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>Thomas R. Holtz, Jr. wrote:
>> (Incidentally, I'm back, but at a new e-mail address)
>Hmm, then you may have missed the beginning of out FAQ discussion.
>One of the questions that came up was "what is a dinosaur?".
>The one bit I could not remember the answer to was what are the
>derived features distinghuishing dinosaurs from stem archosaurs.
>Would it be possible for you to elaborate on that?
>swf@elsegundoca.ncr.com                sarima@netcom.com

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. 
Currently, the definitions of taxa are phylogenetic definitions
(definitions based on ancestry), 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. 
For example, even though snakes and cetaceans have lost some or all of
their limbs, they are still members of Tetrapoda (the four-legged

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

Dinosauria = Saurischia + Ornithischia

or, for the small but significant minority out there:

Dinosauria = Theropoda + Phytodinosauria

(phytodinosaurs being a group suggested by Bakker, Paul, Olshevsky, and
(for a while) Sereno composed of the Sauropodomorpha and Ornithischia).

Dinosauria is a group within Dinosauromorpha, which is defined as all
ornithodiran archosaurs sharing a more recent common ancestor with
dinosaurs than with pterosaurs.

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).  This character was noted by
Owen.  Marasuchus (formerly Lagosuchus) does not exhibit this character,
but Herrerasaurus does (I can't recall if Eoraptor does or not).  Some
other potential dinosaurian synapomorphies (double-headed ribs, for
example) await more detailed description (or better fossils) of the
non-dinosaurian dinosauromorphs.

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.

I hope that helps.  In any case, I have a question of my own.  What is an

Take care, everyone.


Thomas R. Holtz, Jr.
Vertebrate Paleontologist in Exile
U.S. Geological Survey
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092

email:  tholtz@geochange.er.usgs.gov 
Phone:  703-648-5280
FAX:            703-648-5420