Proterochampsia Paper

David Dilkes was kind enough to send his new paper on Proterochampsa (Dilkes and Arcucci 2012). His tree follows traditional nestings (Fig. 1). Dilkes and Arcucci (2012) also describe the long journey this odd branch has taken as new taxa were slowly added over the years. Makes interesting reading.

Proterochampsia tree

Figure 1. Proterochampsia tree by Dilkes and Arcucci (2012). Green added to highlight relationships recovered by the large reptile tree (Fig. 2). Some notes added in blue and red highlight missing and “by default” taxa that should not be included. Not sure why parasuchians don’t nest closer to proterochampsids here as they often do in other trees, including the large reptile tree. Euparkeria seems out of place there, but does nest close to Riojasuchus in other trees.

Unfortunately, one again, too few taxa were added to this tree to recover the same relationships recovered by the large reptile tree (Fig. 2) in which all sister taxa share larger suites of traits. In the Dilkes and Arcucci (2012) tree you get such odd pairings as Doswellia and Vancleavea, Riojasuchus and Aetosaurus, Euparkeria and Parasuchus among others. Only the taxa within the focus group, the Proterochampsia (node D), are true sisters also recovered by the large reptile tree.

Wisely, Dilkes and Arcucci (2012) left out pterosaurs, which are often nested close to parasuchians and proterochampsids. Unfortunately they left out Lagerpeton, members of the Choristodera and several Youngina/Youngoides specimens, all of which would have helped clarify relationships, according to the large reptile tree.

Segment of the large reptile tree.

Figure 2. Left: A segment of the large reptile tree showing what happens when more taxa are included. The Pararchosauriformes form a branch separate from the Euarchosauriformes and develop an antorbital fenestra and foss by convergence. Right: Reducing the branch on the left to include only those taxa chosen by Dilkes and Arcucci (2012) with the addition of the thalattosaur, Vancleavea, mistakenly chosen for inclusion by Dilkes and Arcucci (2012). Here more parsimony in sister taxa, but several forced nestings further toward the base of the tree.

A segment of the large reptile tree (Fig. 2) recovers a different topology because more taxa are included. In the large reptile tree the Proterochampsia were more closely related to parasuchians and choristoderans. All share a dorsal, posteriorly-displaced naris (reversed in Champsosaurus as a snorkel), and several other synapomorphies.

A Distinct Convergent Antorbital Fenestra
We discussed earlier the four times the antorbital fenestra was developed. Check it out. We also earlier discussed the nesting of the large proterochampsid (Fig. 3),

A new specimen attributed to Proterochampsa

Figure 3. A new specimen attributed to Proterochampsa alongside the holotype specimen.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Dilkes D and Arcucci A 2012. Proterochampsa barrionuevoi (Archosauriformes: Proterochampsia) from the Late Triassic (Carnian) of Argentina and a phylogenetic analysis of Proterochampsia.  Palaeontology (advance online publication) 1-33. doi: 10.1111/j.1475-4983.2012.01170.x

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