Osteology of the Middle Triassic pseudosuchian archosaur Arizonasaurus babbitti

Abstract

Arizonasaurus babbitti is rediagnosed from a well-preserved partial skeleton from the early Middle Triassic Moenkopi Formation of northern Arizona. The skull, excluding the braincase, is described and compared to other “rauisuchians” and basal archosaurs. Two possible autapomorphies include a deep fossa hidden from lateral and medial views on the posteroventral edge of the ascending process of the maxilla and a tongue (pubic peduncle of the ilium) and groove (proximal portion of the pubis) articulation between the pubis and the ilium.

The postcrania is described in detail. Tall neural spines form a sail that is similar to those of other basal archosaurs with unknown affinities (Ctenosauriscus, Lotosaurus, Bromsgroveia, “Hypselorhachis”). Additional characters in the pelvic girdle in Arizonasaurus unite this taxon with Ctenosauriscus, Lotosaurus, Bromsgroveia, and Hypselorhachus to form a group, the ctenosauriscids. Pelvic characters of the ctenosauriscids are also found in Poposaurus and “Chatterjeea”, suggesting that ctenosauriscids are closely related. A possibly monophyletic group containing Arizonasaurus, other ctenosauriscids, Poposaurus, Sillosuchus, and “Chatterjeea” the exclusions of most other rauisuchians is proposed based on a new understanding of pseudosuchian characters.

Keywords:

• Arizonasaurus
• Rauisuchia
• Middle Triassic
• Moenkopi formation
• ctenosauriscid

Acknowledgements

I thank Kevin Padian for invaluable help and guidance in preparing this manuscript. I thank the following people for fieldwork assistance: Robert McCord, Robin Whatley, Randall Irmis, William Parker, Doug Jewel, Brenda Chinnery and Tom Olson. I am most grateful to Dylan Rust for finding the specimen. Useful discussions critical to the completion of this manuscript were made with David Gower, Rainer Schoch, Mathew Wedel, Karin Peyer, Jonathan Weinbaum, William Parker, Axel Hungerbuhler and Randall Irmis. I thank Pat Holroyd (UCMP), Sankar Chatterjee (TTUP), Rainer Schoch (SMNS), Janet Gillette (MNA), Andrew Heckert (NMMNH), and Chuck Schaff (MCZ) for access to collections. I thank Kevin Padian, Sam Hopkins, Edward Davis and particularly David Gower for improving the manuscript.