 Ambulocetids had relatively long limbs with particularly strong hind legs , and they retained a tail with no sign of a fluke . The hindlimb structure of Ambulocetids shows that their ability to engage in terrestrial locomotion was significantly limited compared to that of contemporary terrestrial mammals . The skeletal structures of the knee and ankle indicates that the motion of the hindlimbs was restricted into one plane . This suggests that , on land , propulsion of the hindlimbs was powered by the extension of dorsal muscles . Although they could walk on land , in addition to swimming , it is clear that they were not fast in either environment . It has been speculated that Ambulocetids hunted like crocodiles , lurking in the shallows to snatch unsuspecting riparian prey and fish . They probably swam by pelvic paddling ( a way of swimming which mainly utilizes their hind limbs to generate propulsion in water ) and caudal undulation ( a way of swimming which uses the undulations of the vertebral column to generate force for movements ) , as otters , seals and modern cetaceans do . This is an intermediate stage in the evolution of cetacean locomotion , as modern cetaceans swim by caudal oscillation ( a way of swimming similar to caudal undulation , but is more energy efficient ) . 
