Re: Protoavis

["T. Mike Keesey" <tkeese1@gl.umbc.edu>]

On Tue, 14 Oct 1997 charger72@juno.com wrote:

> 1.) If crocodiles aren't ancestral to dinosaurs, but instead a parallel
> branch from the thecodonts should we not presume their physiology
> (including metabolism) would be similar?

What do you mean by "parallel" branch? Crocodilians certainly aren't
convergent with dinosaurs -- their niche is totally different from that
of any dinosaur.

I seem to remember hearing a theory that the ancestral archosaur had a
high metabolism which persisted in most lineages, but crocodilans lost it
as it was not conducive to their niche. Any truth to this?
 
> 2.) If dinosaurs are an intermediary step between thecodonts and birds,
> why did the first birds appear *way* before birdlike features became
> prevalent in dinosaurs?

Just a note on your terminology: In modern classifications, "Thecodontia"
has been abandoned (it's rather arbitrary -- non-crocodilian,
non-pterosaurian, non-dinosaurian archosaurs) and Dinosauria is
monophyletic, therefore including Aves. A better way to say this is that
some non-avian dinosaurs are intermediate between more primitive
archosaurs and birds.
 
Anyway, there are other coelurosaurs that co-existed with or even
pre-dated _Archaeopteryx_. Not many -- _Ornitholestes_ and
_Proceratosaurus_ are all that come to mind. The best way to view it is
probably that there was an explosion of new coelurosaur forms right before
the Jurassic-Cretaceous boundary, one of these forms being avians.

> 3.) It is hypothesized that ornithichians and saurichians are not
> separate groups, but that several herbivorous dinos developed the
> ornithisian pelvis independently when they branched off from carnivorous
> ancestors.

Ornithischia is a single group with a single ancestor. The only
independent line of herbivorous, "bird-hipped" dinosaurs is
therizinosauroids, and they aren't called "ornithischians".

Ornithischian pelves aren't really bird-like, although they look like it.
Instead of the pubis being rotated backwrd, I believe they had a process
on the pubis which grew backward, or something like that.

Therizinosauroid pubes, OTOH, are rotated backward, and therefore somewhat
convergent with birds.

> This demonstrates a proclivity toward this pelvic arangement in dinosaur
> anatomy. Is there any evidence for the same in thecodonts or crocodiles? 
Interesting question...
 
--T. Mike Keesey
tkeese1@gl.umbc.edu
http://umbc.edu/~tkeese1 -- Dinosaur Web Pages, Kubrick in the Simpsons,
Awesome Opossum, Lots o' more Fun!