I disagree that it is far too back to demonstrate - perhaps you could clarify why you think this?
Right, but those same species share common ancestors with reptiles, which in turn (as I recall) share common ancestors with Amphibians, which in turn share common ancestors with Lung fish and other lobe finned fish, including Tiktaalik - the point being that if the archosaurs were one way breathers (which is suggested by the evidence) but their ancestors (in this case lobe finned fish - a common ancestor to all tetrapods) were two-way breathers, then somewhere between Tiktaalik and the emergence of the Archosaurs - a span of 125 million years, one way breathing has to have evolved. The fact that Alligators, and Crocodiles are both one way breathers suggests that it was
probably in place in some form before the last common ancestor of crocodilians and Theropods.
Agreed - one of the points that I was making is that the split may have arisen early in the time line, at a point where lungs were still relatively simple.
I agree to some extent with this, that's why I did the research regarding the evolution of the lung and made the comment about its evolution and its character in amphibians. The counter point that I also made was that, as near as we can tell, lungs evolved as a seperate organ to gills, rather than neccessarily evolving from gills - I suggest this on the grounds that there are lobe finned fish to this day which still have both sets of organs and are still able to use both sets of organs. This is suggestive that Tiktaalik probably had both lungs and gills (I'm sure i've seen something to that effect but I can't recall when or where).
That is also the point I was making. It's why I mentioned the timelines and it's why I linked to the image of the crocodile lungs.
Consider this illustration, for example:
Compared to this:
Then it seems to me that, at least at the macroscopic level the change that it required amounts to the seperation and migration of one of the secondary bronchial tubes. The mutations for doing this exist, and are (occasionally) expressed in humans, most of which lead normal lives - so there is no valid reason to discount the possibility that it was possible in a simpler lung that required fewer changes at lower levels (it also requires changes in the structure of bronchioles, something that's easier with simpler lungs but I don't imagine as being terribly likely to happen in humans).
Is that clearer? The main points that I was making were that:
1. Evidence suggests that air-sacs were present in Dinosaurs, including Tyranosaurs, and so were already in place by the time birds evolved.
2. Evidence suggests that one way breathing evolved from two way breathing very early in the tree (375-250 MYA) and was already in place by the time archosaurs evolved. The corrollary of this is that it evolved at a time when lungs are expected to have been structurally simpler than they are now.
3. One way breathing in alligator lungs appears to be achievable with the migration of a secondary bronchus within a lung, with one way breathing arising spontaneously as a consequence of Bernoulis law and the configuration of the junction where this secondary bronchus meets the primary.
4. The mutations for some of these changes still occur (from time to time) within humans (along with a mutation that indicates a potential pathway to air-sacs) and generally those mutations are not detrimental.