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Difference between sea mammal and fish tails?

Something I've always noticed is that a sea mammal (dolphin, whale)'s tail is horizontal, but any kind of fish I've seen has a vertical tail. I was just wondering if anyone knows why it is that their tails are different, and possibly some advantages/disadvantages of the different tail shapes.

Good observation Both the vertical fins of fish and the horizontal tails of sea mammals are used to generate thrust for locomotion in water. In both cases, body shape responds to mechanical requirements that basically entail forming a "paddle" of some kind.
How this "paddle" is oriented is not so important because the caudal peduncle and tail provide propulsion only - the direction of movement is controlled by other fins. So the differences you have noticed are due to the evolutionary history of these groups of vertebrates.

The vertical tail of fishes is the basic tail shape of all chordates; it's present not only in jawed fishes but also in the jawless lamprey and hagfish, in lancelets, and even in the swimming larva of sea squirts. All these animals move with lateral body undulations thanks to the contraction of muscle "packs" on both sides of the body.
Naturally, when the body axis undulates from side to side, the "paddle" needs to be on the vertical plane.
So why don't whales or manatees have similar tails?
Because in the course of evolution from the first reptilian amniotes to the mammals, the lateral body movement (that you can still see in crocodiles, lizards or snakes) was lost. Mammals only move their vertebral axis up and down, not from side to side. So when some mammals went back to the water, the tails they evolved worked with the same up-down movement of their spine, and thus developed a horizontal tail.

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