Arizonasaurus babbitti Welles, 1947

Nesbitt, Sterling J., 2011, The Early Evolution Of Archosaurs: Relationships And The Origin Of Major Clades, Bulletin of the American Museum of Natural History 2011 (352), pp. 1-292 : 27-28

Arizonasaurus babbitti Welles, 1947 ( fig. 8E)

AGE: Anisian, Middle Triassic ( Lucas,

1998a).

OCCURRENCE: Holbrook Member of the Moenkopi Formation, Arizona; Anton Chico Member of the Moenkopi Formation, New Mexico ( Schoch et al., 2010).

HOLOTYPE: UCMP 36232, maxilla.

REFERRED MATERIAL: MSM 4590, skull and partial skeleton; see Nesbitt (2003, 2005a) and Schoch et al. (2010) for other specimens.

REMARKS: Arizonasaurus represent one of the most completely documented sailbacked archosaurs from the Anisian. Additionally, Arizonasaurus is the most common reptile found in the Holbrook and Anton Chico Members of the Moenkopi Formation ( Nesbitt, 2005b; Schoch et al., 2010). The holotype and the referred specimen (MSM 4590) share two characters: a uniquely shaped ascending process of the maxilla that is triangular in cross section and a deep pit at the posterior side of the base of the ascending process of the maxilla. Both these characters are present in Xilousuchus suggesting that the two taxa are closely related (see below). The partial ‘‘lacrimal’’ described by Nesbitt (2005) is actually the prefrontal.

Nesbitt (2003) found Arizonasaurus as a close relative of Poposaurus and Shuvosaurus (5 Chatterjeea ) within Suchia. Other analyses found a similar position ( Nesbitt and Norell, 2006; Irmis et al., 2007a; Weinbaum and Hungerbühler, 2007; Brusatte et al., 2008). Nesbitt (2003, 2005a) hypothesized that Arizonasaurus formed a clade with other sail-backed suchians including Ctenosauriscus , Lotosaurus , Bromsgroveia , and Hypselorhachis .

Arizonasaurus differs from all other archosaurs except Xilousuchus , Lotosaurus , Hypselorhachis , and Ctenosauriscus by the presence of a sail created by the elongation of the neural spines of the presacral vertebrae. It differs from Xilousuchus by the absence of a deep pit in the parabasisphenoid ventral to the descending process of the opisthotic. Arizonasaurus differs from Ctenosauriscus in anteroposteriorly wide neural spines of the midposterior cervical vertebrae. It differs from Lotosaurus by the presence of teeth.

KEY REFERENCES: Welles, 1947; Nesbitt, 2003, 2005a; Gower and Nesbitt, 2006; Schoch et al., 2010.