Arizonasaurus

Description

It had a sail made of tall neural spines, that was similar to those of other basal archosaurs like Ctenosauriscus, Lotosaurus, Bromsgroveia, and Hypselorhachis.

Arizonasaurus is described from two specimens of braincases. Some ancestral features of the braincase of Arizonasaurus are plesiomorphic for crurotarsans.

Below is a list of characteristics found by Nesbitt in 2005 that distinguish Arizonasaurus:

Discovery and naming

The type species, Arizonasaurus babbitti, was named by Samuel Paul Welles in 1947 on the basis of a few teeth and a maxilla, labelled as specimen UCMP 36232. A fairly complete skeleton was found in 2002 by Sterling Nesbitt.

Classification

Arizonasaurus was closely related to Ctenosauriscus, and together with a few other genera they make up Ctenosauriscidae. The ctenosauriscids were closely related to the poposaurids, as shown by a few shared derived characteristics. The pelvic girdle in Arizonasaurus unites this taxon with Ctenosauriscus, Lotosaurus, Bromsgroveia, and Hypselorhachus. Together, newly identified pseudosuchian features act as evidence that poposaurids, like Poposaurus, Sillosuchus, and Chatterjeea, and ctenosauriscids form a monophyletic group and are both derived rauisuchians.

Below is a phylogenetic cladogram simplified from Butler et al. in 2011 showing the cladistics of Archosauriformes, focusing mostly on Pseudosuchia:

Biogeography

Arizonasaurus is from the middle Triassic Moenkopi Formation of northern Arizona. The divergence of birds and crocodiles occurred earlier than previously thought, as the presence of a poposaurid in the early Middle Triassic suggests. Ctenosauriscids from the Middle Triassic allow the distribution of Triassic faunas to be more widespread, now in Europe, Asia, North America and Africa. The fauna of the Moenkopi Formation represent a stage transitional fauna between faunas of older and younger age.