Re: Why Air Sacs?

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

> Theropods evolved bipedal posture or simply inherited it from a
> ancestral saurischian dinosaur which in turn inherited from a
> ancestral dinosaur which in turn inherited from a ancestral
> dinosauriform organism?

   Well, I think that the dinosauriforms you probably have in mind
(_Marasuchus_, etc.) were facultatively bipedal, not necessarily fully so.
But yes, you are correct -- at least some dinosauriforms seem to have been
on the route to bipedalism before producing the first bona fide dinosaur.
However, the point of the paper isn't that bipedalism was impossible without
air sacs -- just that the evolution of air sacs _helps_ place a biped's
center of gravity in an optimal position to make bipedal locomotion more
efficient. (They also propose that the evolution of the pubic boot had a
similar function in keeping the center of gravity as low to the ground as
possible -- it certainly couldn't hurt, but I don't think that _alone_ was
sufficient.) They thus might have evolved in tandem -- this wouldn't explain
why sauropods also evolved them (presuming that whatever "prosauropods" were
closest to or ancestral to sauropods didn't have them, which is also
arguable), but then again, there's nothing that requires the evolution of
convergent features to have the same selective pressures every time.  Air
sacs do many things in extant birds, and presumably did so as well in
extinct taxa.  It's quite feasible that more than one selective pressure
existed to promote air sac evolution (I'm not a big fan of "THIS is the
reason why this feature evolved" types of arguments.)  (I also like
parentheses...could you tell?)

> The ptero hollow bones was lined with air-sacs?

   Pterosaur bones are pneumatic, yes, implying the existence of pneumatic
diverticula and, thus, air sacs. Many pterosaur bones (especially cervical
vertebrae) don't exhibit big lateral pneumatic fossae, though -- they don't
in many modern birds, either.  This, of course, leads to the question of
whether or not we should automatically assume that because a bone
(particularly a vertebra -- limb bones, when pneumatic, always seem to have
characteristic openings) doesn't have large pneumatic fossae, the animal to
which it belonged necessarily lacked air sacs and diverticula. The question
of whether or not theropods and sauropod evolved air sacs and diverticula
convergently, or whether or not it is a saurischian trait (within the
Dinosauria) hasn't been sufficiently investigated, though of course people
are working on it (I expect Matt Wedel might jump in here, if he's still on
the list...and has any free time!). It's possible that air sacs are a
synapomorphy of the Ornithodira, too -- this would require them to either be
lost or never, ever leave an osteological trace in the Ornithischia, but
since basal ornithischians are bipedal, this would demonstrate that
bipedality in a dinosaurian bauplan did not _require_ air sacs, as noted
above.  Even so, this would require air sacs to have evolved convergently
twice, since the dinosauromimic suchian _Effigia_ reportedly demonstrates
it, too...or, air sacs could be an Archosaurian trait with multiple
subsequent losses. Presently, as odd as it seems, it seems most parsimonious
that air sacs evolved convergently at least twice, and perhaps three or four
times. Farmer does discuss this a bit in her paper -- she prefers the
quadruple convergence, but doesn't discuss other options.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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)