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Re: FAQ (Ornithodira)



 From: tholtz@geochange.er.usgs.gov
 > >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.
 >

What, then, would be an appropriate name for a taxon consisting of
the stem ("basal") ornithodires?  Until recently I have called it
Ornithosuchia, but there is too much doubt about the placement of
Ornithosuchus in the phylogeny of the archosaurs for me to feel
comfortable  with that term.

My tendency in cases like this is to use the cladistic name
for a group as a name for the stem group.  Still, I can see how
that could be confusing.  In this case, though, I do not know
of any remaining alternative names.  (Unless Ornithosuchus is
definitively and emphatically restored to the ornithodires).

Since, in my classification, the stem ornithodires form a taxon
at the same rank as one of Pseudosuchia or Crurotarsi, they
must have a name.

 >  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.

Yep, that is my primary diagnostic character seperating the
ornithodiran thecodonts from the other thecodonts.  

 > 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).
 > 
Ah, there's a possible answer.  Good idea.  I suppose I missed
it because I tend to think of it as a more restricted group,
but with Ornithosuchus removed, that is no longer really true.

[Not only do I desire such a name, I insist on one]

So, "ornithodire" and its variants in my suggested answers should be
replaced by "lagosuchian", or the appropriate variant of it, except
where I am clearly talking about the whole lineage.

(Note, I do not accept Ornithodira as defined as an actual taxon).

swf@elsegundoca.ncr.com         sarima@netcom.com

The peace of God be with you.