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

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

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Probably something to do with needing to surface every so often to breathe, that’s probably easier with a horizontal tail

By “other marine life”, do you mean fish? Cos they aren’t mammals obviously. I don’t know of a marine mammal with a vertical tail

The tails in these animals developed completely independently of each other. In the case of fish the tail developed from the tail. And they therefore ended up with this vertical tail. You can see some of the similar features in alligators even though this is a separate development of a swimming tail again. Mammals on the other hand have a tail which have developed from their hind legs. First as webbing between the toes like most aquatic animals but then this webbing grew together and bigger to form the tail you see today. Because of this it ended up horizontal. This advantage of this is that they use the same bone structures as all mammals feet so it is a much shorter development path. However the vertical tail of fishes means they can use muscles along the entire body to swim instead of shorter muscles connected to the rear body as mammals do.

I don’t actually know but if I had to guess it is because marine mammals were not always marine mammals. They were land mammals originally. If you think about swimming with a pair of legs an up and down motion is significantly easier to do than side to side. Over the course of evolution with small incremental changes, it is fewer changes with more immediate results for the legs and feet to fuse while maintaining the same spine and hip structure that makes up and down motion easier than side to side.

So basically the up and down motion is because marine mammals effectively have legs and feet structure inside their tails where as fish do not.

Compare this to the evolution of fish, their side to side motion fins are flattened out tails instead of fused legs and feet. If you look at many fish they have small guiding fins where there would have been legs. Tail motion works better side to side (watch how an alligator or snake swims) so evolution’s small incremental changes would have favored side to side purely because fish evolved swimming using a tail while mammals evolved swimming using legs.