New study on sphenodontians excludes sphenodontians

Simoes, Caldwell and Pierce 2020
bring us their views on sphenodontian phylogeny. Traditionally, Sphenodontia (= Rhynchocephalia) is the clade that includes the extant tuatara, Sphenodon (Figs. 1–3) and related taxa.

The dorsal spines of Tuatara (Sphenodon).

Figure 2. The dorsal spines of Tuatara (Sphenodon).

From the abstract:
“Here, we investigate the impact of a wide array of model choices on the inference of evolutionary trees and macroevolutionary parameters (divergence times and evolutionary rates) using a new data matrix on sphenodontian reptiles. Specifically, we tested different clock models, clock partitioning, taxon sampling strategies, sampling for ancestors, and variations on the fossilized birth-death (FBD) tree model parameters through time.”

“Here, we investigate the impact of a wide array of model choices on the inference of evolutionary trees and macroevolutionary parameters (divergence times and evolutionary rates) using a new data matrix on sphenodontian reptiles. Specifically, we tested different clock models, clock partitioning, taxon sampling strategies, sampling for ancestors, and variations on the fossilized birth-death (FBD) tree model parameters through time.”

“We provide a new hypothesis of sphenodontian classification, along with detailed macroevolutionary patterns in the evolutionary history of the group.”

Backstory from Simoes, Caldwell and Pierce:
“The concept of the name Rhynchocephalia was initially conceived to include Sphenodon punctuates (Fig. 1) upon the realization of its quite distinct systematic placement relative to squamates among reptiles by Günther 1867. However, the concept was later expanded and for decades it was used to include both sphenodontians and rhynchosaurs (Romer 1956, Williston 1925), which was later refuted since the first computer-based phylogeny of reptiles placed rhynchosaurs as a lineage of archosauromorphs (Benton 1985).

That Benton 1985 study was flawed due to taxon exclusion and other factors common at the genesis of software analysis in 1985. More recent work based on adding taxa overturned that aspect of Benton 1985 by returning rhynchosaurs to the lepidosauria and the sphenodontia.

Figure 2. Subset of the LRT focusing on Sphenodontia.Blue bars are taxa omitted by the authors.

Figure 2. Subset of the LRT focusing on Sphenodontia.Blue bars are taxa omitted by the authors.

Unfortunately, 
by leaving out so many sphenodontian taxa (Fig. 2) Simoes, Caldwell and Pierce bring us only 60% of a study.

Furthermore,
the authors chose an outgroup taxon that was not a lepidosaur, lepidosauriform nor lepidosauromorph. They chose Prolacerta, which, despite its name did not ‘come before Lacerta‘ (a squamate nesting with Eolacerta). Prolacerta nests in the large reptile tree (LRT, 1772+ taxa) basal to archosauriforms.

Furthermore,
by cherry-picking their inclusion set, the authors did not realize their outgroup taxon, Megachirella, was actually a in-group basalmost sphenodontian.

Furthermore,
the authors did not realize their in-group taxa Homeosaurus (2 specimens) nested outside the  Sphenodontia among the Protosquamata.

Furthermore,
two of their outgroup taxa, Ardeosaurus and Eichstättisaurus, are squamates in the ancestry of snakes in the LRT.

Furthermore, 
none of their four published cladograms agree with each other or the LRT. So, taken in concert, those are shaky phylogenetic grounds to base any study on.

Figure 1. The best data I have been able to found to document the origin of rhynchosaurs like Scaphonyx and Hyperodapedon. Despite their apparent (from the literature) commonality, there is precious little in the literature about rhynchosaurs.

Figure 1. The best data I have been able to found to document the origin of rhynchosaurs like Scaphonyx and Hyperodapedon. Despite their apparent (from the literature) commonality, there is precious little in the literature about rhynchosaurs.

How did rhynchosaurs leave the rhynchocephalia?
We looked at that many years ago. Carroll (1988) revisiting Carroll (1977) reported, “It was long thought that rhynchosaurs were closely related to modern sphendontids not the basis of general similarities of the skull and dentition. The common presence of primitive features such as the lower temporal bar only points to their common origin among early diapsids. Although the dentition appears to be vaguely similar, it is fundamentally different. Sphenodontids have only a single row of acrodont teeth in the maxilla, but rhynchosaurs have multiple rows of teeth set in sockets. Sphenodontids have a second row of teeth in the palatine, but this bone is edentulous in the rhynchosaurs. What appear to be long premaxillary teeth in the rhynchosaurs are actually processes from the premaxillary bones. Sphenodontids have true premaxillary teeth.”

Do you hear Carroll’s colleague, Larry Martin, laughing?
Clades and interrelationships are determined not by possibly convergent individual traits, but by phylogenetic analysis. The last common ancestor and all of its descendants is the only valid and irrefutable way to recognize and organize clades.

Getting back to Carroll (1977, 1988) the problem is:
the rhynchosaur outer tooth row is indeed the maxilla, the inner row is the palatine, as in rhynchocephalians, especially easy to see in Priosphenodon. That the palatine fuses to the maxilla does not take away the identity of either. Thus, the palatine is not edentulous in rhynchosaurs.

The premaxillary is also toothless
in Priosphenodon, currently considered a rhynchocephalian. Mesosuchus is considered a proto-rhynchosaur, not a rhynchocephalian, yet it has socketed teeth on the premaxilla. So there’s a transition zone developing between basal and derived rhynchocephalians, which are distinct, but decidedly related — because — no other taxa are closer to them than these two are to each other in the LRT.

