''Arizonasaurus'' was a

ctenosauriscid Ctenosauriscidae is an extinct family (biology), family of pseudosuchian archosaurs within the clade Poposauroidea. Ctenosauriscids existed in Africa, Asia, Europe and North America during the Early Triassic to the Middle Triassic period (latest ...

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-avian d ...

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 and ...

(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. Part ...

of northern Arizona. A fairly complete skeleton was found in 2002 by

Sterling Nesbitt Sterling Nesbitt (born March 25, 1982, in Mesa, Arizona) is an American paleontologist best known for his work on the origin and early evolutionary patterns of archosaurs. He is currently an associate professor at Virginia Tech in the Department of ...

. 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 Samuel Paul Welles (November 9, 1907 – August 6, 1997) was an American palaeontologist. Welles was a research associate at the Museum of Palaeontology, University of California, Berkeley. He took part in excavations at the Placerias Quarry in ...

in 1947.

Discovery and naming

The

, ''Arizonasaurus babbitti'', was named by

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 t ...

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

Description

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

s, such as other

ctenosauriscids Ctenosauriscidae is an extinct family of pseudosuchian archosaurs within the clade Poposauroidea. Ctenosauriscids existed in Africa, Asia, Europe and North America during the Early Triassic to the Middle Triassic period (latest Olenekian to An ...

like ''

Ctenosauriscus ''Ctenosauriscus'' is an extinct genus of sail-backed poposauroid archosaur from Early Triassic deposits of Lower Saxony in northern Germany. It gives its name to the family Ctenosauriscidae, which includes other sail-backed poposauroids such ...

'', ''

Lotosaurus ''Lotosaurus'' is an extinct genus of sail-backed poposauroid known from Hunan Province of central China. Discovery ''Lotosaurus'' is known from the holotype IVPP V 4881 (or possibly V 4880), an articulated and well-preserved sk ...

'', ''

Bromsgroveia ''Bromsgroveia'' is an extinct genus of predatory ctenosauriscid archosaur from the Middle Triassic Bromsgrove Sandstone of England. Ctenosauriscids were a group of rauisuchians that was related to the ancestors of modern crocodiles and allig ...

'', and ''

Hypselorhachis ''Hypselorhachis'' is a genus of extinct reptile, possibly a ctenosauriscid archosaur related to ''Ctenosauriscus''. It lived during the Triassic Period. It is currently known only from a single vertebra found from the Middle Triassic Manda Bed ...

''. ''Arizonasaurus'' is described from two

braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, or brain-pan is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calvaria or skul ...

specimens. Some ancestral features of these braincases are

plesiomorph In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, and ...

ic for

crurotarsan Crurotarsi is a clade of archosauriform reptiles that includes crocodilians and stem-crocodilians and possibly bird-line archosaurs too if the extinct, crocodile-like phytosaurs are more distantly related to crocodiles than traditionally thoug ...

s. 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.

Classification

''Arizonasaurus'' was closely related to ''

''; and, together with a few other genera, they make up

Ctenosauriscidae Ctenosauriscidae is an extinct family of pseudosuchian archosaurs within the clade Poposauroidea. Ctenosauriscids existed in Africa, Asia, Europe and North America during the Early Triassic to the Middle Triassic period (latest Olenekian to Anis ...

. 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

pseudosuchia Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Prior to ...

n features act as evidence that poposaurids, such as ''

Poposaurus ''Poposaurus'' (" Popo Agie reptile") is an extinct genus of pseudosuchian archosaur from the Late Triassic of the southwestern United States. It belongs to the clade Poposauroidea, an unusual group of Triassic pseudosuchians that includes sail- ...

'', ''

Sillosuchus ''Sillosuchus'' is a genus of shuvosaurid poposauroid archosaur that lived in South America during the Late Triassic period. Shuvosaurids were an unusual family of reptiles belonging to the group Poposauroidea; although their closest modern rel ...

'', and ''

Chatterjeea ''Shuvosaurus'' (meaning "Shuvo's lizard") is a genus of beaked reptile from the Late Triassic of western Texas. Despite looking superficially similar to a theropod dinosaur, it is actually more closely related to crocodilians. Discovery and ...

'', and ctenosauriscids form a monophyletic group that is derived

rauisuchia "Rauisuchia" is a paraphyletic group of mostly large and carnivorous Triassic archosaurs. Rauisuchians are a category of archosaurs within a larger group called Pseudosuchia, which encompasses all archosaurs more closely related to crocodilia ...

ns. Below is a phylogenetic cladogram simplified from Butler ''et al.'' in 2011 showing the cladistics of

Archosauriformes Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Latest Permian (roughly 252 million years ago). It was defined by Jacques Gauthi ...

, focusing mostly on

Pseudosuchia Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Prior to ...

Biogeography

''Arizonasaurus'' is from the middle Triassic

of northern

Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...

. 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