taxa solves that problem, as the LRT demonstrates.


References
Gray JE 1855. (1856). New Genus of Fish-scaled Lizards (Scissosaræ), from New Guinea. Annals and Magazine of Natural History, Second Series 18: 345–346.

wiki/Solomon_Islands_skink
wiki/Carusia
wiki/Carusioidea
http://www.markwitton.com
http://tetzoo.com

https://www.researchgate.net/publication/328388754_A_new_lepidosaur_clade_the_Tritosauria

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

Tree topology change separates protosquamates from tritosaurs

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

New data
added to the large reptile tree has changed the topology of the Lepidosauria. This is the biggest change I’ve seen in the last four years, yet it occurred in a fairly dark alley of the Amniota where very few researchers lurk.

Now the Tritosauria has been halved,
extending only from Tijubina to pterosaursPalaegama is still the outgroup.

Figure 1. Palaegama is basal to Coelurosauravus ('rib' gliders), Megachirella (rhynchocephalians), Lacertulus (protosquamates) and Tijubina (tritosaurs)

Figure 1. Palaegama is basal to Coelurosauravus (‘rib’ gliders), Megachirella (rhynchocephalians), Lacertulus (protosquamates) and Tijubina (tritosaurs). Taxa to scale. Note the miniaturization at the base of the Lepidosauria and the similarity of all these sister taxa.

Palaegama is still basal to the so-called ‘rib’ gliders (not ribs, but dermal extensions as we discovered here). It is also basal to all Lepidosaurs.

Among the Lepidosaurs…
Former tritosaurs now form a grade of protosquamates extending from Lacertulus and Bavarisaurus to the primal division of Iguania and Scleroglossa.

Figure 2. Protosquamates to scale. Click to enlarge. Colors indicate clades.

Figure 2. Protosquamates to scale. Click to enlarge. Colors indicate clades.

Tiny Scandensia remains as the last common ancestor of the Lepidosauria and the MFSN 19235 specimen is its larger predecessor.

Other protosquamates include the Daohugou lizard, Carusia, Meyasaurus, Homoeosaurus, Dalinghosaurus and Hoyalacerta.

This change followed the addition of several basal rhynchocephalians, like Megachirella, that shed new light on the origin of the Lepidosauria and the radiations that succeeded them. We looked at those additions here.

Nomenclature
The Tritosauria remains a clade distinct from Squamates. We’ll need a new name for the clade that includes Lacertulus and Iguana, their last common ancestor and all of its descendants. That clade does not include the Tritosauria. A clade that includes the new clade and the Tritosauria is also needed. The Rhynchocephalia is a sister taxon and Palaegama is the current outgroup.

The interesting thing, and we’ve seen this before…
The order of the protosquamates is exactly the opposite as I originally thought. The logic behind this is like folding a piece of paper. The sister taxa at the fold are the same. The beauty of the new topology places the Permian Lacertulus at the base of its clade, where it belongs chronologically and nests the most derived taxon, Scandensia from the Early Cretaceous where it belongs. We’ve seen this before when M. Mortimer nested beaked dinosaurs basal to theropods and I pointed out the hypothesis that the tree was upside-down. See. It can happen to anybody.

The problem with Scandensia is, it looks a lot like the basal rhynchocephalian, Gephyrosaurus and it has long digits like Palaegama. So, I was readily fooled by what appeared to be the real deal — but it was not the real deal. When you look at the protosquamates (Fig. 2), there is no obvious direction to this clade or grade of lepidosaurs. The details of the skeleton reveal their relationships.