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From: tholtz@geochange.er.usgs.gov
 > >
 > >One of the questions that came up was "what is a dinosaur?".
 > Okay, I'll give it a shot.  But first, a bit of systematic theory -
 > it has
 > long been known that there is a difference between the definition of a
 > taxon (a named group of organisms) and the diagnosis of the taxon.

This needs some added notes for the FAQ.  In taxonomic parlance,
the term "diagnosis" means the set of features by which a taxon
(group) is distinguished from other similar taxa (groups).

 > Currently, the definitions of taxa are phylogenetic definitions
 > (definitions based on ancestry),

This is the cladistic approach.  The evolutionary taxonomists use a
combination of ancestry and extent of evolutionary divergence.

[Indeed, the above is as good a short *definition* of the cladistic
approach as almost any]

 >  while diagnosis are based on
 > synapomorphies (shared derived characters).  However, the definition 
 > is the key, since key derived features of a group can be lost during
 > evolution. ...

This is largly what I wanted - the set of synapomorphies establishing
the basal boundary of Dinosauria.  The rest of my notes already
discuss the inclusion or exclusion of birds.
 > Anyway, my personal definition of Dinosauria would in fact be based on
 > Owen's 1841/2 definition, modified for modern phylogenetic theory. 
 > Specifically, I would define Dinosauria as the common ancestor of
 > Megalosaurus bucklandi, Iguanodon anglicus, and Hylaeosaurus armatus,
 > and all of that ancestor's descendants.  This definition is equivalent
 > to the statement:
 > Dinosauria = Saurischia + Ornithischia

This is fairly good, and probably should be added to my notes.
 > Okay, but what about the bones?
 > Some of Owen's dinosaurian characters (upright stance, medial orientation
 > of the femoral head, etc.) turn out to be ornithodiran or dinosauromorph
 > characters.  However, one hallmark dinosaur feature which is lacking in
 > other dinosauromorphs is the addition of one or more extra vertebrae into
 > the sacrum (given a total or three or more).

So, the main derived character distinguishing Dinosauria from
the basal Archosaurs is the presence of an enlarged hip joint
(that is sacrum), containing at least three *fused* vertebrae.

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.
 > In any case, it is become clear that there is no major marked change
 > between very primitive dinosaurs like Eoraptor and advanced nondinosaurian
 > dinosauromorphs like Marasuchus.  More advanced hips (and maybe ribs) mark
 > the origin of the true dinosaurs.

In what way are the hips of herrerasaurs and Eoraptor more "advanced"
than those of basal ornithodires?  I would suggest that these
features might be added to the diagnosis of Dinosauria.
 > I hope that helps.  In any case, I have a question of my own.  What is an
 > "FAQ"?

Frequently Asked Questions.  A sort of beginners intro. to an Internet
discussion group.  (It is a term widely used in usenet news groups).

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

The peace of God be with you.