Innominate bone
Flat and irregular bone; part of the appendicular skeleton
Also known as: Hip bone, pelvic bone, or oscillating bone
Classification
Flat and irregular bone; part of the appendicular skeleton
Dimensions
Broad, flared, and relatively flat. In many medium-to-large land mammals, it forms a propeller-like shape with a deep circular socket in the middle and large openings for muscle attachment.
Key Features
The most obvious feature is the deep, moon-shaped socket (the cup for the thigh bone) and a large, smooth-edged hole known as the 'hidden window.' It often has a thickened, flared ridge along the top edge that looks like the crest of a wave.
Similar Bones
It is sometimes confused with the shoulder blade (scapula) because both are flat. However, the hip bone is much thicker, has a deep circular socket instead of a shallow one, and contains a large natural hole, which a shoulder blade lacks.
Across Species
In quadrupeds like deer or horses, these bones are elongated and narrow to support powerful running muscles. In birds, the pelvic bones are fused into a rigid frame called a synsacrum to stabilize the body for flight. In whales, the hip bones have shrunk into tiny, free-floating remnants hidden within the body, as they no longer need to support limbs for walking.
Evolutionary History
This bone evolved from simple plates in early fish-like ancestors into a robust tripod system as vertebrates transitioned to land. The shifting of the bones toward the back of the body allowed for the transfer of force from the legs to the spine, enabling the giant leaps and strides seen in modern terrestrial animals.
Photography Tips
Place a common object like a coin or a key next to it for size comparison. Shoot from the side to capture the depth of the socket, and take a top-down view to show the overall 'butterfly-wing' or 'propeller' shape.