Arizonasaurus, the fin-backed rauisuchian

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

Greetings,

This just in the online Biology Letters of Proceedings of the Royal Society
of London B:
Nesbitt, S.J. 2003. Arizonasaurus and its implications for archosaur
divergence

Previously known only from a maxilla, Arizonasaurus babbitti Welles 1947 of
the early Anisian (Middle Triassic) Moenkopi Formation turns out to be a
poposaurid pretending to be Dimetrodon!   The new specimen is about 50%
complete, with much of the skull, the axial column from the cervicals
through the sacrals, and the limb girdles.  The dorsal and sacral neural
spines form a Dimetrodon-like sail.

The new Arizonasaurus material shows similarities to Ctenosauriscus koeneni
(Germany), Lotosaurus adentus (China), Bromsgroveia walkeri (England) and
'Hypselorhacis mirabilis' (Tanzania), suggesting a possible 'ctenosauriscid'
clade. Phylogenetic analysis unites Arizonasaurus, Chatterjeea, and
Poposaurus in an unresolved trichotomy, with Postosuchus, Saurosuchus,
crocodyliforms, and others as more distant outgroups.

As an earliest Middle Triassic rauisuchian, it shows that the
Pseudosuchia-Ornithosuchia split (i.e., the croc-line and bird-line split)
had to have occurred prior to the beginning of the Middle Triassic.

Cool critter.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796