At the start of his career, Benton (1983) reported, 
“Rhynchosaurs have no special relationship with the sphenodontids. The supposed shared characters are either primitive (e.g. complete lower temporal bar, quadratojugal, akinetic skull, inner ear structure, 25 presacral vertebrae, vertebral shape, certain character of limbs and girdles) or incorrect (e.g. rhynchosaurs do not have acrodont teeth, the ‘beak-like’ premaxilla of both groups is quite different in appearance, the ‘tooth plate’ is wholly on the maxilla in rhynchosaurs but on maxilla and palatine in sphenodontids).”

At the time, and perhaps to this day,
Benton did not realize the lower temporal bar was derived in sphenodontians. Early lepidosaurs don’t have it.

Acrodont teeth are also derived from socketed teeth,
so all sphenodontids had to do was stop fusing their teeth to their skull in order to go back to the socketed teeth found in rhynchosaurs.

Rhynchosaurs stop fusing their ankles
at the same time that they stop fusing their teeth to their jaws. That’s just what they do. It doesn’t mean they get kicked out of their family. That would be “Pulling a Larry Martin.”

Figure 1. Rhynchocephalian and Rhynchosaur palates. That's Priosphenodon in the middle leading to Mesosuchus and Howesia, to Trilophosaurus and Azendohsaurus and rhynchosaurs. That's where the palatine grows as large as and alongside the maxilla. In derived taxa these two bones fuse creating the illusion that the maxilla has the entire tooth pad. Look at those palatine stems on Priospbenodon, which really come out on rhynchosaurs.

Figure 4. Rhynchocephalian and Rhynchosaur palates. That’s Priosphenodon in the middle leading to Mesosuchus and Howesia, to Trilophosaurus and Azendohsaurus and rhynchosaurs. That’s where the palatine grows as large as and alongside the maxilla. In derived taxa these two bones fuse creating the illusion that the maxilla has the entire tooth pad. Look at those palatine stems on Priospbenodon, which really come out on rhynchosaurs.

What can one learn from Simoes, Caldwell and Pierce 2020? 

  1. Stop cherry-picking taxa
  2. Stop trusting others for your cladograms and inclusion sets
  3. Add more taxa to let your wider gamut cladogram tell you which taxa are in the inclusion set
  4. Show fossils and reconstructions ,so readers can judge the accuracy of your scoring choices

It’s all comes down to taxon exclusion
and inappropriate taxon inclusion. Sure it’s faster and easier to trust others. But when you do, you and your team are now liable for their mistakes. Whatever your results, they depend on a valid phylogenetic context, missing in the sphenodontian study of Simoes, Caldwell and Pierce 2020.

Once you have a wide gamut cladogram,
it’s yours forever. Every worker should have their own.

The LRT has shown since 2011
that rhynchosaurs are derived sphenodontians. Additional taxa since then have only reinforced that nine-year-old hypothesis of interrelationships.


References
Benton M J 1983. TheTriassic reptile Hyperodapedon from Elgin: functional morphology and relationships. Phil. Trans. R. Soc. Lond. B 302, 605^717. Carroll, R. L. 1988 Vertebrate paleontology and evolution. New York: W. H. Freeman & Co.
Benton MJ 1985. Classification and phylogeny of the diapsid reptiles. Zool J Linnean Soc. 84(2):97–164.
Carroll RL 1977. The origin of lizards. In Andrews, Miles and Walker [eds.] Problems of Vertebrate Evolution. Linnean Society Symposium Series 4: 359–396.
Carroll RL 1988. Vertebrate Paleontology and Evolution. W. H. Freeman and Co. New York.
Günther A 1867. Contribution to the anatomy of Hatteria (Rhynchocephalus, Owen). Ann. Mag. Nat. Hist. (3) 20: 128-129.
Romer AS 1956. Osteology of the reptiles. 1st ed. Chicago: University of Chicago Press;
Simoes TR, Caldwell MW and Pierce S 2020. Sphenodontian phylogeny and the impact of model choice in Bayesian morphological clock estimates of divergence times and evolutionary rates. BMC Biol 18, 191 (2020). https://doi.org/10.1186/s12915-020-00901-5
Williston SW 1925. The osteology of the reptiles. Cambridge: Harvard University Press; 1925.

wiki/Sphenodon
wiki/Rhynchocephalia

https://pterosaurheresies.wordpress.com/2013/07/29/kaikaifilusaurus-and-the-origin-of-the-rhynchosaurs/

https://pterosaurheresies.wordpress.com/2013/03/23/sapheosaurus-bridges-the-sphenodontidtrilophosaurrhychosaur-gap/

https://pterosaurheresies.wordpress.com/2011/09/24/moving-rhynchosaurs-and-trilophosaurs-back-into-the-rhynchocephalia-sphenodontia/

https://pterosaurheresies.wordpress.com/2016/01/13/early-evolution-of-rhynchosaurs/

https://pterosaurheresies.wordpress.com/2014/02/18/the-origin-of-rhynchosaurs-revisited/

https://pterosaurheresies.wordpress.com/2014/03/20/arguments-against-the-rhynchosaurrhynchocephalian-rhylationship/

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