Arizonasaurus

Arizonasaurus was a ctenosauriscid archosaur from the Middle Triassic (243 million years ago).[1] Arizonasaurus is found in the Middle Triassic Moenkopi Formation 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, Arizonasaurus babbitti, was named by Samuel Paul Welles in 1947.

Arizonasaurus
Temporal range: Middle Triassic, 243 Ma
↓
Model, Museum am Löwentor, Stuttgart
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Paracrocodylomorpha
Branch: †Poposauroidea
Family: †Ctenosauriscidae
Genus: †Arizonasaurus
Welles, 1947
Species
  • †A. babbitti Welles, 1947 (type)

Discovery and naming

The type species, Arizonasaurus babbitti, was named by Samuel Paul Welles in 1947 on the basis of a few teeth and a maxilla, labelled as specimen UCMP 36232.[2] A fairly complete skeleton was found in 2002 by Sterling Nesbitt.[3]

Description
Restoration

Arizonasaurus had a sail made of tall neural spines. This sail was similar to those of other basal archosaurs, such as other ctenosauriscids like Ctenosauriscus, Lotosaurus, Bromsgroveia, and Hypselorhachis.[4]

Arizonasaurus is described from two braincase specimens. Some ancestral features of these braincases are plesiomorphic for crurotarsans.[1]

Below is a list of characteristics found by Nesbitt in 2005 that distinguish Arizonasaurus:[4]

  • a deep fossa hidden from view on the posteroventral edge of the upward-pointing process of the maxilla;
  • and a tongue-groove attachment between the pubis and the ilium.

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.[3] The pelvic girdle in Arizonasaurus unites this taxon with Ctenosauriscus, Lotosaurus, Bromsgroveia, and Hypselorhachus.[4] 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.[3]

Below is a phylogenetic cladogram simplified from Butler et al. in 2011 showing the cladistics of Archosauriformes, focusing mostly on Pseudosuchia:[5]

Poposauroidea
 

Qianosuchus

 
 
Ctenosauriscidae
 

Arizonasaurus

 
 

Xilousuchus

 
 

Hypselorhachis

 
 

Ctenosauriscus

 
 

Bromsgroveia

 
 

Waldhaus Taxon

 
 
 
 
 

Poposaurus gracilis H

 
 

Poposaurus gracilis Y

 
 
 
 

Lotosaurus

 
 
 

Sillosuchus

 
 
 

Shuvosaurus

 
 

Effigia

 
 
 
 
 
 
 

Biogeography

Arizonasaurus is from the middle Triassic Moenkopi Formation of northern Arizona. 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.[3]

References
  1. Gower, D.J. & Nesbitt, S.J. (2006). "The braincase of Arizonasaurus babbitti-further evidence for the non-monophyly of 'rauisuchian' archosaurs". Journal of Vertebrate Paleontology. 26 (1): 79–87. doi:10.1671/0272-4634(2006)26[79:TBOABE]2.0.CO;2.
  2. Welles, S.P. (1947). "Vertebrates from the Upper Moenkopi Formation of northern Arizona". University of California Publications in Geological Sciences. 27 (7): 241–294.
  3. Nesbitt, S.J. (2003). "Arizonasaurus and its implications for archosaur divergence". Proceedings of the Royal Society B. 270: S234–S237. doi:10.1098/rsbl.2003.0066. PMC 1809943. PMID 14667392.
  4. Nesbitt, S.J. (2005). "Osteology of the Middle Triassic pseudosuchian archosaur Arizonasaurus babbitti". Historical Biology. 8 (1): 19–47. doi:10.1080/08912960500476499.
  5. Butler, R.J.; Brusatte, S.J.; Reich, M.; Nesbitt, S.J.; Schoch, R.R.; Hornung, J.J. (2011). Andrew A. Farke (ed.). "The Sail-Backed Reptile Ctenosauriscus from the Latest Early Triassic of Germany and the Timing and Biogeography of the Early Archosaur Radiation". PLoS ONE. 6 (10): e25693. doi:10.1371/journal.pone.0025693. PMC 3194824. PMID 22022431.
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