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RE: Function Talks at Ostrom Symposium
On Saturday, February 20, 1999 3:51 PM, Dinogeorge@aol.com
> Birds therefore must have developed their present lung system >alongside<
> their original diaphragmatic lung system, at first simply to increase
> ventilatory efficiency as they became better fliers. Gradually the original
> system was lost (or became modified into, say, part or all of their air sac
> system--to figure out the details requires examination of extant-bird
> entrails) as the newer system took over more of the respiratory function.
> didn't happen overnight, and I look forward to the discovery of possible
> flightless, very birdlike theropods with well-preserved innards showing signs
> of >both< kinds of respiratory systems--say in Brazil.
This is not quite what Ruben's group have in mind. Ruben's point is that you
*cannot* go from hepatic pump to avian without a step that is so debilitating
as to be nonviable. Fortunately, the hepatic pump (diapghragm) system and the
avian system are not the universe of possible respiratory mechanisms. In
small, lizard-like forms, the lungs can be operated simply by rib contractions
using the intercostal muscles. From here, it is not difficult to develop an
avian style of respiration. The moral of the story is that birds could not
have developed from theropods if they had a hepatic pump.
DISCLAIMER: I get this from some correspondence with students of his. It is
third-hand hearsay, and I may have it wrong, or his views may have changed.
Frankly, it looks as though all of the various ideas are converging. If
maniraptorans diverged very early -- this seems to be the current feeling at
least on this List -- then there isn't a heck of a lot of difference between a
small, basal maniraptoran and a small, basal "thecodont" of some other type.
It would even be quite possible that the original divergence was of a small,
rib-breathing form and that at least a *proto*-bird came first. If my
suspicion is correct, all three theories are pretty close to equivalent and we
can all quit getting our feathers ruffled.