A new look for Arizonasaurus and a new nesting

As a rauisuchid,
Arizonasaurus stands out as an oddball. The dorsal fin, first of all. No other rauisuchid has anything like it. The cervical centra are longer than tall, not taller than long as in other rauisuchids. The skull was restored by Nesbitt (2011) as dinosaurian, but the deep maxilla with its sharp ventral angle and extensive pre-ascending process appears to be more like the skull of Qianosuchus.

Due to a paucity of elements, the skull of Arizonasaurus has to be largely restored and there are several ways to do that.

Xilosuchus (Fig. 1) was associated with Arizonasaurus, but Xilosuchus has a much longer neck, again, very odd for a rauisuchid. Here (Fig. 1) the cervicals match pretty well with Yarasuchus, a sister to Qianosuchus.

Xilosuchus and Yarasuchus. Red arrows point to similar spacing of cervicals.

Figure 1. Xilosuchus and Yarasuchus. Red arrows point to similar spacing of cervicals.

There is another clade of archosauriforms with elongate neck centra and large neural spines. At present only two taxa occupy this clade, Yarasuchus and Qianosuchus. They happen to be sisters to Ticinosuchus + aetosaurs in the large reptile tree. Here’s my case for inclusion of Xilosuchus and Arizonasaurus within the Yarasuchus clade. I’m not the first to do so. Brusatte et al. (2012 their figure 5) have already nested them together. So did Nesbitt (2011, his figure 51.)

But they didn’t reconstruct the skull of Arizonasaurus as if they were sisters. Here (Fig. 2) is a new restoration of Arizonasaurus.

The skull of Arizonasaurus reconstructed with available data, including maxilla and dentary bones from Nesbitt (2003) and postcranial drawings from Nesbitt (2011).

Figure 2. The skull of Arizonasaurus reconstructed with available data, including maxilla and dentary bones from Nesbitt (2003) and postcranial drawings from Nesbitt (2011). So much is missing from the Arizonasaurus skull that several interpretations can be created. The distinctly angled maxilla is closest to Qianosuchus and was interpreted differently by Nesbitt (2011) as a more dinosaurian-type skull. The Xilosuchus skull is also by Nesbitt. Would be nice to see the actual materials.

When we put it all together (Fig. 3), using all available data, Arizonsaurus has a new look and a new nesting. The long neural spines of the Yarasuchus clade are simply that much longer in Arizonasaurus, along with a shorter neck — but not so short that the centra are taller than wide. In Arizonasaurus the neural spines might be tall, but the centra, at least some of them, appear to be longer than tall.

Arizonasaurus with a new Qianosuchus-like skull.

Figure 3. Arizonasaurus with a new Qianosuchus-like skull. The ventral pubis is common to the Qianosuchus clade. The scapula is typically larger and dorsally broader than in Arizonasaurus.

As a yarasuchid or qianosuchid, Arizonasaurus still stands out as a oddball, but less of one. Arizonasaurus appears to have a smaller scapula than the others, but it is incomplete and could be larger. The depth of the pubis also foretells longer hind limbs than in the short-limbed yarasuchid/qianosuchids. The large reptile tree will reflect this new nesting the next time I update it. Still no relation to the other finback, Lotosaurus, though, which continues to nest with the plant-eating dinosaurs.

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.

Butler RJ, Brusatte SL, Reich M, Nesbitt SJ, Schoch RR, et al. 2011. The Sail-Backed Reptile Ctenosauriscus from the Latest Early Triassic of Germany and the Timing and Biogeography of the Early Archosaur Radiation. PLoS ONE 6(10): e25693. doi:10.1371/journal.pone.0025693 Plos One paper
Nesbitt SJ 2003. Arizonasaurus and its implications for archosaur divergence
Sterling J. Nesbitt Proceedings of the Royal Society, London B (Suppl.) 270, S234–S237. DOI 10.1098/rsbl.2003.0066
Welles SP 1947 Vertebrates from the Upper Moenkopi Formation of the Northern Arizona. Univ. California Publ. Geol. Sci. 27, 241–294.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.