A new, tiny choristodere, Mengshanosaurus, enters the LRT

It’s tiny and probably a hatchling because sister taxa are much larger.
Mengshanosaurus minimus (Yuan et al. 2021; Early Cretaceous, China; skull length 3.5cm) nests between Ikechosaurus and Champsosaurus in the LRT (subset Fig. 3). Note the indented remnants of the antorbital fenestra in the hatchling model (Fig. 1). Apparently the post-frontal fontanelle is not a pineal opening. Sister taxa do not have a pineal opening.

Figure 1. Full scale model of CT-scanned Mengshanosaurus skull.

Traditional skull misinterpretations continue in Yuan et al.
Yuan et al did not properly label several fused bones (corrected Fig. 2 right) because they don’t know which taxa are choristodere outgroups and last common ancestors. That remains a traditional academic enigma that no one else seems to want to resolve, confirm or refute (Fig. 3) by simply adding taxa to find out.

Figure 2. Holotype of Mengshanosaurus.

Similarly,
the authors had no idea where to nest choristoderes as reptiles. In the large reptile tree (LRT, 1879+ taxa; subset from 2013 in Fig. 3) choristoderes nest as derived proterosuchids. Tiny transitional taxa, like the BPI 2871 specimen, lose the antorbital fenestra. Sister clades within the Pararchosauriformes include the Parasuchia and Proterochampsia. Euarchosauriformes derived from Euparkeria evolve to Archosauria, Rauisuchia, Erythrosuchia, etc.

Figure 3. Subset of the large reptile tree from 2013 focusing on the pararchosauriformes and the Choristodera. This has not changed much, but for the addition of taxa, like Mengshanosaurus between Ikechosaurus and Champsosaurus.

If you don’t know where your clade resides,
keep adding taxa until it becomes apparent and all candidate sister taxa are considered. Or just sneak a peek at the LRT. Don’t overlook tiny taxa. Often tiny taxa bridge gaps, forming transitions at the genesis of major clades in a process known as phylogenetic miniaturization. This time a tiny taxon just turned out to be a hatchling.

Figure 4. The choristodere, Champsosaurus laramiensis (USNM PL 544147) has a vestige antorbital fenestra in the usual place, anterior to the orbit. Here the frontal fontanelle is also present, as in Mengshanosaurus.

PS
Sometimes adult choristoderes also retain a vestige of the antorbital fenestra (Fig. 4).

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References
Yuan M, Li D-Q, Ksepka DT and Yi H-Y 2021. A juvenile skull of the longirostrine choristodere (Diapsida: Choristodera), Mengshanosaurus minimus gen. et sp. nov., with comments on neochoristodere ontogeny. Vertebrata PalAsiatic in press DOI: 10.19615/ j.cnki.2096-9899.210607

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