I don't understand. Are you saying it would be impossible for marine mammals to gradually "evolve" side-to-side movement similar to that seen in fish? I'd love to hear your explanation for that.
I'ld say that for marine mammals to have a skeletal anatomy like fish, would be a real problem.
Side-to-side movement by itself wouldn't necessarily be a problem. But remember, the side-to-side movement is just the end result. What matters is the mechanics causing / allowing for that movement.
A side-to-side moving marine mammal isn't necessarily a problem, if there is a logical pathway for the underlying mechanics from land dwelling up/down moving creatures to the anatomy allowing for side-to-side swimming.
Whatever anatomy would cause those mechanics, would have to be the result of a series of adaptions starting with the anatomy of an up-down moving, land dwelling creature (if evolution is accurate).
Evolution can't go back in time or return to the drawing board. It needs to work with the body plans that are currently present.
A good example is the human eye with the blind spot.
Our ancestors' eyes originally evolved "backwards" wich resulted in all the "wiring" having to cross the retina. This gives us a blind spot.
Evolution can't go back to reverse this and remove the blind spot. Nope. That's how the wiring originally evolved, so that's the "eye blueprint" that will have to be used for further development, if any. The "solution" consisted of our brains filling in the parts we can't see with what is most likely what it should be. Our brains create the illusion that the blind spot isn't there.
Compare that with the octopus. It doesn't have this problem.
Yet, we both see pretty much the same thing and neither of us has a black spot in the middle of its vision field. But we both accomplish
the same result but by
different mechanisms.
I would expect a side-to-side swimming marine mammal to show this same phenomena.
It would accomplish the
same result as actual fish, but by
different mechanisms. In the case of marine mammals, it would have to be a mechanism that makes sense in light of its land-dwelling ancestry.
To finish, I'ld like to point out that we are
again talking about hypothetical finds and speculating about the hypothetical reactions of the scientific community.
If you have any
real finds you wish to discuss where we actually KNOW what the scientist reaction was instead of having to speculate about it, go ahead.