[sauropod@ou.edu: Re: Greg Paul, Athletic Sauropods and Air Sacs]

[Mike Taylor <mike@tecc.co.uk>]

People,

Matt Wedel asked me to forward this message to the list.

Enjoy!

 _/|_    _______________________________________________________________
/o ) \/  Mike Taylor -- <mirk@mail.org> -- http://www.miketaylor.org.uk/
)_v__/\  "The pet-shop man's brother was lying!" -- Monty Python.



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Date: Tue, 24 Apr 2001 16:35:16 -0500
From: Matt Wedel <sauropod@ou.edu>

Regarding air sacs in sauropods, one thing that they did *not* do was hold
up the neck, assuming that the sauropod system was comparable to the bird
system.  First, there's no way to pressurize them.  I have spent quite a bit
of time dissecting and generally playing around with ostrich necks, and
there are no valves.  Muscular contraction to seal of the cervical
diverticula can't be invoked, either, because the lateral diverticula which
run through the transverse foramina are connected to the supramedullary
airway(s), which run through the neural canal above the spinal cord--all the
way to the pelvis.  There simply are no muscles capable of squeezing off the
supramedullary airways, so if you compress the lateral diverticula, the air
will just run into the supramedullary canal and shortly find itself
elsewhere, possibly all the way down at the animal's rear end, in the
abdominal air sac.  I'm not guessing about this; Kent Sanders and I took a
section of an ostrich neck, wrapped surgical gloves around either end,
inflated it via an air tube, and spent some time moving it around into
various positions and taking x-rays.  Bend it one way, and the air rushes
off to some area that isn't being compressed.  This hold for either lateral
or vertical motion.

This whole passive support thing stems from the often-made observation that
if you run an air hose into a dead bird and inflate it, the neck straightens
out and the wings unfold.  This is because the cervical and thoracic
diverticula, along with the entire respiratory system, are being actively
inflated by an external pump.  I haven't actually done the math to determine
how much air pressure would be required to support a sauropod neck, but I'll
bet it far exceeds the pressure that can be generated by the respiratory
system.  Outside of which, given the interconnectedness of the entire avian
respiratory and diverticular system and the lack of valving mechanisms
(outside of the aerodynamic valving of the lungs, natch), there is simply no
way to pressurize the neck to the exclusion of the rest of the system.  If
sauropods did pressurize their necks, they did it with novel structures that
didn't fossilize.

Just because an anatomical structure could be used for a purpose doesn't
necessarily mean that it was.  If the structure is skeletal, one should look
for osteological correlates of the presumed soft tissues, and make sure that
the osteological structure, inferred soft tissue structure, and hypothetical
function are all in agreement.  Furthermore, the acquisition of the hard
tissue, soft tissue, and function should make sense when placed in
phylogenetic context (see John Hutchinson's bitchin' papers in the April
ZJLS--HP Holtz discussed them in an April 10 post).  Extant analogues can be
tremendously helpful here, but one has to be careful in choosing analogues.
Ungulates are horrible models for sauropod neck mechanics because their
necks are constructed very differently.  Birds are great models; outside of
the diarthroidal joints and robust diapophyses, their verts are almost
identical to those of sauropods.

Finally, can't we just leave sauropod necks alone?  The muscle attachment
points on their vertebrae range from prominent to humongous and the
vertebrae themselves were probably *at least* 50% air.  I see no reason to
invoke novel mechanisms like giant nuchal ligaments, diverticular inner
tubes, leaf-spring cervical ribs, or orbital skyhooks to hold up sauropod
necks.  Spectacular?  Definitely.  Unfathomably mysterious?  No.

Feel free to respond to me at [sauropod@ou.edu].

Best,

Matt Wedel
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