Procompsognathus – What does it look like?

The small Late Triassic archosaur Procompsognathus (~60 cm length, von Huene 1921, Fig. 1) was earlier and convincingly revealed to be a chimaera by Sereno and Wild (1992). The croc skull (Figs. 2,3) did not belong to the dino post-crania. Unfortunately no reconstruction was provided. Here (Fig. 1) is a Procompsognathus reconstruction , along with Segisaurus (~1 m length, Camp 1933) an early Jurassic dinosaur, to which it was allied.

Figure 1. Procompsognathus (below) along with Segisaurus (not to scale). We don't have the actual skull of Procompsognathus, but it was likely small, but taller than wide.

Figure 1. Procompsognathus (below) along with Segisaurus (not to scale). We don’t have the actual skull of Procompsognathus, but it was likely small, but taller than wide.

Procompsognathus post-crania
The post-cranial portion of the specimen (SMNS 12591) was considered close to Segisaurus (Fig. 1) and here nests close to it, but closer to the tiny Middle Triassic theropod, Marasuchus.  Pedal digit 1 rides a little higher on the metatarsus in Procompsognathus and Marasuchus among only a few distinguishing traits.

Distinct from Segisaurus, Procompsognathus has longer, more robust hind limbs and essentially vestigial forelimbs. It is also half as large with a much longer pubis, longer cervicals with smaller cervical ribs, a higher metatarsal 1 and shorter, more robust phalanges on pedal digit 4, which also has a very long ungual.

Figure 2. SMNS 12591a, a basal croc skull close to the ancestry of dinosaurs.

Figure 2. SMNS 12591a, a basal croc skull close to the ancestry of dinosaurs. The premaxilla is unknown and has been restored here. The palatine appears in the antorbital fenestra.

SMNS 12591a – the croc skull
A basal croc, the SMNS 12951a skull, is twice as wide as tall. The quadrate leans anteriorly. Phylogenetically the skull nests in the large reptile tree at the base of the Gracilisuchus + Scleromochlus clade and next to the Terrestrisuchus + Saltoposuchus clade. So there is a good chance that the SMNS 12951a skull was attached to gracile bipedal crocodylomorph post-crania, along the morphological lines of Procompsognathus, and not too far from the base of the Archosauria.

Figure 3. The SMNS 12591a skull reconstructed. It is twice as wide as tall, a croc feature.

Figure 3. The SMNS 12591a skull reconstructed. It is twice as wide as tall, a croc feature.

Sereno and Wild (1992) described postfrontals (blue in Fig. 2), but strangely did not illustrate them (Fig. 3). Gracilisuchus and Scleromochlus also retain postfrontals but most other crocs do not. What appears to be a post dividing the antorbital fenestra in situ is actually the displaced palatine, as described by Sereno and Wild (1992).

References
Camp C 1936. A new type of small bipedal dinosaur from the Navajo sandstone of Arizona. Univ. Calif. Publ., Bull. Dept. Geol. Sci., 24: 39-56.
Huene F von 1921.
Neue Pseudosuchier under Coelurosaurier aus dem württembergischen Keuper. Acata Zoologica 2:329-403.
Sereno P and Wild R 1992. Procompsognathus: theropod, “thecodont” or both? Journal of Vertebrate Paleontology 12(4): 435-458.

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