Why do most marine mammals have horizontal tails, while other marine life tend to have vertical tails?

Question

Why do most marine mammals have horizontal tails, while other marine life tend to have vertical tails?

In: 75

Answer 1

Mammals have backbones (and associated muscles, of course) that are good at bending sort of “under” the length of the backbone (picture a cheetah running; that backbone is flexing back and forth, helping the back legs move forward and backwards like crazy), where fish have backbones (and attached muscles) that are good at moving “side to side”. When a mammal goes aquatic, that backbone (and muscles) is still best at that same “running like a cheetah” movement, and the new for-water tail works best oriented perpendicular to the backbone’s movement.