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Published online by Cambridge University Press: 19 July 2017
The order Cetacea was the earliest, and is the most diverse mammalian group to adapt to a marine existence. The first fossil whales appear in the fossil record in the Middle Eocene. Interestingly, cetaceans developed in some ways that are parallel to earlier marine reptiles and appear to have filled some of their niches. Some cetaceans later re-adapted to fresh water.