Several kinds of mammals are aquatic or semi aquatic, but only sea cows and whales, order Cetacea, are so thoroughly aquatic that they cannot come out onto land. Fossils discovered in Middle Eocene rocks in Pakistan indicate the land-dwelling an¬cestors of whales were among the artiodactyls, but some paleontologists think the ancestors were wolf-sized, meat eating mammals. During the transition from land-dwelling animals to aquatic whales, the front limbs modified into paddle like flippers, the rear limbs were lost, and the nostrils migrated to the top of the head. In addition, whales have a large, horizontal tail fluke used for propulsion.
For many years, paleontologists had little fossil evidence that bridged the gap between land-dwelling animals and fully aquatic whales. This important transition took place in a part of the world where the fossil record was poorly known. Beginning about 20 years ago, paleontologists have made some remarkable finds that resolved this evolutionary enigma. For instance, the Lower Eocene whale Ambulocetus still had limbs capable of support on land, whereas Basilosaurus, a 15 m long Upper Eocene whale, had only tiny, vestigial rear limbs. The latter had teeth similar to those of their ancestors, and its nostrils were on the snout, but it was truly a whale, although differently proportioned from those living now.
By Oligocene time, both presently existing whale groups-baleen whales and toothed whales-had evolved.