 Both basilosaurids have skeletons that are immediately recognizable as cetaceans . A basilosaurid was as big as the larger modern whales , with genera like Basilosaurus reaching lengths of up to 60 ft ( 18 m ) long ; dorudontines were smaller , with genera like Dorudon reaching about 15 ft ( 4 @.@ 6 m ) long . The large size of basilosaurids is due to the extreme elongation of their lumbar vertebrae . They had a tail fluke , but their body proportions suggest that they swam by caudal undulation and that the fluke was not used for propulsion . In contrast , dorudontines had a shorter but powerful vertebral column . They too had a fluke and , unlike basilosaurids , they probably swam similarly to modern cetaceans , by using caudal oscillations . The forelimbs of basilosaurids were probably flipper @-@ shaped , and the external hind limbs were tiny and were certainly not involved in locomotion . Their fingers , however , retained the mobile joints of their ambulocetid relatives . The two tiny but well @-@ formed hind legs of basilosaurids were probably used as claspers when mating . The pelvic bones associated with these hind limbs were not connected to the vertebral column as they were in protocetids . Essentially , any sacral vertebrae can no longer be clearly distinguished from the other vertebrae . 
