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