Evolution of Air-breathing Tetrapods

Thanks for this topic caffeine, it is a subject of great interest to me, although one I do not keep as currently informed as I would like. For example, it is well established that aquatic barnacles change into air breathing geese. Bad bio jokes aside...One concept I believe is misunderstood by many people is the relationship between gills, lungs, and bony fish. I have seen many creationist claims that appeal to the misunderstanding by the implication that a fish gill would somehow need to transform into a lung. One could list many difficulties with that transition (although if one remains wet and very small it can be done).However, the missing piece in fish evolution is that early in their evolutionary history the bony fishes developed lungs. That is, lungs are considered to be primitive. Their early adaptive radiation seems to have been freshwater and air breathing as a supplement to gills was probably advantageous (even today it is in fish found in shallow, warm freshwater systems where we see numerous air breathing species). At the same time other gnathostomes (sharks, placoderms, etc) dominated the oceans.As bony fish invaded the ocean their was no need for the lung as a breathing apparatus. In the majority of fishes the lung is transformed into a swim bladder. In primitive forms the swim bladder is connected to the mouth and must be filled manually by gulping air. In advanced forms the esophagus is gone and the bladder fills solely by blood carried gases. The most advanced on this continuum are the bony fishes that have lost the swim bladder altogether (flatfishes, Stichaeids, etc)So to summarize this part we need to understand that what we might think of as a fish today is not the same as a Devonian fish. At that time (in the teleosts) lungs were the norm. It has been said many times by cladists that we might think of ourselves as highly modified bony fishes. I would clarify that we are highly modified primitive bony fishes because we retain that ancient lung thingy.Bony fishes, and especially the advanced bony fishes, are the dominant aquatic vertebrates today. Throughout this group we find diverse members who have 'found themselves' in situation akin to that of the primitive species long ago. they are living in marginal conditions where oxygen-depleted water and/or waterless conditions exist. The intertidal zone, ephemeral ponds and streams, marshes, etc. are examples. And in these conditions we repeatedly find fishes 're-inventing' the lung. They often have vascularized mouth and throat tissues. Some, like the mudskippers that caffeine showed, have gone beyond his and have invaginations to increase the surface area.The central point of this is to envision water to land transition as one involving creatures that were already breathing air. It is not the creationist strawman of something like a salmon jumping onto dry land and hoping an accidental miracle gives it lungs.

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Doctor Bashir: "Of all the stories you told me, which were true and which weren't?"
Elim Garak: "My dear Doctor, they're all true"
Doctor Bashir: "Even the lies?"
Elim Garak: "Especially the lies"

Hi caffeine,I have the article. That issue of P&BZ contains a special collection of articles about tetrapod evolution. Your article is mainly about comparing surfactants between extant teleosts and tetrapods. The authors conclude that the pharyngeal surfactants used by teleosts (proposed as antibiotics) without lungs are homologous to those used by lungfish and tetrapods as an antiadhesive (prevents the primitive open-sack style lung from collapsing and not being able to inflate). They conclude that the surfactants evolved early, before lungs/swim bladders. They also cite a ref I don't have access to regarding the placoderm swim bladder as being independant in origin. I will post their diagram:

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Doctor Bashir: "Of all the stories you told me, which were true and which weren't?"
Elim Garak: "My dear Doctor, they're all true"
Doctor Bashir: "Even the lies?"
Elim Garak: "Especially the lies"