Arizonasaurus

Arizonasaurus, meaning "Arizona reptile," is a genus of ctenosauriscid archosaur that lived during the Middle Triassic, approximately 243 million years ago, in what is now Arizona. Its fossils were discovered in the Moenkopi Formation. The animal is most notable for a large sail along its back, formed by greatly elongated neural spines extending from its vertebrae. This sail resembled those of other early archosaurs, including related ctenosauriscids such as Ctenosauriscus, Bromsgroveia, and Hypselorhachis.

The type and only known species, Arizonasaurus babbitti, was named and described in 1947 by Samuel Paul Welles, based on a few teeth and a maxilla (specimen UCMP 36232). A much more complete skeleton, about fifty percent of the animal, was later discovered in 2002 by Sterling Nesbitt. Two braincase specimens have also been described, which retain some ancestral features that are plesiomorphic for crurotarsans. Distinctive characteristics identified by Nesbitt in 2005 include a deep, hidden fossa on the posteroventral edge of the upward-pointing process of the maxilla, and a tongue-groove attachment between the pubis and the ilium.

Classification places Arizonasaurus as a close relative of Ctenosauriscus, and together with several other genera they form the family Ctenosauriscidae. These ctenosauriscids share derived characteristics with the poposaurids. Features of the pelvic girdle link Arizonasaurus with Ctenosauriscus, Lotosaurus, Bromsgroveia, and Hypselorhachis. Newly identified pseudosuchian traits support the idea that other poposauroids, such as Poposaurus, Sillosuchus, and Shuvosaurus, along with ctenosauriscids, form a monophyletic group more derived than rauisuchians.

The presence of a poposauroid in the early Middle Triassic suggests that the divergence of birds and crocodiles occurred earlier than previously thought. Ctenosauriscids from this time also indicate that Triassic faunas were more widespread across Europe, Asia, North America, and Africa. The fauna of the Moenkopi Formation represents a transitional stage between older and younger faunas.