The terrestrial "vertebrates" of my planet must breathe air, and therefore must have lungs of some kind. They are descended from fish-like creatures, which breathe through gills.
However, I would like to - if possible - have an alternative origin for the animals' lungs. On Earth, lungs developed from the swim bladder of bony fish, which was an air-filled organ lined with blood vessels.
So, what organ:
- Could be beneficial to fish-like creatures
- Could evolve into a lung(s)
- Would not be a swim bladder
On the end result:
- It must be capable of sustaining megafauna
- It must lead to a single passage in the mouth and two nostrils (So no tracheae)
- It should, preferably, not be a copy of any invertebrate lungs either (e.g. book lungs)
The ancestral organ mustn't be some really specialized adaptation, it has to be something that a lot of fish should have. Otherwise, it evolving into the respiratory system of a whole superclass of diverse land fauna just doesn't seem realistic.
What I mean by that is; take fish. Air bladders aren't a fringe phenomenon found in one species, neither are gill arches and suction feeding or living in anoxic waters. We don't have to resort to obscure evolutionary steps in order to explain something that is often used as proof of evolution being impossible. Similarly, my aliens' lungs should be developed from a wide-ranging organ.