''Arizonasaurus'' was a ctenosauriscid

archosaur Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avi ...

from the

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma ...

(243 million years ago). ''Arizonasaurus'' is found in the Middle Triassic

Moenkopi Formation The Moenkopi Formation is a geological formation that is spread across the U.S. states of New Mexico, northern Arizona, Nevada, southeastern California, eastern Utah and western Colorado. This unit is considered to be a group in Arizona. Par ...

of northern Arizona. A fairly complete skeleton was found in 2002 by Sterling Nesbitt. The taxon has a large sailback formed by elongated neural spines of the vertebrae. The

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen( ...

, ''Arizonasaurus babbitti'', was named by Samuel Paul Welles in 1947.

Discovery and naming

The

, ''Arizonasaurus babbitti'', was named by Samuel Paul Welles in 1947 on the basis of a few teeth and a

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The ...

, labelled as specimen UCMP 36232. A fairly complete skeleton was found in 2002 by Sterling Nesbitt.

Description

''Arizonasaurus'' had a sail made of tall neural spines. This sail was similar to those of other basal

s, such as other ctenosauriscids like '' Ctenosauriscus'', '' Lotosaurus'', '' Bromsgroveia'', and '' Hypselorhachis''. ''Arizonasaurus'' is described from two braincase specimens. Some ancestral features of these braincases are plesiomorphic for crurotarsans. Below is a list of characteristics found by Nesbitt in 2005 that distinguish ''Arizonasaurus'': *a deep fossa hidden from view on the posteroventral edge of the upward-pointing process of the

; *and a tongue-groove attachment between the pubis and the

ilium Ilium or Ileum may refer to: Places and jurisdictions * Ilion (Asia Minor), former name of Troy * Ilium (Epirus), an ancient city in Epirus, Greece * Ilium, ancient name of Cestria (Epirus), an ancient city in Epirus, Greece * Ilium Building, a ...

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, such as '' Poposaurus'', '' Sillosuchus'', and '' Chatterjeea'', and ctenosauriscids form a monophyletic group that is 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

of northern

Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a U.S. state, state in the Southwestern United States. It is the list of U.S. states and territories by area, 6th largest and the list of U.S. states and territories by population, 14 ...

. 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 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 represents a stage transitional fauna between the faunas of older and younger age.

References

External links

Artistic rendering of ''Arizonasaurus babbitti''
*
Initial description of complete skeleton by Sterling Nesbitt in Royal Society Biology Letters

Braincase of ''Arizonasaurus babbitti''
{{Taxonbar, from=Q134797 Ctenosauriscids Middle Triassic reptiles of North America Triassic archosaurs Fossil taxa described in 1947 Taxa named by Samuel Paul Welles Prehistoric pseudosuchian genera