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 long; dorudontines were smaller, with genera like "Dorudon" reaching about 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.