Hip bone first appeared on fish. It consists of a simple, usually triangular bone connected to the ventral fin. Hips on both sides are usually connected at the front end, even in lungfish and sharks, but they never attach to the spine.
In early quadrupeds, this early hip bone evolved into the ischium and pubis, while the ilium formed a new structure, initially rod-shaped, but soon added a larger bone blade. The acetabulum already exists where the three bones meet. In these early forms, the connection with the spine is incomplete, and a pair of ribs connect two structures; However, in most later forms, the pelvis has formed a complete ring.
Extended data
Modern amphibians and reptiles have substantially changed this ancestral structure based on their different forms and lifestyles. The obturator foramen of these animals is usually very small, although most reptiles do have a large gap between the pubic bone and the ischium, called the thyroid window, which is similar in appearance to the obturator foramen of mammals. In birds, pubic symphysis only exists in ostriches, and the two hip bones are usually far apart, which makes it easier to lay large eggs.
In therapsids, the hip rotates counterclockwise, which makes the ilium move forward and the pubic bone and ischium move backward relative to their positions in reptiles. The same pattern can be seen in all modern mammals, and the thyroid fenestration and obturator foramen will merge to form a single space. In mammals, the ilium is usually a narrow triangle, but it is larger in ungulates and humans, where it fixes powerful gluteal muscles.
Marsupials also have a fourth pair of bones, the prepreg tube or "bag bone", which extends forward from the pubic bone to help support the abdominal muscles and the bag of marsupials. In mammals with placenta, the entire pelvis of women is usually wider than that of men to allow young people to be born.