Ornithodira, breathing with long necks

[MarkEShelly@aol.com]

Thoughts on breathing

   The clade Ornithodira is characterized by an s-shaped neck.    How did they 
breath with the additional air way dead space?  The longer neck seems to imply 
that they would have had to compensate for the additional used air trapped in 
their throats while breathing.  Obviously, ostriches, camels, and giraffes can 
do it.  I however, put a snorkel on and I have to reduce my activity level.  If 
the snorkel is too long, I cannot even clear water from it, let alone breath 
fresh air.
   As an engineer, I would design an air exchange device as follows.  
1.  Place the air intake in the air stream (nostrils) with a means on 
controlling flow.
2.  Filter large particles out of the air (nose hairs, cilia, moist surfaces).
3.  Adjust temperature, moisture for optimal gas exchange (moist surfaces).
4.  Generate air flow (ventilator ribs, diaphragms, contractions, positioning).
5.  Optimize gas exchange surfaces (thin wall, large surface area).
6.  Counter current gas exchange surfaces (flow through lungs).
7.  Moisture and heat recovery. (Moist surfaces)
8.  Keep exhaust air separate from intake, out of air stream.

Obviously, animals have evolved their systems that do much more (e.g. sample 
air, cool heads, and create sounds) with their breathing apparatus.   
   Mammals evolved comparatively large exchange surfaces and many possibly 
could use the flexing motion of the back to help compress and expand their 
lungs during maximum exertion, a fast gallop, to provide the oxygen needed at 
high exertions.  However, they do not have a counter current system or a 
separate exhaust pathway.  Therefore, they will be breathing in a large 
proportion of used air each breath.  A long neck would increase the volume of 
the used air.  When combined with constant body temperature, night vision, a 
running frame, 4 chambered hearts, and heat retention coverings, they appear to 
have been (and some still are) ideal night animals!
   Crocs have not expanded their air exchange surfaces and do not have one way 
flow through lungs.  They can not warm cold air into their lungs (unless warm 
themselves – warm water absorbs more gas. Does it improve gas exchange?). 
 They must wait until warm for active anaerobic exertion.  My iguana must rest 
after bursts of speed to catch his breath.
   Birds, with flow through lungs, air sacs, hearts, color vision, and wings or 
upright gait can be active at all times, primarily daylight and grow fast.  
Those with long necks still have extra dead air they must breathe again.  The 
relatively small lung is compensated by the counter current air exchange.  The 
pre and post lung air sacs and their one way lung air flow suggests that they 
exhale the air that has been in their lungs the longest first, compensating for 
the dead space.  Ventilator ribs help control ventilation and possibly 
sequence.  Air sacs in bones are possibly a lightening feature that prevented 
hollow bone collapse during flight due to pressure differentials.  Bird bones 
needed to be hollow for the most bending resistance to weight (I can expand if 
needed).
   The bipedal gait combined with the long neck IMHO probably went with the 
flow through (one way) lung.  I say this regardless of the means of 
ventilation, metabolism, or heat conservation methods employed. With an 
(ancestral?) air sac aft of the lungs, inhaling would bring air through a 
comparatively stiff lung.  The most used air would be exhaled first.  The key 
is adequate oxygen exchange for above crocodile exertion levels.  The 
ventilator ribs and pre-lung air sacs later allowed oxygen exchange while 
breathing out also. 
   When combined with a four-chamber heart, flight and aerobic hunting could 
possibly have occurred before homothermy.  
   The additional steps towards homothermy in theropods and Pterosaurs could 
have included:
1.  Heat released during elevated exertion levels would allow exertion after 
external air started cooling.
2.  Insulation on extremities would extend the late exertion time by reducing 
heat losses in early evenings.  This insulation on arms/wings could be wrapped 
around the non-insulated body surfaces and eggs at the end of the day to help 
retain heat, increasing growth time.
3.  Non-insulated surfaces could still be exposed to sunlight to warm up the 
body.
4.  Increased metabolism, especially when small or young would push for more 
insulation.
5.  Freed hands due to the bipedal gait, with skin or feathers, would lead to 
flight.  Small size for flight would push for a higher metabolism, more 
efficient gas exchange, better heart and heat conserving methods.  
6.  A higher metabolism would mean a more rapid growth.
7.  Flight would have allowed bipedal running flying dinosaurs to populate 
isolated areas and islands where flight would be lost (as occurs on island 
birds).  Some of the later flightless dinosaurs, retaining teeth and hand 
claws, would eventually compete with other, flightless theropods, possibly with 
lower metabolisms, especially in wooded areas.
8.  A rapid metabolism, would allow flying animals to grow fast.  Their need 
for light-weight would work to prevent continuous growth.

   Herbivores that did not become armored or large, would feel evolutionary 
pressure for a higher metabolism (not necessarily homothermy) and aerobic 
exertion levels from the theropods.
   Sauropods became so large that, except while young, a high metabolism would 
not be needed (but may have occurred in the ancestral or youth stages).  
However, they would need exertion levels adequate to move to new feeding areas 
and for protection from theropods.  I wonder if their necks brought air in the 
throats in one tube and out the neck air sacs back to the throat near the neck 
to prevent breathing as much used air. 

Mark Shelly