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More on air-sacs versus piston pumps (long)

I've been working on the respiratory systems of archosaurs for many years
(some information is in PDW), and the evidence that pterosaurs, sauropods and
theropods had air-sac ventilated lungs is very good. 

It is important to understand that the air-sac systems of modern birds are
highly modified by flight, even in ratites which are secondarily flightless.
Flight has two effects. First, the aerobic demands of flight are extreme so
the efficiency of the respiratory system must be unusually high. Second, the
sternal complex is highly modified in order to support the massive flight
muscles (in particular the sternum is hyperenlarged in order to support the
wing elevating supracoracoideus muscles) and the pubes are retroverted in
order to rebalance the tail less body. Therefore, if air-sacs evolved in
terrestrial nonavian archosaurs, then they should be expected to have
functioned differently than those of any modern bird, and had less capacity
than in flying birds. 

In ratites the posterior air-sacs are less well developed than in flying
birds, and the sternum is shorter. This is most true of kiwis (T. Huxley 1882
Proc. Zool. Soc. Lond. 64:561), and apparently elephant birds also. In flying
birds aerobic exercise capacity remains well above the reptile level even
when 70% of the air-sacs are disabled (J. Brackenbury & J. Amaku 1990 J. Exp.
Biol. 152:93); flight is no longer aerobically sustainable (and blood gas
levels are messed up although the birds are not profoundly ill). In fact, the
abdominal air-sacs of females are regularly deflated by enlargement of the
ovaries without impairing the birds. 



Pneumatic bones, especially the presacrals (some birds lack pneumatic leg

Shortening of the trunk (dorsals and ribs fewer in number)

Shortening of first dorsal ribs (because the small avian type lung is not
ventilated by surrounding ribs)

Elongation and increased mobility (indicated by well developed double heads)
of the posterior ribs in order to ventilate posterior air-sacs. Lumbar region
therefore absent

Uncinate processes (may not be critical in nonfliers)

Posteriorly elongated sternal complex (helps ventilate posterior airsacs, but
this may not be critical in nonfliers)

[Retroverted pubes are present in archosaurs without air-sacs (herrerasaurs,
ornthischians) and are probably not critical for air-sacs] 


No pneumatic skeletal elements

Trunk remains long, shallow

Long anterior ribs reatained

Lumbar region with very short or absent posterior ribs is present (allows
abdominal region to move with piston action)

Gastralia retained

Pubes mobile (only in more advanced crocodilians)

[An anteriorly projecting  pubic boot is not necessarily associated with this
system because at least some crocodilians (such as Terrestrisuchus) lack any
significant distal expansion. Besides, distal expansion may strengthen pubis
for ground resting] 


No pneumatic skeletal elements 

Lumbar region present (allows abdomen to contract and expand as diaphragm
ventilates lung)

Lumbar region preceeded by strongly descending curve of mid-dorsal ribs
(supports vertical diaphragm)

Now to look at extinct archosaurs - 

Early tetradactyl predatory dinosaurs - no pneumatic skeletal elements, no
lumbar region, no evidence for any of the above lung systems. Increased
mobility of trunk ribs does suggest aerobic capacity increased above lizard
level, but not by much. 

