Os Coxae (Pelvis)
Irregular bone; part of the appendicular skeleton (limb support system)
Also known as: Hip Bone, Pelvic Bone, Innominate Bone
Classification
Irregular bone; part of the appendicular skeleton (limb support system)
Dimensions
Typically wide and flat with a propeller-like twist. In mammals, the size varies greatly from the palm-sized hips of a raccoon to the massive, yard-wide pelvis of a draft horse.
Key Features
Look for a large cup-shaped socket on the outer side where the leg bone attaches. It usually features a large, distinctive hole in the bottom half and a wide, flared wing at the top. The texture is often thin and blade-like in some areas while being thick and reinforced around the socket area.
Similar Bones
At a glance, a weathered hip bone could be mistaken for a shoulder blade (scapula). However, the shoulder blade is usually much flatter and lacks the deep, circular cup-and-ball socket and the large circular hole found in the hip bone.
Across Species
In four-legged animals like deer or dogs, the hip is narrow and elongated to help with stride length. In contrast, heavy-bodied animals like elephants have massive, broad basins to support their internal organs. Birds have a highly modified pelvis that is fused to their spine to create a lightweight, rigid structure for flight, often featuring a long, thin shape.
Evolutionary History
The hip originated from three separate bones that fused over millions of years into a single unit. This structure allowed early vertebrate ancestors to move from water to land, providing a sturdy anchor for the back legs to push against the ground. Over deep time, the shape has constantly shifted to accommodate different ways of moving, whether climbing, running, or swimming.
Photography Tips
Lay the bone on a flat, neutral surface with a ruler next to it for scale. Take one photo directly from above to show the overall silhouette, and another from the side to capture the depth of the leg socket and the curves of the bone.