Why do some aquatic animals have tails that move left and right, and others that go up and down?

Question

Why do some aquatic animals have tails that move left and right, and others that go up and down?

In: 6

Answer 1

Fish have vertically articulated tails. They evolved a long time before mammals.

Aquatic mammals evolved horizontally articulated tails, because the geometry of the hips and limbs the tails evolved from means it’s much easier to move them up and down than left and right.

An aquatic mammal’s “tail” isn’t actually a tail, but modified legs.

Flatfish appear to have tails that move up and down, but fun fact, they tend to start life as normal upright fish and flatten out as they get older. So they’re just fish on their sides.