Scientifically, the group called dinosaurs includes all the animals that evolved from the very first dinosaur, the common ancestor. A key evolutionary innovation of dinosaurs is that they walk with a fully erect posture, holding their hind legs vertically under their hips. The top of the thighbone (or femur) has a knob or head that sticks out to the side of the rest of the bone and fits into the hip socket (or acetabulum) in the pelvis. The hip socket has a hole in the center, as well as a rim of bone along the upper margin, which helps support the weight of the body on the leg. Since these features are present in all known dinosaurs, we deduce that they first evolved in the common ancestor.