Early Evolution of Rhynchosaurs

A new open access paper
by Ezcurra, Montefeltro and Butler 2016 provides several first time ever color photos of rhynchosaur skulls and a cladogram of rhynchosaur relationships (Fig. 1). It’s a good paper, with good interrelationships. Unfortunately the wrong outgroup, the Protorosauria, was chosen.

Figure 1. Rhynchosaur cladogram by Ezcurra et al. 2016. Note the outgroup includes two protorosaurs. The large reptile tree recovers protorosaurs elsewhere and has a long list of outgroup taxa among the trilophosaurs and rhynchocephalians. See figure 2.

Figure 1. Rhynchosaur cladogram by Ezcurra et al. 2016. Note the outgroup includes two protorosaurs. The large reptile tree recovers protorosaurs elsewhere and has a long list of outgroup taxa among the trilophosaurs and rhynchocephalians within the Lepidosauromorpha, not the Archosauromorpha. See figure 2. Carmel area includes taxa matching the large reptile tree among rhynchosaurs and proximal outgroups.

By contrast,
the large reptile tree nests rhynchosaurs with trilophosaurs and rhynchocephalians (sphenodontids, Fig. 2), not protorosaurs. Taxon inclusion will help you recover this relationship, too, if you wish to repeat the experiment. Ezcurra et al. (2016) relied on untested tradition, but that tradition brings with it a certain air of credulity as Prolacerta does indeed converge with Mesosuchus in several regards. But parsimony prevails when the following lepidosauromorphs (Fig. 2) are included in analysis. This is a relationship best left to software, not eyeballs and paradigms.

Figure 2. This subset of the large reptile tree nests rhynchosaurs with trilophosaurs and rhynchocephalians, not protorosaurs.

Figure 2. This subset of the large reptile tree nests rhynchosaurs with trilophosaurs and rhynchocephalians, not protorosaurs. Where is Priosphendon in the Ezcurra study? 

In the transition from rhynchocephalians to rhynchosaurs,
this clade had an interesting radiation that included Leptosaurus, Sapheosaurus, Trilophosaurus and Azendohsaurus (which also nests with protorosaurs when the taxa in figure 2 are excluded, before producing rhynchosaurs. Priosphenodon (Fig. 3), typically considered a Cretaceous rhynchocephalian, is a transitional taxon for some reason left off of the Ezcurra et al. 2016 taxon list that nests closer to rhynchosaurs than Mesosuchus does in the large reptile tree. Probably because all rhynchosaurs died out by the Jurassic.

Figure 3. Priosphenodon nests closer to rhynchosaurs than Mesosuchus does, yet it was not included in the Ezcurra et al. 2016 study.

Figure 3. Priosphenodon nests closer to rhynchosaurs than Mesosuchus does, yet it was not included in the Ezcurra et al. 2016 study. Perhaps because the only known fossils are a hundred million years too late. 

References
Ezcurra MD, Montefeltro F and Butler RJ 2016. The Early Evolution of Rhynchosaurs. Frontiers in Ecology and Evolution 3:142 (23 pgs) doi: 10.3389/fevo.2015.00142 http://dx.doi.org/10.3389/fevo.2015.00142

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