Arizonasaurus (meaning "Arizona reptile") was a ctenosauriscidarchosaur from the Middle Triassic (243 million years ago) Moenkopi Formation of what is now Arizona.[1] The taxon is known for having a large back sail formed by elongated neural spines of its vertebrae. The type and only known species is A. babbitti, named in 1947.
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
Fossil
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 Hypselorhachis.[4] Together, newly identified pseudosuchian features act as evidence that other poposauroids, such as Poposaurus, Sillosuchus, and Shuvosaurus, and ctenosauriscids form a monophyletic group that is more derived than rauisuchians.[3]
Below is a phylogenetic cladogram simplified from Butler et al. in 2011 showing the cladistics of Archosauriformes, focusing mostly on Pseudosuchia:[5]
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 those of older and younger age.[3]
References
1 2 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. S2CID55338555.
↑ Welles, S.P. (1947). "Vertebrates from the Upper Moenkopi Formation of northern Arizona". University of California Publications in Geological Sciences. 27 (7): 241–294.
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