Think dolphins evolving the same fins and tail as sharks, despite having bones and needing to breathe air.
I will grant you that dolphins and sharks share the (main) dorsal fin, and I will also grant you that the pectoral fins of sharks and the flippers of dolphins are cases of convergent evolution.
But as far as tails are concerned, cetaceans differ from sharks, most fish, and ichthyosauria in that they have flukes (horizontal fins, like wings on a plane) while the others have caudal fins (vertical). Basically, ceteans generate thrust by moving their tail up and down, while fish (et al) generate thrust by moving their tail left and right.
I will grant you that dolphins and sharks share the (main) dorsal fin, and I will also grant you that the pectoral fins of sharks and the flippers of dolphins are cases of convergent evolution.
But as far as tails are concerned, cetaceans differ from sharks, most fish, and ichthyosauria in that they have flukes (horizontal fins, like wings on a plane) while the others have caudal fins (vertical). Basically, ceteans generate thrust by moving their tail up and down, while fish (et al) generate thrust by moving their tail left and right.