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Why do Marine animals have fins.

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Do you mean fish or marine mammals? I guess it doesn't matter - they both have fins because selection gave the ones with fins huge advantages in their marine environment. –  shigeta Jan 22 '13 at 22:27
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Welcome to the site! You'll get better answers (and generate more interest) if you can provide a bit more context to your question! –  Kyle. Jan 23 '13 at 0:22
    
Wikipedia has a pretty decent article on fins which seems to answer your question. –  Jack Aidley Jan 23 '13 at 14:46
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It is more efficient to use fins than feet, hooves, or other similar body parts. It is the same reason that you can swim faster while wearing flippers. Having a larger surface area allows animals to push against more water, so that they have more force when swimming. Here is a picture of the bones in a dolphin fin. They are extremely similar to the bones in a hand, because dolphins evolved from land animals that had individual fingers.

Amphibians like frogs and salamanders have webbed feet, which is a "compromise" between feet and fins. These feet allow them to push against water with more power, since they have webbing instead is separate fingers. It also allows them to grip the ground on land, since their fingers can move independently of each other.

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