The large reptile tree nests choristoderes, both large and small, with archosauriformes, derived from a long series of proterosuchids ending with the small, former younginid, BPI 2871 (Fig. 1). This tiny transitional taxon at the base of the Choristodera documents yet another case of phylogenetic miniaturization and independent loss of the antorbital fenestra.
All proterosuchids had an antorbital fenestra,
but tiny BPI 2871 had a dimple, sealed in back (Figs. 1, 7), not a fenestra.
I ran across this image
of the skull of Champsosaurus (Fig. 2), a large, long-snouted choristodere. It also appears to retain a small dimple (pink arrow). The dimple is otherwise undocumented (Fig. 4) and not always duplicated, as shown here (Fig. 4).
Getting back to ‘did Champsosaurus have an antorbital dimple’?
Apparently only sometimes. In this clade the dimple is a vestige at best, and not always present. Most other choristoderes do not have an antorbital dimple. But some do.
living crocs also lack an antorbital fenestra. By convergence with choristoderes, crocs lost what their ancestors had. So there is precedence for such an evolutionary change. Brown 1905 did not illustrate a preorbital dimple in his treatise on Champsosaurus (Fig. 4).
Tchoiria is a basal choristodere
and this specimen (Fig. 5) appears to have a preorbital dimple along the same lines as the Triebold Champsosaurus cast (Fig. 3). But another Tchoiria (T. kauseni; “choy-er-ee-ya”??, Fig. 6, Ksepka Gao and Norell 2005) does not have such a dimple.
What Tchoiria tells us about champsosaurs
Unlike other tetrapods, champsosaurs created an upper temporal arch from the postfrontal contacting the squamosal. In Tchoiria you can see that transition taking place (overlooked in the original paper, Fig. 6). Tchoiria also shows the separation of the prefrontal and nasal in their traditional places. By doing so, the very long ascending process of the premaxilla (not a narrow fused set of nasals) is revealed.
A closeup of the dimple on BPI 2871 might be instructive.
The dimple here looks like the origin of the antorbital fenestra in Youngoides romeri (FMNH UC 1528 seen here), but it arrives at a much more derived node on the large reptile tree cladogram.
Brown B 1905. The osteology of Champsosaurus Cope. Memoirs of the AMNH 9 (1):1-26. http://digitallibrary.amnh.org/dspace/handle/2246/63
Ksepka, DT, Gao K-Q and Norell MA 2005. A New Choristodere from the Cretaceous of Mongolia. American Museum Novitates 3468. 22pp.