Re: Ornithodira, breathing with long necks

["David Marjanovic" <david.marjanovic@gmx.at>]

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.