Ambulocetus natans, a long-extinct species of cetacean from the Eocene (about 59-60 mya) and creationist nightmare extraordinaire, is one of the most fascinating fossil species known. Ambulocetus natans literally means “The walking whale that swims”, so as to provide an uppercut reply to any creationist who ignorantly inquires: “What I don’t see in the fossil record is walking whales that swim!”.
Ambulocetus would probably appear a bit like a monstrous crocodile, and this is also an indication for his amphibious characteristics. He’s got your garden variety walking legs, alright, but they appear to be more suited for swimming, just as they are with crocodiles.
Ambulocetus’ nose appears to have a similar adaptation as whales do, and by that I mean that it could, just as whales can, swallow underwater. Its hearing is well-suited for being underwater and it also has similar dentition. Ambulocetus, then, appears to be a whale in many respects, barring the fact that its got ambulatory legs. It also had eyes and nose at the top of its skull, to allow it to see above water when slightly submerged. This shows both terrestrial and aquatic adaptations, all in what appears to be an organism with cetacean characteristics.
It’s important to note that ambulocetus is not a whale as such, since it shows transitional characteristics common to both artiodactyls (hoofed animals) and cetaceans as well. It serves more as a marker for one of the stages in which land-walking artiodactyls eventually evolved into sea-faring cetaceans, and it does so exquisitely because it is effectively both.