The Sailback Arizonasaurus – a Good Bipedal Candidate

Figure 1. Arizonasaurus configured as a biped. The depth of the pubis suggests a similar length for the femur and tibia. The gracile pectoral girdle suggests a gracile forelimb. The long deep tail is based on the related Yarasuchus.

Figure 1. Arizonasaurus configured as a biped. The depth of the pubis suggests a similar length for the femur and the tibia follows. The gracile pectoral girdle suggests a gracile forelimb, perhaps smaller than shown here. The long deep tail is based on the related Yarasuchus. Looks a little like Spinosuchus, doesn’t it? And this clade is known for fish-eating.

Like the cheese, Arizonasaurus stands alone. Almost.
Phylogenetic analysis nests what is known about Arizonasaurus with Yarasuchus and Qianosuchus two sail-less ticinosuchians (not poposaurs, as envisioned by Nesbitt 2011).

Another sailback, Lotosaurus does nest with poposaurs, though, but it’s quite different and a herbivore.

Figure 2. Xilousuchus and  Yarasuchus compared.

Figure 2. Xilousuchus and Yarasuchus compared. Yarasuchus is a sister to Arizonasaurus, but has a much more robust pectoral girdle.

We don’t have any fore limbs or hind limbs for Arizonasaurus,
but we do have its pectoral and pelvic girdles.

The pelvic girdle is very deep, compared to the ancestral Vjushkovia, and sister taxa like Yarasuchus, Qianosuchus and Ticinosuchus. In these taxa the femur extends at least as far as the pubis depth and sometimes a little further. If we add such femora to the reconstruction of Arizonasaurus, it becomes essentially bipedal.

The pectoral girdle is quite small and gracile. It would be odd for massive or elongate forelimbs to be attached to such a small pectoral girdle, so here (Fig.1) an appropriate gracile short forelimb is added.

But wait, that’s not all.
That sail adds leverage and strength to the back bone, helping to hold up the elevated anterior half. Moreover, if we add on the deep tail of the related Yarasuchus, we reconstruct a substantial counterbalance (Fig. 1).

References
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
Nesbitt SJ, Liu J and Li C 2010. A sail-backed suchian from the Heshanggou Formation (Early Triassic: Olenekian) of China. Transactions of the Royal Society of Edinburgh 101 (Special Issue 3-4):271-284.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
Welles SP 1947 Vertebrates from the Upper Moenkopi Formation of the Northern Arizona. Univ. California Publ. Geol. Sci. 27, 241–294.
Wu X-C 1981. The discovery of a new thecodont from north east Shanxi. Vertebrata PalAsiatica 19: 122–132.

wiki/Arizonasaurus

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