There you are, 50 million years ago, enjoying a hot Eocene afternoon at the seashore and watching a foxlike prehistoric rodent cool its paws in the mud at the water's edge. Suddenly, a monstrous beast explodes from the depths, seizes the hapless fox-thing in its massive jaws and disappears beneath the waves. All is quiet again. The local birds return to their business. It's as though nothing ever happened. You've just glimpsed the probable hunting methods of an Ambulocetus natans, a 400-pound (180 kilograms), 10-foot-long (3 meters) "walking whale."
Basically, an Ambulocetusnatans was a mammalian version of a crocodile but with longer hind legs to power it through the water. Like crocodiles and hippos, it had widely spaced eyes that sat near the top of the skull — ideal for lurking invisibly in the shallows until some tasty victim comes along. Paleontologists think it waddled around a bit on land but probably spent the bulk of its time in shallow coastal waters. With webbed feet and hearing mechanisms well adapted for life underwater, A. natans forms part of the important fossil record that tells us how whales inched their way from land to sea over millions of years [source: Thewissen et al.].