Pelvis (Innominate Bone)
Irregular bone; part of the appendicular (limb) skeleton that connects to the axial (core) skeleton.
Also known as: Hip bone, basin, or haunch bone
Classification
Irregular bone; part of the appendicular (limb) skeleton that connects to the axial (core) skeleton.
Dimensions
The pelvis is a broad, bowl-shaped structure consisting of two symmetrical halves joined at the front and to the wedge-shaped tailbone at the back. It is typically wider than it is deep.
Key Features
Look for a deep, circular cup (socket) on the outer side where the thigh bone fits. The top has a broad, flared wing shape, while the bottom contains a large, prominent hole on each side and a thick bridge of bone across the front. From above, it looks like a hollow ring or basin.
Similar Bones
The shoulder blade (scapula) is also a broad, flat-style bone, but it lacks the deep circular socket and the ring-like basin structure that defines the pelvis. The sacrum (tailbone) sits in the middle of the pelvis but is shaped like a flattened, perforated wedge rather than a flared wing.
Across Species
The shape of the pelvis varies drastically by species depending on how they move. In four-legged mammals like dogs or deer, the pelvis is long and narrow to support horizontal movement. In birds, the pelvis is often fused more extensively with the spine to provide a rigid foundation for flight. In humans, the pelvis is exceptionally broad and bowl-like, having evolved into a short, wide structure to support the weight of the trunk during upright walking.
Evolutionary History
The pelvis originated in early fish as small plates supporting pelvic fins. Over millions of years, as vertebrates moved onto land, these plates grew and anchored to the spine to allow legs to carry the body's weight. The move to two-legged walking in our lineage led to a significant shortening and widening of the pelvis to provide a stable center of gravity and attachment points for powerful gluteal muscles.
Photography Tips
Capture a top-down view to show the 'basin' shape and a side view to clearly show the deep leg socket. Use a neutral background like grey or brown to help the complex curves and holes stand out, and place a common object like a coin or ruler nearby for size context.