Re: Ornithodira, breathing with long necks

["Jerry D. Harris" <dinogami@hotmail.com>]

> However, I'm not familiar with the term 'passive
> stay devices', so I don't know if we're talking about the same thing.  
> Please
> elaborate a bit on 'passive stay devices'.

  To the extent of my knowledge, it means (broadly, and in my own words) "a 
means of maintaining a supportive stiffness of a body part without (or with 
absolute minimal) muscular effort."  The standard example is the horse 
hindlimb: the patella can slide onto a special facet so that the hindlimb 
cannot bend at the knee or the ankle -- a somewhat complex arrangement of 
various ligaments and tendons ensure this.  Thus, the horse can rest its 
weight on the hindlimbs without having to use muscles to prevent the joints 
from bending (thus tiring the animal).

  In the avian wing, inflating the air sacs provides a means of keeping the 
limb extended, again without muscular effort; my reading of what Akersten 
proposes is that the parallel cervical air sac system of both birds and 
sauropods acts the same way.  I tend to think of it in this (very 
simplistic!) way: imagine you have a series of models (made of something 
like balsa wood) of articulated cervical vertebrae.  You thread one of those 
kid's-party-style, long, hot-dog-shaped balloons through the transverse 
foramina on either side of the column.  When you inflate each balloon, it 
becomes rigid with air pressure and rises to a horizontal (or so) position; 
when both are inflated simultaneously, they turgidly hold the cervical 
vertebral column out horizontally with it.  Thus, simply inflating the 
balloons holds the vertebrae horizontally; no muscles are needed -- the neck 
is held out horizontally (as in, for example, flight) passively, not 
actively.

  Of course, it's really more complicated than this, especially in 
sauropods, because each vertebra, plus all the muscle, is really quite heavy 
-- I am curious as to the nature of the proposed air sac membrane to support 
all that weight (as opposed to, for example, balsa wood!), particularly 
since the membrane in chickens is really, really thin, flimsy, and easily 
punctured! (As an aside, this may be a problem for any sauropod sustaining a 
bad neck injury -- if the air sacs are punctured, the entire neck support 
system might collapse!)  That is to say, I don't know what the maximum 
weight that an air sac system _alone_ could support, but it certainly has 
something to do with the overall mass of the neck, the nature of the air sac 
membrane, and the kinds of pressures normally maintained in said sacs, 
which, in turn, speaks to the ability to get air in and out of the system!

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Jerry D. Harris
Dept of Earth & Environmental Science
University of Pennsylvania
240 S 33rd St
Philadelphia PA  19104-6316
Phone: (215) 573-8373
Fax: (215) 898-0964
E-mail: jdharris@sas.upenn.edu
and     dinogami@hotmail.com
http://www.sas.upenn.edu/~jdharris

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