Re: Why Air Sacs?

["Jerry D. Harris" <jharris@dixie.edu>]

> All ornithischians that was not fully quadrupedal, was facultative
> bipedal or there was fully bipedal ornithischians (the basal
> ornithischian that you mention bellow?)?

   The basalmost ornithischians (_Pisanosaurus_, _Lesothosaurus_, even 
heterodontosaurids) seem to be fully bipedal, even when not known from 
terrific specimens.  Very ornithischian-like dinosauriforms, which I suppose 
have the potential to expand what we know of the term "ornithischian" (e.g., 
_Silesaurus_, possibly _Sacisaurus_) may have been quadrupedal or 
facultatively bipedal.  But as before, air sacs are being touted as only 
_one_ possible component of bipedalism, not the _only_ possible one.

> Hmmm... A huge muscular tail was not more important to place the
> center of gravity well above the area between hindfeet?

   I think the long tails are thought to function, at least initially (in 
addition to a place to anchor caudofemoralis musculature, etc.), as a 
_balance_ to keep the center of gravity as close as possible to the actual 
locomotory organs, not to keep the center of gravity very high off the 
ground.  As has been pointed out numerous times elsewhere, a higher center 
of gravity makes roll-overs and overall balance much more difficult than a 
lower center of gravity, as anyone who's ever owned a Jeep C-5 of C-7 can 
attest.  So we're not talking here about the craniocaudal position of the 
center of gravity, but the dorsoventral position, although the former is, of 
course, important, too.

> If we did not stick hard with evidences we'll end up with another
> "just so" history.

   Perhaps, but all hypotheses such as these are necessarily hypotheses, 
not theories or "proof," simply because the only way to tell if they really 
are correct would be to build a time machine and go back and see for 
ourselves.  But this particular hypothesis does explain much of the evidence 
as it appears presently in the fossil record, so it's a viable one.  Perhaps 
not the _only_ one, but one nonetheless.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Jerry D. Harris
Director of Paleontology
Dixie State College
Science Building
225 South 700 East
St. George, UT  84770   USA
Phone: (435) 652-7758
Fax: (435) 656-4022
E-mail: jharris@dixie.edu
and     dinogami@gmail.com
http://cactus.dixie.edu/jharris/

"Trying to estimate the divergence times
of fungal, algal or prokaryotic groups on
the basis of a partial reptilian fossil and
protein sequences from mice and humans
is like trying to decipher Demotic Egyptian with
the help of an odometer and the Oxford
English Dictionary."
              -- D. Graur & W. Martin (_Trends
                  in Genetics_ 20[2], 2004)