A New Proterochampsa? This Time with Postcrania!

For several decades all we knew of Proterochampsa barrionuevoia (Reig 1959) consisted of several skulls and cervical vertebrae. The rest of the postcranium remained unknown until now. A new report by Trotteyn (2011) reveals much of the missing post-crania. PVSJ 606 was at least twice as large as the holotype (Fig. 1). It was found in the Ischigualisto Formation of the Late Triassic.

A new specimen (PVSJ 606) attributed to Proterochampsa by Trotteyn (2011)

Figure 1. A new specimen (PVSJ 606) attributed to Proterochampsa by Trotteyn (2011). Lateral view from Trotteyn (2011). Dorsal view after tracing the skull image in Trotteyn (2011).

Proterochampsa nodosa
A third specimen and a separate species, P. nodosa, I have not seen yet. The abstract distinguishes the two species, “..snout becoming narrow anteriorly in a less gradual manner than in P. nodosa, lower occiput, nares lanceolate with narrow anterior and posterior ends, and frontal less irregular that in P. nodosa.”

Proterochampsa? dorsal view.

Figure 2. Bone identification in a dorsal view of the skull PVSJ 606.

Comparisons
Due to the compressed skull of PVSJ 606 and its attribution, I expected the post-crania to be likewise compressed with short legs, perhaps most similar to basal parasuchians, like Parasuchus. As it turns out, the post-crania more closely resembled that of Chanaresuchus.

The Pararchosauriformes.

Figure 3. The Pararchosauriformes. Click to see entire tree. Note the nesting of the new specimen attributed to Proterochampsa as a sister to the holotype, but at the base of the chanaresuchids.

Description
The skull of PVSJ 606 (Fig. 2) was relatively enormous! It was as long as the presacral vertebral column. The various fenestrae were relatively smaller. The premaxilla was shorter and the post-orbital area was more robust. The premaxilla was not squared off. Do these differences, plus the major size difference mean the new specimen is a new species? Unfortunately Trotteyn (2011) completely ignored comparisons to the holotype skull, focusing all her efforts on a description of the post-crania alone.

The cervicals and caudals were more gracile in PVSJ 606 than in sister taxa. This seems improbable considering the size of the skull, but true. The torso was rather short and the legs were rather long.

Ecology and Behavior
No one has figured out the ecology and behavior of Proterochampsa or this specimen. Not sure what to make of it yet. It’s a bizarre creature!

Analysis
Trotteyn (2011) did not published a cladistic analysis having opted instead to synonymize the new specimen with the holotype of Proterochampsa. Under cladistic analysis PVSJ 606 nested as a sister to Proterochampsa (Fig. 3) at the base of the Cerritosaurus clade that also includes Chanaresuchus. So, PVSJ 606 is likely not congeneric or conspecific with Proterochampsa. It takes 8 extra steps to create a sisterhood with the holotype.

pes of Proterochampsa

Figure 4. DGS reveals two other digits and metatarsal 5 not described in the pes of Proterochampsa hidden among the skull bones. See Figure 1 for a reconstruction.

The Pes
Only two toes (including their metatarsals) were described by Trotteyn (2011). Examination of the photograph (Fig. 2) appears to show two more toes plus metatarsal 5 on the skull, which is somewhat fragmented, but largely intact. The  reconstructed elements create what appears to be a valid pes with continuous PILs (Fig. 1). Are these interpretations valid or not? Send data! We’ll figure this out. I’m only working from published photos in black and white.

An Ancestral Sister to Pterosaurs and Dinosaurs?
A recent report on archosaur relations (Brusatte et al. 2010) placed Proterochampsa at the base of the clade that produced dinosaurs and pterosaurs. Ridiculous for dozens of reasons, yet this was one of many such studies with similar results. Unsupportable and illogical results such as these are a direct result of inappropriate exclusions and inclusions in the taxon list resulting in “by default” nestings.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Brusatte SL , Benton MJ , Desojo JB and Langer MC 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida), Journal of Systematic Palaeontology, 8:1, 3-47.
Reig OA 1959
 Primeros datos descriptivos sobre nuevos Reptiles Arcosaurios del Triasico de Ischigualsto (San Juan, Argentina): Revista de la Asociacion Geologica Argentina, tomo 13, n. 4, p. 257-270.
Trotteyn MJ 2011. Material postcraneano de Proterochampsa barrionuevoi Reig, 1959 (Diapsida: Archosauriformes) del Triásico Superior del centro-oeste de Argentina. Ameghiniana 48:424-446.

wiki/Proterochampsa

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