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In marine mammals, there are two different body types for two different niches:

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A long, strong tail for all-marine whales...

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...and all four legs modified into flippers for pinnipeds that feed in the water but breed on land.

So as you can see, many marine mammals have one form or another, but why not both? Couldn't a semi-aquatic mammal have a long, strong tail alongside its modified hindlegs? (Granted, the hindlegs would be smaller, but the point still stands.)

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    $\begingroup$ Whale ancestors were like that $\endgroup$ – Renan Oct 12 '18 at 0:34
  • $\begingroup$ The hindlegs would be sort of useless wouldn't they? It won't improve stability or speed since the larger tails movements would render its movements useless. Sort of like snakes and lizards. The side to side body motion renders the leg motions useless and so the legs fell off. $\endgroup$ – Shadowzee Oct 12 '18 at 0:40
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We have this already. Otters, beavers, platypus and others use their tails and legs for propulsion and steering.

At some point in their evolution whales also would have had both tails and hind limbs, even when almost if not fully aquatic since whale fossils have been found with hips and limbs.

It's actually harder to work out how and why whales lost their hind limbs and their tails evolved with flukes. Although we know this has happened millions of years ago with marine reptiles as well.

The probable reason is that instead of swimming they undulated their bodies which would make paddling hind limbs a hindrance, and since their hind limbs were detached from the spine at the hip they didn't use them as paddles. Still no clear consensus on why the tail took precedence, but my assumption is streamlining and power since the tail continues from the spine if it flattened it would create more power.

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    $\begingroup$ Crocodilians too. Big tail, strong legs as well. $\endgroup$ – Willk Oct 14 '18 at 3:22
  • $\begingroup$ @Willk yep, I didn't want to go into reptiles too much since the question is mammals $\endgroup$ – Kilisi Oct 14 '18 at 3:48
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During the late Triassic and early Jurrasic periods, crocodiles were radiating outwards into many niches and competing with dinosaurs and mammals. There were a number of seagoing crocodiles which seem to fit your criteria:

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Sea going Crocodiles

Now they were becoming well adapted to a fully oceanic lifestyle (and indeed one of their main competition, the Ichthyosaurs were also undergoing similar evolutionary adaptations as they transitioned from land dwellers to highly adapted oceanic predators:

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Cartorhynchus, one of the ancestral Ichthyosaurs

So there is a possibility of transitional species having both tails and flippers for moving in the water, and at least two species have done so in the past before the evolution of the Whales.

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for a semi-aquatic animal a large tail would be a deal-breaker. such an adaptation would have to await a fully aquatic transition.

for a fully aquatic animal the superior propulsion of tail elaborating into a fluke is selected for while the hind limbs, no longer needed for walking, or weight-bearing on land and inferior for propulsion, become vestigal.

i think what you are describing is, as others have noted, a transition phase which did not persist long with ancient whale species. from ambulocetus to dorudon, the total transition took less than 5Ma.

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