What’s the difference between marine animals with a horizontal tail and a vertical one?

Question

This is for a book I’m writing where mermaids and sirens are more of a natural plausible predators instead of magic, so i more want to know the movement of a animal having a certain tail aswell as the possibility of a mammal having it.

In: Biology

Answer 1

I believe that sea mammals have horizontal fins due to the shape of the bones from ancestor land mammals, i.e., sea mammals developed from land mammals that went marine.