Monday, May 10, 2010


Arizonasaurus babbitti. Scale bar = 0.5 meters. (Nesbitt 2005)

Meaning: "Arizona lizard"
Species: A. babbitti
Nominal Author: Welles 1947
Age: early Middle Triassic (Anisian - 240 Ma)
Location: Arizona, southwestern USA (Moenkopi Formation)
Physical Characteristics: a large (3+ meters) sail-backed predator (Rauisuchian)

Arizonasaurus was the dominant predator of the Moenkopi fauna, which represented an important transition into a modern fauna of crocodiles, dinosaurs(birds), mammals, lizards, and turtles, as opposed to the previous fauna, dominated by more primitive tetrapods (Nesbitt 2003). Arizonasaurus helps to close the gap between the primitive fauna of the Lower Triassic and the more modern fauna of the Upper Triassic. The type specimen for Arizonasaurus consisted solely of a mostly complete left maxilla (Welles 1947), but the far more complete skeleton found by Nesbitt in 2002 has helped to resolve relationships between fragmentary and more complete taxa within Rauisuchia.

Rauisuchian Phylogenetics:
The holotype of Arizonasaurus babbitti has been considered a dinosaur, a stagonolepid, a rauisuchian, a proterosuchian, a trilophosaurid, and a erythrosuchid, but the new remains discovered by Nesbitt suggest that Arizonasaurus is indeed a poposaurid (Hunt 1993, Nesbitt 2003). A more recent archosaur phylogeny places Arizonasaurus within Poposauroidea (bootstrap <50/4) which, combined with its sister taxon Rauisuchoidea, makes up Rauisuchia (Brusatte et al 2010). Within Poposauroidea, resolution decreases, resulting in a polytomy between Arizonasaurus, Bromsgroveia, Lotosaurus, Poposaurus, Sillosuchus, and Shuvosauridae (Effigia + Shuvosaurus). When ankle characters were removed, Arizonasaurus and Bromsgroveia were united into a single taxon.


  1. One of the strangest prehistoric "crocs", that's for sure... Any idea why he developed that sail-like structure on his back?

  2. Well, I would have to look into it a lot further, but my first guess would be for thermoregulatory purposes, like the mid-Cretaceous dinosaurs Spinosaurus, Ouranosaurus, and Amargasaurus. I'll have to look into the paleoclimatology of the early Middle Triassic tho. Such structures have also been suggested to have been used for display.

  3. Yeah, i think that's the most plausible theory about those structures ;)

  4. Don't forget the Permian synapsid Dimetrodon! Also, Bullyland makes a pretty sweet Arizonasaurus figure. I have one somewhere.

  5. And Edaphosaurus! So many cool sail-backs.

  6. I think the sail on its back was used mostly for display, but also thermoregulatory purposes.