Innominate bone (consisting of the Ilium, Ischium, and Pubis)
Irregular and Flat bone; part of the limb (appendicular) skeleton forming the pelvic girdle
Also known as: Hip bone, Pelvic bone, or Haunch bone
Classification
Irregular and Flat bone; part of the limb (appendicular) skeleton forming the pelvic girdle
Dimensions
In large mammals like deer or cattle, these can range from several inches to over a foot in length, featuring a broad, flared wing and a deep, cup-like socket.
Key Features
The most recognizable feature is the acetabulum—a deep, circular, cup-shaped socket where the thigh bone fits. The bone also features a broad, flattened 'wing' and several large openings that allow major nerves and blood vessels to pass through to the legs.
Similar Bones
Large fragments of the shoulder blade (scapula) can look similar because they are also flat and flared, but the shoulder blade has a shallower socket and a distinct raised ridge or 'spine' running across its surface which the hip bone lacks.
Across Species
The hip bone varies wildly across species to reflect locomotion. In humans, the upper wing (ilium) is broad and bowl-shaped to support walking upright. In four-legged runners like horses or deer, the bone is more elongated and narrow to accommodate powerful leg muscles for sprinting. In aquatic mammals like whales, the pelvis has largely disappeared or remains only as a tiny, floating internal vestige.
Evolutionary History
This bone structure represents the vital bridge between the spine and the hind limbs. It evolved from simple plates in early fish to a robust, three-part fused structure that allowed the first tetrapods to haul themselves onto land. Over millions of years, the point of connection for the legs moved from the sides of the body to directly underneath, shifting the shape of the hip to support more efficient weight-bearing.
Photography Tips
Lay the bone on a neutral background next to a ruler or common coin for scale. Take one photo looking directly into the cup-shaped socket and another from a side-on view to show the flatness and curvature of the 'wing' sections.