Theropods - No lumbar region, pubes immobile. Lack of both precludes piston
ventilation, nor is it likely that the group ancestoral to birds would
possess an usual and highly specialized system otherwise probably limited to
crocodilians.  Instead, increasingly well developed air-sac system appears to
have evolved. Note that B. Britt (1994 J. Vert. Paleo 14[SVP abstracts];18A)
and R. Reid (1996 Brigham Young Geol. Stud. 41:25) have both emphasized that
the cavities in dinosaur vertebrae are so similar to those of birds that they
must be concluded they contained pulmonary air-sacs. Lightning of the
skeleton is not an explanation because there is no need to do so in
terrestrial animals. (Development of the pubic boot varies wildly in
theropods. Extremely large boots probably associated with belly resting
rather than lung ventilation. Boot virtually absent in troodonts, anterior
process in present in dromaeosaurs.)
Coelophysian-ceratosaur grade - cervicals pneumatic, anterior ribs long.
Probably the very beginning of what would become the avian air-sac system,
aerobic capacity should have been further boosted above reptile level. 
Tetanurans (including Archaeopterx) - Dorsals becoming pneumatic, trunk
further shortened, anterior ribs short, posterior ribs elongated and double
headed (rib change begins in sinraptors and yangchuanosaurs, is fully
developed in allosaurs, tyrannosaurs, etc). The increasingly pneumatic
vertebral series indicate expansion of the air-sacs. The shift of rib length
and mobility from the chest to the abdomen strongly suggests a shift of
ventilation from smaller lungs to abdominal air-sacs, probably no better
developed than in kiwis (my restoration of very large posterior air-sacs in
theropods in PDW is in error, so it is questionable whether the gastralia
were involved in ventilating these air-sacs). This complex is extinct in that
there are no entirely nonflying theropods around anymore, and it is therefore
nonavian. It is probably pre-avian, and could probably ventilate aerobic
exercise about as well as in ground mammals and birds. Archaeopteryx probably
could not aerobically power fly as well as modern birds, but this is not
surprising in such a primitive flier. 
Bird-like theropods (oviraptors, troodonts, dromaeosaurs) and some early
Cretaceous birds - Sternal plate much larger than in Archaeopteryx, larger
than in kiwis and elephant birds. Dromaeosaurs DID have uncinate processes
(clearly present in photos of fully prepared fighting Velociraptor, also
pers. comms.), this must be an independent adaptation because some Cretaceous
birds lack these processes. These adaptations MAY have evolved in earlier
flying ancestors. In any case these advanced theropods and primitive birds
clearly had sophisticated near-avian respiratory systems. (Pubic retroversion
in dromeosaurs was associated with lightening of the slender tail, as in
short tailed birds, not with respiration.)

Sauropods - Presacrals and sometimes even caudals usually pneumatic, often
highly so, and must have contained pulmonary air-sacs (see Britt & Reid; not
possible for long bones to be pneumatic in such giants). Trunk shortened (not
a size feature because giant mammals have long dorsal series). Anterior ribs
remain long, but this was necessary to support shoulder girdle in these
quadrupeds. No lumbar region, because posterior ribs were elongated, as well
as highly mobile because they have well developed double heads. The latter
indicates ventilation of air-sacs, because if the ribs were intended to
primarily support the enormous belly they should have been immobilized as in
ankylosaurs. Looking up into the ribcage of a sauropod reveals a remarkably
bird-like complex. Air-sac complex was not avian because it was independently
developed and probably functioned differently, but may have approached or
equalled ratite capacity, especially in diplodocids which had the shortest
trunks, most mobile posterior ribs and most pneumatic vertebrae. Improved
respiration efficiency may have been in part a way to overcome the dead space
of a very long trachea.  

Pterosaurs - Some presacrals pneumatic, long bones highly so; peumacity is
not critical for weight saving in fliers because bats bones are more solid,
again indicates pulomonary air-sacs. Trunk very short. Posterior ribs
elongated, double headed. Sternal plate very large. Because there is no
lumbar region a piston pump is not possible. Air-sac ventilated lungs very
strongly indicated. Prepubes of may have helped ventilate abdominal air-sacs,
so function of this system may have differed very much from birds because
these are independently developed systems. Ventilation may have been as
efficient as in flying birds, if not then blood may have had very high oxygen
affinity as in bats. 

Ornithischians - Pneumatic bones absent, so no air-sacs. Perry has argued
 that retroverted pubes supported abdominal wall muscles that used abdomen to
ventilate lungs. Because there are no descendents it is not possible to
assess whether lungs were septate as in reptiles and birds, or alveolar as in
mammals. Aerobic exercise capacity of most ornithischians probably lower than
in birds and many mammals. 
Ornithopods - Well developed lumbar region preceeded by steeply descending
middle ribcage, gastralia absent. This arrangement is very mammal-like and
probably supported a vertical diaphragm. If lungs were alveolar then aerobic
exercise capacity may been as high as in most mammals.