Why do marine mammals sway their tails vertically, while fish sway them horizontally?

Question

Why do marine mammals sway their tails vertically, while fish sway them horizontally?

In: Biology

Answer 1

“It reflects the evolutionary history of locomotion. Our wormy ancestors slithered on the sea floor, so undulated side-to-side. Fish inherited that movement, for which a vertical tail is best.

Their distant land mammal descendants evolved to run with limbs underneath: an unstable gait allowing rapid direction changes. To extend the stride, their spine flexes up-and-down. Marine mammals kept this movement, for which a horizontal tail is best.”

Source:
https://www.sciencefocus.com/nature/why-do-fish-have-vertical-tail-fins-and-whales-have-horizontal-ones/