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DIPLODOCUS NECKS -Reply



Brother Pat (and other interested parties out there),

Your description of the avian ventilatory cycle is essentially correct
except for the statement about unidirectional flow, which occurs through
the lung and not in the trachea.  Airflow in the trachea is bidirectional, in
with inspiration, out with expiration.

The "problem" with inspiring and expiring through a long neck and the
possible increase in resistance to airflow that such a neck would
represent is not as big a problem as one might think.  The resistance
associated with a quadrupling of trachea length would be offset by only
a 40% increase in radius.  The increase in tracheal volume that this
would produce would, however, increase the tracheal dead space
proportionately.  An increase in tidal volume and a reduction (or
elimination) of dead space in the lung by a prarbronchial lung architecture
would minimize this problem.  I believe that the extreme lengths of
sauropod necks would only have been possible with an avian-style
parabronchial fixed-volume lung with associated highly compliant air
sacs to minimize the work of breathing.

A secondary benefit of the air sacs would be to extend them into bone to
form air cavities, thereby lightening the skeleton.  Sauropods may have
exploited both the parabronchial lung architecture to minimize dead space
and the air sac system to reduce weight.

Discussions of evidence for pneumatization of bone should distinguish
between _fluid-filled_ cavities and _air-filled_ cavities.  The former could
be used to reduce weight in any creature (water being lighter than
bone), but the latter could only be used by animals with air-sac-like
anatomical structures.  Closed cavities filled with gas either shrink and
disappear or become filled with fluid;  Gas-containing body cavities
persist only if they communicate with the outside would or if they are
maintained by the expenditure of metabolic energy (swim bladders in
fish, for example).   The soft tissues clues that might help distinguish
between these two situations are rarely if ever preserved.  Other clues
might be the presence on foramina in bones that might have allowed
penetration of air sacs into the bone.  The open structure of some bones
such as sauropod vertebrae might make it very difficult to tell if these
were filled with air or fluid.

The apparent absence of pneumatized bone in ornithopods suggests that
the parabronchial lung with air sacs was poorly developed or absent in
this group.

JMNorton

>>> <PTJN@aol.com> 07/26/98 02:52pm >>>

My understanding of the avian respiratory system leads me to believe
that it
does, to a large degree, eliminate the problem of dead air space posed
under
other respiratory schemes by an elongated trachea.  By a long shot, I'm
not
the best person to make this case, but here are my thoughts:

Unlike mammals and non-dinosaurian reptiles that have a bellows-like
ventilation system, it takes *two* inhalation/exhalation cycles for air to
move in and out of a bird.  In a bird lung, new air comes in one end of the
lung and the used air goes out the other end.  One set of air sacs store
air
before it's ready to enter the lungs, and another set of air sacs store air
until it's ready to be breathed out.  The sequence of bird breathing, as I
understand it, is as follows:

Inhalation #1: Air inhaled into posterior air sacs;
Exhalation #1: Spent air is expelled via the mouth from anterior sacs and
previously inhaled air moves from posterior sacs into the lungs;
Inhalation #2: New air is inhaled into posterior sacs and previously
inhaled
air moves out of lungs to anterior sacs; 
Exhalation #2: Inhaled air #1 is expelled from anterior sacs via the mouth.

This means that, at any one time, air flow is essentially unidirectional in
the trachea; thereby eliminating (or, at least greatly reducing) the dead air
problem.

As I said before, maybe sauropods had this type of lung and maybe they
didn't.
I have no idea.  But it certainly seems obvious that the avian lung evolved
either within the Saurischia or perhaps even prior to the divergence of
the
Saurischia from the Ornithischia.  Personally, I think an avian-like lung
was
an evolutionary innovation of the basal dinosauria.  Sereno and Novas
have
certainly shown that Herrarosauridae and Eoraptor--perhaps two of the
earliest
known dinosaurs--had already evolved pluerocoels (or at least
hollowing) in
the long bones and the centra of the vertebrae.

Comments?