How do we know none of the fish species never had lungs? Isn’t it impossible to define a fish biologically without excluding a whole bunch of land living species?
We probably can’t. It’s just lungs are more complex than gills and usually evolution goes towards to more complex systems. If you’re talking about classification, I wouldn’t say impossible. Though I’m not a taxonomist.
Evolve into fins, then gills, then come back to the ocean
Probably not the gills. Once you go lungs you never go back, looking at all those mammals in the ocean.
How do we know none of the fish species never had lungs? Isn’t it impossible to define a fish biologically without excluding a whole bunch of land living species?
We probably can’t. It’s just lungs are more complex than gills and usually evolution goes towards to more complex systems. If you’re talking about classification, I wouldn’t say impossible. Though I’m not a taxonomist.