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. Both basilosaurids and dorudontines are relatively closely related to modern cetaceans, which belong to parvorders Odontoceti and Mysticeti. However, according to a 1994 study done by Fordyce and Barnes, the large size and elongated vertebral body of basilosaurids preclude them from being ancestral to extant forms. As for dorudontines, there are some species within the family that do not have elongated vertebral bodies, which might be the immediate ancestors of Odontoceti and Mysticeti. The other basilosaurids became extinct. Evolution of modern cetaceans Baleen whales All modern baleen whales or mysticetes are filter-feeders which have baleen in place of teeth, though the exact means by which baleen is used differs among species (gulp-feeding within balaenopterids, skim-feeding within balaenids, and bottom plowing within eschrichtiids). The first members of both groups appeared during the middle Miocene. Filter feeding is very beneficial as it allows baleen whales to efficiently gain huge energy resources, which makes the large body size in modern varieties possible. The development of filter feeding may have been a result of worldwide environmental change and physical changes in the oceans. A large-scale change in ocean current and temperature could have contributed to the radiation of modern mysticetes. The earlier varieties of baleen whales, or "archaeomysticetes", such as Janjucetus and Mammalodon had very little baleen and relied mainly on their teeth. There is also evidence of a genetic component of the evolution