Tiny toothy Navajosphenodon enters the LRT basal to Trilophosaurus, not with beaked Sphenodon

According to the NY Times:
“While sifting through a backlog of fossils at Harvard University’s Museum of Comparative Zoology, Stephanie Pierce, the museum’s curator of vertebrate paleontology, and her team recently discovered the nearly complete skeleton of a lizardlike animal on a slab of stone small enough to fit in the palm of their hands. The remarkable fossil was discovered in 1982 during an expedition to the Kayenta Formation, a fossil-rich outcrop in northern Arizona.”

Always makes a great story whenever overlooked fossils come under study.

The NY Times headline promised,
“Fossil reveals secrets of the Nature’s most mysterious reptiles.”

An edited second draft might have read, …the secrets of Nature’s..
PS. A day later the NY Times edited their headline (Fig. x).

Figure x. The title was revised a day later. See, you can fix and erase mistakes if you make them online.

Spoiler alert: This teasing headline has a limited payoff.
Pierce concludes, “We need more fossils.” Moreover, some of their revelations are wrong.
Also, the subhead is wrong.
That ancestor that lived 190 mya (Figs 1–3) looks so different from extant Sphenodon that three nodes in the LRT (Fig 4) separate the two, and the size is way off.

Unfortunately,
more powerful phylogenetic headlines were overlooked due to taxon exclusion in this study. After reconstruction (Figs 2,3) and analysis, Navajosphenodon has an importance greater than its diminutive size.

Figure 1. The subadult holotype of Navajosphenodon shown actual size at lower left.

That ‘remarkable fossil’
was named Navajosphenodon (Figs. 1–3) by Simões, Kinney-Broderick and Pierce 2022 based on its resemblance to the extant tuatara, Sphenodon. The tiny relatively complete holotype specimen (Fig 1) was about half the size of far less complete adult specimens of Navajosphenodon found nearby. Even so, that makes even a large Navajosphenodon smaller than any sphenodontian relatives, with the single exception of the related, Leptosaurus, a taxon not mentioned by Simões, Kinney-Broderick and Pierce.

Figure 2 from Simões, Kinney-Broderick and Pierce 2022. New colors, pelvis and skull reconstructions added here. Note the elongate, Trilophosaurus-like torso and lack of a Sphenodon-like beak.

Navajosphenodon sani
(Simões, Kinney-Broderick and Pierce 2022; MNA.V.12442 formerly MCZ VP 9016, Kayenta Fm, Early Jurassic) was originally considered a sphenodont close to coeval and highly fragmentary Cynosphenodon. In turn, those two were considered close to extant Sphenodon.

By simply adding omitted pertinent taxa,
the LRT (subset Fig 4) pulls Navajosphenodon away from Sphenodon. Cynosphenodon will not be tested in the LRT unless more complete specimens are discovered. It does have a mandible similar to Navajosphenodon.

Figure 3. The first reconstructed skull of Navajosphenodon. Lacking the beak of Sphenodon, this taxon has the premaxillar lateral ascending processes of Trilophosaurus (Fig 5), along with a long list of homologous traits, yet retains many sphenodontian traits.

Here
in the Large Reptile Tree (LRT, 2058 taxa, subset Fig. 4) tiny Navajosphenodon (Figs 1–3) nests as a tiny transitional taxon at the base of the Trilophosaurus (Fig 5) + Rhynchosaurus clade. Those taxa were not included in the original study. In the LRT this clade is derived from the Sphenodontia (= Rhynchocephalia). Navajosphenodon further cements this LRT hypothesis of interrelationships arrived at by testing taxa together that were not tested together in prior studies.

Lacking a front half,
Noteosuchus is a traditional basal rhynchosaur, but in the LRT (Fig 4) i Noteosuchus nests basal to Navajosphenodon, closer to traditional sphenodontians. Taxon exclusion remains the number one problem in paleontology. Adding taxa solves those paleo problems.

Figure 4. Subset of the LRT focusing on the Sphenodontia (= Rhynchocephalia) terminating in extant Sphenodon and extinct rhynchosaurs, trilophosaurs and kin, taxa omitted from sphenodontian studiies. Tiny Navajosphenodon links them together. Noteosuchus is a traditional basal rhynchosaur, but note it nests basal to Navajosphenodon here.

Due to its tiny size
Navajosphenodon is a late-surviving, phylogenetically miniaturized taxon documenting the shift from a beaked sphenodontian morphology to a toothy trilophosaur morphology. Based on the stratigraphic presence ofTrilophosaurus (Fig 5) in the Late Triassic, that radiation occurred earlier.

The last known survivor of the clade Rhynchosauria,
Priosphenodon, exteded that clade to the Cenomian (early Late Cretaceous). Priosphenodon is traditionally considered closer to Sphenodon than to Rhynchosaurus, but this is due to taxon exclusion. In the LRT Priosphenodon nests at the base of the otherwise Triassic Rhynchosauria, which have homologous premaxillary beaks.

Figure 5. Trilophosaurus has filled in the lateral temporal fenestra with an enalrged quadrate, reduced the orbit and increased the upper temporal fenestra. Compare to Navajosphenodon (Figs 1–3).

Notably, perhaps oddly,
in the transition zone from beaked sphenodontians to beaked rhynchosaurs there are several toothed taxa. Among them, Azendohsaurus, Trilophosaurus and odd Teraterpeton bridge this transition zone. So does toothed and tiny Navajosphenodon at their base. The reappearance of premaxillary teeth in these transitional taxa should be considered reversals since precursor taxa (see next paragraph) had similar teeth. Phylogenetic miniaturization in Navajosphenodon indicates reversal via neotony was at play at the phylogenetic genesis of these toothy transitional taxa.

Simões, Kinney-Broderick and Pierce wrote,
“The left premaxilla has three acrodont teeth preserved in situ and not forming the ventrally expanded dentigerous beak observed in most sphenodontians—a condition more commonly observed in early sphenodontians, such as Gephyrosaurus and Diphydontosaurus.”

Here Simões, Kinney-Broderick and Pierce recognized this reversal.

Phylogenetic context is every bit as important as stratigraphic context.
Let’s keep adding taxa to our cladograms (e.g. Fig 4) until taxon exclusion is no longer the endemic problem it remains today.

References
Simões TR, Kinney-Broderick G and Pierce SE 2022. An exceptionally preserved Sphenodon-like sphenodontian reveals deep time conservation of the tuatara skeleton and ontogeny. Communications Biology 5, 195; doi: 10.1038/s42003-022-03144-y

wiki/Rhynchosaur
wiki/Trilophosaurus
wiki/Rhynchocephalia

Publicity:
sci-news.com/paleontology/navajosphenodon-sani-10594.html