Innominate Bone
Flat/Irregular bone and a core component of the appendicular skeleton's girdle.
Also known as: Hip bone, Pelvic bone, or Coxal bone
Classification
Flat/Irregular bone and a core component of the appendicular skeleton's girdle.
Dimensions
In mammals, this is typically a broad, flat bone with a complex three-dimensional shape. Its width and curvature vary greatly depending on the animal's method of locomotion and size.
Key Features
The most hallmark feature is the deep, cup-shaped socket on the outer side, which looks like a smooth 'bowl' where the leg bone fits. Another key cue is the large, flat, ear-shaped blade that flares out at the top. You will also typically see a large circular opening or 'window' near the bottom and a roughened area where it connects to the tailbone.
Similar Bones
At first glance, a fragment of a hip bone might be confused with a shoulder blade (scapula). However, the shoulder blade is usually much thinner and flatter, lacking the deep, circular bowl-like socket and the large 'window' opening found in the hip.
Across Species
In nature, the hip bone is a master of adaptation. In birds, it is fused into a single rigid structure called a synsacrum to support the forces of flight. In marine mammals like whales, the hip bones have shrunk into tiny, unattached remnants deep within the body, as they no longer need to support legs. Large land mammals like elephants have massive, plate-like hips to support their immense weight, while agile jumpers like deer have long, narrow hips that act as efficient levers for powerful leg muscles.
Evolutionary History
The hip bone began as simple paired plates in early fish-like ancestors, providing a basic anchor for pelvic fins. As vertebrates moved onto land, these plates grew and merged with the spine to support the body's weight against gravity. Over millions of years, the three separate parts of the hip—the flared upper blade, the sturdy sitting bone, and the front crossing bone—fused together to create the modern, solid cup-and-ring structure seen in land-dwelling animals today.
Photography Tips
To help with identification, take one photo looking directly 'top-down' at the widest part and another from the side to show the depth of the leg socket. Using a ruler for scale is excellent, but ensure the lighting is soft to avoid harsh shadows that might hide the subtle curves of the bone's surface.