Arizonasaurus
Middle Triassic archosaur with a prominent back sail.
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 have been recovered from the Moenkopi Formation. The animal is most distinctive for the large sail along its back, formed by greatly elongated neural spines on its vertebrae. This sail was similar to those seen in other early archosaurs, including the ctenosauriscids Ctenosauriscus, Bromsgroveia, and Hypselorhachis.
The type and only known species, *Arizonasaurus babbitti*, was named and described by Samuel Paul Welles in 1947 based on a few teeth and a maxilla (specimen UCMP 36232). A much more complete skeleton, representing about half of the animal, was discovered in 2002 by Sterling Nesbitt. Two braincase specimens have also been described, which retain several ancestral features that are plesiomorphic for crurotarsans. Distinctive characteristics of the genus 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. Features of the pelvic girdle link this taxon with *Ctenosauriscus*, *Lotosaurus*, *Bromsgroveia*, and *Hypselorhachis*. Newly identified pseudosuchian traits indicate that ctenosauriscids and other poposauroids such as *Poposaurus*, *Sillosuchus*, and *Shuvosaurus* form a monophyletic group more derived than rauisuchians. Biogeographically, the presence of a poposauroid in the early Middle Triassic suggests that the divergence of birds and crocodiles occurred earlier than previously thought. The ctenosauriscids of this period show that Triassic faunas were widespread across Europe, Asia, North America, and Africa. The fauna of the Moenkopi Formation itself represents a transitional stage between older and younger faunal assemblages.
- location
- Arizona, USA
- classification
- Ctenosauriscid archosaur
- type_species
- Arizonasaurus babbitti
- known_for
- Large back sail formed by elongated neural spines
Lore & Background
Arizonasaurus is known for a prominent back sail formed by greatly elongated neural spines on its vertebrae, a feature shared with other early archosaurs such as Ctenosauriscus, Bromsgroveia, and Hypselorhachis. The type and only species, A. babbitti, was first described in 1947 by Samuel Paul Welles based on a few teeth and a maxilla. A much more complete skeleton, about fifty percent intact, was discovered in 2002 by Sterling Nesbitt. The animal is also represented by two braincase specimens, which retain several ancestral traits that are plesiomorphic for crurotarsans. Distinctive characteristics that set Arizonasaurus apart include a deep, hidden fossa on the posteroventral edge of the upward-pointing process of the maxilla, and a tongue-groove connection between the pubis and the ilium. The pelvic structure links this genus closely with Ctenosauriscus, Lotosaurus, Bromsgroveia, and Hypselorhachis. These ctenosauriscids, together with poposaurids such as Poposaurus, Sillosuchus, and Shuvosaurus, form a monophyletic group more derived than rauisuchians. Arizonasaurus lived during the Middle Triassic, about 243 million years ago, in what is now northern Arizona, within the Moenkopi Formation. Its presence there, alongside other ctenosauriscids found across Europe, Asia, and Africa, indicates a wide distribution of these faunas during the Middle Triassic.
Reader's Guide
Arizonasaurus is significant as a Middle Triassic ctenosauriscid archosaur that helps clarify the early evolution of pseudosuchians. Its presence in the Moenkopi Formation suggests that the divergence of birds and crocodiles occurred earlier than previously thought. The ctenosauriscids, including Arizonasaurus, allow the distribution of Triassic faunas to be more widespread across Europe, Asia, North America, and Africa. The fauna of the Moenkopi Formation represents a stage transitional between older and younger faunas. Phylogenetic analysis places Arizonasaurus close to Ctenosauriscus, and together with other genera they form Ctenosauriscidae, which are more derived than rauisuchians. The pelvic girdle of Arizonasaurus unites it with Ctenosauriscus, Lotosaurus, Bromsgroveia, and Hypselorhachis, and newly identified pseudosuchian features support a monophyletic group including poposauroids and ctenosauriscids.
Did You Know?
- Arizonasaurus means 'Arizona reptile'.
- A fairly complete skeleton (~50%) was found in 2002 by Sterling Nesbitt.
- Its sail was similar to those of Ctenosauriscus, Bromsgroveia, and Hypselorhachis.
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