complete, with a small pair of ribs connecting the two structures; nonetheless the pelvis already forms the complete ring found in most subsequent forms. in practice, modern amphibians and reptiles have substantially modified this ancestral structure, based on their varied forms and lifestyles. the obturator foramen is generally very small in such animals, although most reptiles do possess a large gap between the pubis and ischium, referred to as the thyroid fenestra, which presents a similar appearance to the obturator foramen in mammals. in birds, the pubic symphysis is present only in the ostrich, and the two hip bones are usually widely separated, making it easier to lay large eggs. in therapsids, the hip bone came to rotate counter-clockwise, relative to its position in reptiles, so that the ilium moved forward, and the pubis and ischium moved to the rear. the same pattern is seen in all modern mammals, and the thyroid fenestra and obturator foramen have merged to form a single space. the ilium is typically narrow and triangular in mammals, but is much larger in ungulates and humans, in which it anchors powerful gluteal muscles. monotremes and marsupials also possess a fourth pair of bones, the prepubes or "marsupial bones", which extend forward from the pubes, and help to support the abdominal muscles and, in marsupials, the pouch. in placental mammals, the pelvis as a whole is generally wider in females than in males, to allow for the birth of the young. the pelvic bones of cetaceans were formerly considered to be vestigial, but they are now known to play a role in sexual selection. additional images position of the hip bones (shown in red). animation. right hip bone. animation. right hip bone. external surface. right hip bone. internal surface. left hip-joint, opened by removing the floor of the acetabulum from within the pelvis. hip bone.medial view. hip bone. lateral view.