This passage discusses fossils that help to explain the likely origins of cetaceans – whales, porpoises, and dolphins. A. Recent discoveries of fossils have helped to show the link between land mammals and cetaceans. B. The discovery of Ambulocetus natans provided evidence for a whale that lived both on land and at sea. C. The skeleton of Basilosaurus was found in what had been the Tethys Sea, an area rich in fossil evidence. D. Pakicetus is the oldest fossil whale yet to be found. E. Fossils thought to be transitional forms between walking mammals and swimming whales were found. F. Ambulocetus' hind legs were used for propulsion in the water.