New Champsosaurus paper perpetuates old myths

Whenever taxon exclusion mistakes are made and reviewed here,
I try to write to the lead author of the paper. Below is a recent email directed to Professor Dudgeon et al. 2020 on their recent review of the well-preserved skull of Champsosaurus (Figs. 1, 3), which they re-examined using computed tomography analysis.

Figure 1. Champsosaurus from Dugeon et al. Here the nasal is the ascending process of the premaxilla. The prefrontal is the nasal fused to the prefrontal. The postorbital is the postfrontal and vice versa.

Figure 1. Champsosaurus from Dugeon et al. Here the nasal is the ascending process of the premaxilla. The prefrontal is the nasal fused to the prefrontal. The postorbital (pro) is the postfrontal (pof) and vice versa.

Dear Dr. Dudgeon:

It’s always good to see new studies on old skulls.

Based on phylogenetic bracketing the bone traditionally identified as the ‘nasal’ is the ascending process of the premaxilla. That makes the purported ‘prefrontal’ a fused nasal + prefrontal. The postorbital and postfrontal are mislabeled with the other bone identity based on Tchoria (Fig. 2), a taxon not mentioned in your text. See attached.

Choristoderes are not ‘neodiapsid reptiles.’ Phylogenetically they are archosauriformes arising from Proterosuchus, Elachistosuchus and Tchoria. Phylogenetic miniaturization in that lineage lost the antorbital fenestra. See links below.

https://pterosaurheresies.wordpress.com/2013/08/13/champsosaurus-and-its-snorkel-nose/
http://reptileevolution.com/reptile-tree.htm
http://reptileevolution.com/champsosaurus.htm
http://reptileevolution.com/youngina-bpi2871.htm
http://reptileevolution.com/hyphalosaurus.htm
http://reptileevolution.com/lazarussuchus.htm

Best regards,

Figure 1. Tchoria and phylogenetic bracketing help identify bones in the skull of Champsosaurus (Fig. 2).

Figure 2. Tchoria and phylogenetic bracketing help identify bones in the skull of Champsosaurus (Fig. 2).

So, the Dudgeon et al. paper
is yet another great example of a situation in which phylogenetic analysis and bracketing (= comparing related taxa) sheds more light on a specimen than high-resolution micro-computed tomography scanning and/or adding characters (= looking more deeply into one taxon to the exclusion of others).

Figure 2. Champsosaurus skull with premaxilla in yellow.

Figure 3. Champsosaurus skull with premaxilla in yellow, nasal + prefrontal in pink. Bone identities determined by phylogenetic bracketing with Tchoria. See figure 2.

The greatest benefit 
available from the large reptile tree (LRT, 1631 taxa) is this sort of phylogenetic bracketing based on the validated nesting of sisters that have never been tested together in prior studies. You can look more deeply into one skull, as Dudgeon et al. did. Or you can examine many skulls, as ReptileEvolution.com and the LRT enable workers to do (Figs. 2, 4). In this case, using computed tomography on one skull did not put an end to traditional myths regarding the identity of bones in Champsosaurus.

Note to readers who like to harp on these issues:
More characters were not needed to resolve these problems. More taxa were needed.

Firsthand access + computed tomography did not help Dudgeon et al. Rather, a century-old drawing (Brown 1905, Fig. 3), access to several sister taxa for comparison (Figs. 2, 4) and Adobe Photoshop were the tools needed to resolve this issue.

It helps to know what you are dealing with.
Only a wide-gamut phylogenetic analysis that minimizes taxon exclusion can tell you where a specimen nests in the cladogram. Too often workers like Dudgeon et al. rely on vague citations, rather than running tests themselves or citing ongoing and self-repairing studies like the LRT. Publishing a mistake is to be avoided no matter how trivial.

Figure 2. Dorsal, lateral and palatal views of BPI 2871 with bones colorized above. Below, reconstructed images of BPI 2871 tracings. It is more complete than illustrated by Gow 1975. Click to enlarge. Note the tiny remnant of the antorbital fenestra. The squamosal has been broken into several parts.

Figure 4. Dorsal, lateral and palatal views of Late Triassic BPI 2871 with bones colorized above. Below, reconstructed images of BPI 2871 tracings. It is more complete than illustrated by Gow 1975. Note the tiny remnant of the antorbital fenestra and the long ascending process of the premaxilla.  The squamosal has been broken into several parts. This is a tiny phylogenetically miniaturized sister to the ancestor of Champsosaurus.

Champsosaurus annectens (Cope 1876, Brown 1905) ~1.5 m in length, Late Cretaceous to Eocene. Champsosaurus was derived from a sister to Tchoiria, and was a sister to other choristoderes, such as Cteniogenys and Lazarussuchus. This clade must have originated in the Late Permian or Early Triassic, but fossils are chiefly from late survivors, hence the wide variety in their morphology.


References
Brown B 1905. The osteology of Champsosaurus Cope. Memoirs of the AMNH 9 (1):1-26. http://digitallibrary.amnh.org/dspace/handle/2246/63
Cope ED 1876.
On some extinct reptiles and Batrachia from the Judith River and Fox Hills beds of Montana: Proceedings of the Academy of Natural Sciences, Philadelphia. 28, p. 340-359.
Dudgeon TW, Maddin HC, Evans DC & Mallon JC 2020. 
Computed tomography analysis of the cranium of Champsosaurus lindoei and implications for choristoderan neomorphic ossification. Journal of Anatomy (advance online publication)
doi: https://doi.org/10.1111/joa.13134
https://onlinelibrary.wiley.com/doi/10.1111/joa.13134

http://reptileevolution.com/champsosaurus.htm

2 thoughts on “New Champsosaurus paper perpetuates old myths

  1. Based on phylogenetic bracketing the bone traditionally identified as the ‘nasal’ is the ascending process of the premaxilla.

    …So how come there’s a suture between it and the premaxilla, then?

    • re: suture…. Good question. Covered that in 2013. Here’s the pertinent paragraph. “Traditionally
      In Champsosaurus (Fig. 1) the dorsal medial bone is traditionally considered the nasal and the paired bones following it are considered the prefrontals. However if you look at all the closest kin to Champsosaurus it becomes clear that the paired bones remain traditional nasals. The prefrontals are simply missing, likely due to fusion with the nasals. That means the tooth-bearing portions of the premaxilla wrapped completely around the rostrum and nares until they came into contact with the ascending process of the premaxilla, which extends beyond the naris in many related taxa.”

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