Os Coxae (Pelvis Half)
Irregular bone; part of the appendicular (limb) skeleton that connects the legs to the main body.
Also known as: Hip Bone, Pelvic Girdle, Haunch Bone
Classification
Irregular bone; part of the appendicular (limb) skeleton that connects the legs to the main body.
Dimensions
This bone is typically broad and flat at the ends with a narrower, thicker middle section. In many mammals, it is several times longer than it is wide, though the specific proportions vary greatly depending on the animal's method of movement.
Key Features
Look for a large, signature hole (the 'obturator foramen') and a deep, cup-shaped socket where the thigh bone fits. It generally has a flattened, blade-like section at the top and rounded, thicker knobs at the bottom.
Similar Bones
The shoulder blade (scapula) is similarly flat but lacks the large deep socket and the prominent enclosed hole found in the hip bone. A single vertebra might have holes and bumps, but it is much more compact and blocky compared to the sprawling, flared shape of the hip.
Across Species
In mammals like deer or dogs, the pelvis is elongated to support powerful running muscles. In birds, these bones are often fused into a single specialized structure to provide a light but rigid frame for flight. Whales possess tiny, vestigial hip bones that 'float' within their body wall, remnants from an era when their ancestors walked on land.
Evolutionary History
The pelvic girdle evolved from the simple bony plates that supported the hind fins of ancient lobe-finned fish. Over millions of years, as ancestors transitioned to life on land, these plates grew larger and more robust to support the full weight of the body against gravity, eventually forming the tripod-like structure seen in modern terrestrial vertebrates.
Photography Tips
Place the bone on a flat, neutral surface and photograph it from directly above. Take a second shot from the side to show the depth of the hip socket and use a coin or ruler next to the bone to provide a clear sense of size.