The most primitive Choristoderes?

Choristoderes
are a difficult clade to figure out both inside and out.

Wikipedia reports, “Champsosaur skulls are actually very similar to lizard skulls, though heavily modified. This has led some researchers to consider champsosaurids as lepidosauromorphs. However, champsosaurs lack the complex quadrate of lepidosaurians. With features of both diapsid groups, the phylogenetic position of Choristodera is highly confused.”

Matsumoto et al. (2007) reports, “This tree confirms the monophyly of Neochoristodera (Evans and Hecht1993) including Champsosaurus, Ikechosaurus, Simoedosaurus, and Tchoiria. The relationships of the non−neochoristoderan taxa have been more controversial. In our analysis, the Jurassic Cteniogenys retains a basal position, with the Chinese Philydrosaurus (Gao and Fox 2005) one node above it.”

Note the loss of resolution at the base of their tree (Fig. 1).

Figure 1. The family tree of the Choristodera according to Matsumoto et al. 2007. Early eras added in color. Note that ancestral taxa were large and long-snouted with four temporal openings, not two.

Figure 1. Click to enlarge. The family tree of the Choristodera according to Matsumoto et al. 2007. Early eras added in color. Note that some ancestral taxa, like Diandongosuchus, were large and long-snouted. Others were small, like the BPI specimen of Youngina, but equally long snouted. So the long-snouted forms may be more primitive. Note the loss of resolution at the clade base.

The question in the Matsumoto et al. 2007 tree is what happened in pre-Jurassic times? In 2007 Youngina was indeed the best outgroup taxon, but several varieties are known and some of these have a longer rostrum. Unfortunately these were overlooked as Matsumoto et al. focused on known choristoderes. In 2012 the large younginidDiandongosuchusbecame known. While it nests at the base of the parasuchia, it also nests at the bases of the Choristodera and Chanaresuchia. Yes, it has an antorbital fenestra, and so do some Youngina specimens. It is possible that the Choristodera secondarily lost their aof. Or they never had one.

In any case, the large reptile tree recovered a similar basal split between the large and small choristoderes, but with complete resolution among the five small ones and three large ones. More precision in the character scoring of the various Youngina specimens should add clues to this mystery. Don’t discount those subtle variations!

References
Matsumoto R, Evans SE and Manabe M 2007. The choristoderan Monjurosuchus form the Early Cretaceous of Japan. Acta Palaeontologica Polonica 52(2):329-350.

 

4 thoughts on “The most primitive Choristoderes?

    • Doesn’t matter, phylogenetically, if they do or not (that’s the cusp when they appear), but I’ve seen some that look like there’s something going on there. Send me the evidence to your position and we’ll straighten this out. I’ve learned to distrust comments and to only trust evidence (which is good Science). And, uh, on that note, you owe me a skull photo from a promise a few year’s back, if you recall. I think it’s the Chicago specimen. Thanks for touching base, Nick. Now let’s get on the good foot.

      • If you’d seen the actual specimen, you’d know that it’s not there. It’s damage from prep that left the overlaying bone too thin which made it not render clearly in the CT image (most of the external surface of the type specimen is chipped, pitted with prep damage and crushed by geology). Those aren’t photographs, they’re volume renderings. Your “evidence” comes from my paper but you didn’t read it to understand what was going on- the volume renderings were only provided to show the braincase in situ.

        A comment on evidence and burden of proof. Numerous authors over time have examined specimens of Youngina (Broom, Watson, Romer, Gow, Carroll, and onward) and no one ever saw any evidence of the AOF. Sure, they could be wrong, but the burden of proof isn’t on them to demonstrate that it isn’t present. You’re the one asserting that it is present, which puts it on you. It’s pretty easy to see that it’s not present (the specimen should be back on display at the AMNH). I’ve explained to you before why it appears that way in those images and why it isn’t the case.

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