Did Champsosaurus and Tchoiria have an antorbital dimple?

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.

BPI2871-3views588

Figure 1. Dorsal 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. A vestige antorbital fenestra may be present here. Compare to figure 2.

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).

Figure 2. This specimen of Champsosaurus appears to retain a tiny antorbital fenestra. Is this replicated in other specimens?

Figure 2. This specimen of Champsosaurus appears to retain a tiny antorbital fenestra. Is this replicated in other specimens? Or is it a shadow? If it is a shadow, this is exactly where the AOF would be if present. The nares (nostrils) are above the jaw tips.

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.

Figure 3. Cast of Champsosaurus from Triebold Palentology. Here again we see that dimple in front of the eye.

Figure 3. Cast of Champsosaurus from Triebold Palentology. Here again we see that dimple in front of the eye, below the lacrimal prefrontal and above the maxilla. Click to enlarge.

Figure 4. How Barnum Brown illustrated the preorbital region of Champsosaurus, without a hint of a dimple or antorbital fenestra.

Figure 4. How Barnum Brown illustrated the preorbital region of Champsosaurus, without a hint of a dimple or antorbital fenestra.

Remember
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.

Figure 5. The skull of Tchoiria appears to also have a preorbital dimple.

Figure 5. The skull of Tchoiria appears to also have a preorbital dimple. This one appears to have a longer rostrum than in figure 6 and indeed, the two species are known to have different tooth counts.

 

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.

Figure 6. Tchoiria klauseni as originally interpreted and as interpreted using DGS. Note this specimen shows the the transition from the postorbital to the postfrontal contacting the squamosal. This also shows the extent of the premaxillary ascending process (yellow), nasals separate from prefrontals.

Figure 6. Tchoiria klauseni as originally interpreted and as interpreted using DGS. Note this specimen shows the the transition from the postorbital to the postfrontal contacting the squamosal. This also shows the extent of the premaxillary ascending process (yellow) and the nasals (pink_ separate from prefrontals (brown) Those would fuse in Champsosaurus (Fig. 4).

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.

Rostral area of BPI 2871, formerly considered a younginid and here nesting at the base of the Choristodera.

Figure 7. Rostral area of BPI 2871, formerly considered a younginid and here nesting at the base of the Choristodera. Colors reappear every 5 seconds. Note the infilling of the antorbital fenestra. This is only known after cladistic analysis. The two sides of the same skull each provide clues as to the in vivo morphology. Splinters are often difficult to identify. This interpretation may change as time goes by. In the lower image, the fragile back of the dimple has been shifted dorsally. 

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
Ksepka, DT, Gao K-Q and Norell MA 2005. A New Choristodere from the Cretaceous of Mongolia. American Museum Novitates 3468. 22pp.

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