Re: Ornithodira, breathing with long necks

["James Norton" <JNorton@une.edu>]

I'm puzzled.  What is the source of pressure in these diverticula that
would allow them to withstand the compressive forces generated by a
heavy neck?  If the diverticula are connected to the air sac system or
the trachea by open connections, the pressure in them would be
atmospheric and any external pressure would force air out through the
connection and cause them to collapse.  If they are separate from, or
closed of from, the air sacs and trachea, then increased pressure within
them would cause the gases to diffuse into the surrounding tissues and
the diverticula would collapse.  Any isolated gas pocket  eventually
disappears due to diffusion of the gas into the adjacent tissue, and any
diverticula openly connected to the air sac system would have a pressure
only equal to that of the surrounding atmosphere.  There is no evidence
(structural or comparative) for any system to generate gas under
pressure, such as with swim bladders.  How could such a system work?

JMN

>>> "David Marjanovic" <david.marjanovic@gmx.at> 04/13/2001 9:43:33
a.m. >>>
Yesterday I wrote...

> > >At the SVP meeting 2000 it was proposed that sauropods held their
necks
> > > stiff not with long, superstrong ligaments (no evidence for these
can
be
> > > found), but with air sacs between and in the vertebrae, just
like
birds do
> > > it. The abstract sounds very convincing.
> >
> > Interesting!  At least it is a theory that could be tested.  How
would
> > they get pressurized?  Could they get enough pressure? Would it be
possible
> >with 2 air sacs?  Could they then bend the neck?
>
> I'll (probably :-] ) dig up the abstract tomorrow. Anyway, it says
that
> birds do all this.

William A. Akersten & Charles H. Trost: Function of avian air sac
diverticula, implications for sauropod cervical biomechanics.
Abstracts
supplement to JVP 20(3) September 2000 p. 25A

"Pressurized cervical, humeral, and femoral air sac diverticula in
living
avians function as passive stay devices for their respective
extremities and
some may have had similar functions in saurischian dinosaurs. For
example,
birds lack nuchal ligaments which, in mammals, provide passie support
for
the head and nek. The analogous function in modern avians is provided
by a
complex network of cervical air sac diverticula which, when inflated,
passively support the extended neck after positioning by musculature.
        While it has long been assumed that sauropods possessed
powerful
nuchal ligaments to support their enormous necks, the morphology and
geometry of their cervical and anterior thoracic regions indicates
that
nuchal ligaments are unlikely. Even if such ligaments were present,
the
forces they could have produced would be so nearly parallel to the long
axis
of the vertebral series that vertical support would be negligible in
practically all head positions [in contrast to hadrosaurs]. The
extensively
pneumatized cervical vertebrae of sauropods suggest a parallel with
the
cervical air sac diverticula of birds. In lateral view, sauropod
cervicals
display large oval spaces, herein termed intervertebral fenestrae,
which
overlap adjoining vertebrae and extend into the vertebral bodies and
transverse canals. The long axes of these fenestrae and most of their
volume
lies ventrad to the articulation between the cervical centra. We
hypothesize
that the intervertebral fenestrae are occupied by sac-like expansions
of the
diverticula which, when inflated, would passively support the neck
after
initial positioning by the cervical musculature. Other anatomical
relationships suggest additional possible functions, such as
thermoregulation and strengthening of vertebral centra."

Reminds me of the hydrostatic skeletons of e. g. earthworms, but these
can't
vary in stiffness.