Why do some aquatic animals have tails that move left and right, and others that go up and down?
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Related question: Are there any non-mammilian sea creatures with vertical-moving tails or vice versa?
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.
Fish move left to right because they’ve lived in water from the beginning and happened to develop stronger muscles that do the side to side motion. They could have done up and down but nature doesn’t care, if something works it stays.
Dolphins and whales move up and down because they evolved from land mammals that walked on four legs. The biomechanics of walking on four legs make it so that the spine has to have a little bit of up and down sway, plus they developed strong muscles to keep the spine up and hold the weight of the belly. Those muscles are responsible for an up and down motion. So when the ancestors of dolphins went back into the water, they already had those nicely developed muscles, and it was easy to modify the tail (not the legs like other comments claim) to propel them forwards by making the already existing, up and down moving muscles bigger and stronger.
The back legs of dolphins became useless and eventually shrunk down until they disappeared because it cost the body too much energy to make and maintain those limbs. Some individuals still have tiny bones that are remains of the hip bones, found near the spine. The tail got longer but there are no bones in the flat part of the tail: that’s mainly fibrous tissue, kind of like the cartilage in your ears.
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