Re: Ornithodira, breathing with long necks

["Matthew Bonnan" <mbonnan@hotmail.com>]

David Marjanovic reported:

from the SVP abstract by 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."

I have never heard this before ?????  However,:

"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."

See Wedel, Cifelli, and Sanders. 2000. Osteology, paleobiology, and 
relationships of the sauropod dinosaur Sauroposeidon. Acta Palaeontologica 
Polonica, 45(4): 343-388.

Whereas birds do not have a continuous nuchal ligament, they do have 
interspinous ligaments between each vertebra:

"A large elastin ligament, the ligamentum elasticum interlaminare, is also 
present [in birds].  Unlike the ligamentum nuchae of artiodactyls, this 
ligament is discontinuous, connecting only the neural spines of successive 
cervical vertebrae.  The ligament attaches to each spine below the level of 
the neural spine, leaving a visible scar.  In groups with simple, undivided 
neural spines, such as brachiosaurids, the interspinous ligament was 
probably discontinuous, as in birds.  In those taxa with bifurcated neural 
spines, the ligament attachment scars are usually at the anterior and 
posterior ends of the cleft.  This could indicate discontinous ligaments, or 
a continous ligament that also anchored to each vertebra.

"The large elastin ligamens of artiodactyls and birds function as 
energy-return systems, helping to raise the neck following ventriflexion.  
These ligaments are taut in all normal positions of the neck, but even at 
maximum ventroflexion the ligaments alone are insufficient to support the 
neck; additional support must be provided by epaxial muscle groups ..."

So, birds DO have a nuchal ligament, it is just divided into many separate 
parts that do the same thing.

"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."

Again, I have not heard of this in bird necks.  Did anyone on the list go to 
this talk/poster?  Did they show anything convincing regarding the 
bird/air-sac/neck thing?

Matt Bonnan
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