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Re: FAQ (Ornithodira)
>Ornithodires, those archosaurs closest to the ancestry of dinosaurs,
>share with dinosaurs the derived character of an upright stance
>(as opposed to the semi-improved stance of crocodilians or the
>sprawling stance of other reptilians).
>An example of an ornithodiran is Marasuchus (ex Lagosuchus).
>Another is Lagerpeton.
Another is Tyrannosaurus rex, and still another is Pteranodon longiceps.
Ornithodira is specifically the clade composed of the most recent common
ancestor of Pterosauria and Dinosauria, and all of that ancestor's
descendants. It is a wholly-cladistic term, being coined by J. Gauthier in
his hallmark 1986 paper on saurischian phylogeny. Thus, ornithodires do not
"share" anything with dinosaurs, since dinosaurs are already one of the
major groups of Ornithodira.
The name "ornithodira" means "bird necked", and indicates one of the other
ornithodiran synapomorphy, the development of elongate cervical vertebrae
capable of assuming an "S"-shape curve. The third hallmark of Ornithodira
is the advanced mesotarsal ankle joint, a simple ankle where the upper
ankle bones (astragulus and calcaneum) are closely attached to the tibia
and fibula, while the lower ankle bones are closely attached to the
metatarsus. In most other tetrapods, the ankle is a highly complex joint
with many planes of movement.
If you wish to use a paraphyletic term for Marasuchus, Lagerpeton, and
Pseudolagosuchus, the work Lagosuchia is floating around for such a
purpose. Ornithodira should not be used in any context other than the
original defintion (i.e. = Pterosauria + Dinosauromorpha).
Thomas R. Holtz, Jr.
Vertebrate Paleontologist in Exile
U.S. Geological Survey
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA 22092