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

They evolved differently than fish. Whereas the ancestors of fish lived in the oceans, you can trace back the ancestry of sea mammals to land mammals that eventually developed tails similar to that of mermaids because their tails evolved from their hind legs.

In fact, in certain pictures of whales like beluga whales, you can actually see their knees.