 Pakicetus had a pelvic bone most similar to that of terrestrial mammals . In later species , such as Basilosaurus , the pelvic bone , no longer attached to the vertebrae and the ilium , was reduced . Certain genes are believed to be responsible for the changes that occurred to the cetacean pelvic structure , such as BMP7 , PBX1 , PBX2 , PRRX1 , and PRRX2 . The pelvic girdle in modern cetaceans were once thought to be vestigial structures that served no purpose at all . The pelvic girdle in male cetaceans is different in size compared to females , and the size is thought to be a result of sexual dimorphism . The pelvic bones of modern male cetaceans are more massive , longer , and larger than those of females . Due to the sexual dimorphism displayed , they were most likely involved in supporting male genitalia that remain hidden behind abdominal walls until sexual reproduction occurs . 
