Pelvis (Innominate Bone)
Irregular bone; part of the appendicular skeleton (limb support)
Also known as: Hip bone, haunch bone, basin bone
Classification
Irregular bone; part of the appendicular skeleton (limb support)
Dimensions
In mammals, the pelvis is typically broad and roughly butterfly-shaped. Its size varies significantly depending on the animal's weight-bearing needs, generally being the widest part of the axial frame.
Key Features
The most distinctive field cue is a large, rounded hole (the obturator foramen) and a deep, cup-shaped socket (the acetabulum) where the thigh bone fits. The bone usually consists of three fused parts that create a complex, twisty shape with flat, blade-like surfaces and thickened ridges.
Similar Bones
The scapula (shoulder blade) is often confused with the pelvis because both have large flat surfaces. To tell them apart, look for the deep, cup-like socket and the large circular hole; the shoulder blade lacks a large central hole and has a much shallower socket for the arm.
Across Species
The pelvis showcases incredible adaptation across species. In cursorial (running) mammals like horses, it is long and narrow to facilitate forward stride. In contrast, heavy mammals like elephants have massive, plate-like pelvises to support vertical weight. Birds possess a fused, elongated pelvis that integrates with the spine to create a rigid 'rig' for the stresses of flight, while in whales, the pelvis has shrunk into small, free-floating bones that no longer connect to the spine.
Evolutionary History
The pelvis first emerged as simple paired plates in early fish to support pelvic fins. As vertebrates moved onto land, these plates enlarged and fused to the spine, creating a sturdy bridge that allowed weight to be transferred from the body to the limbs. This transformation from a loose support to a rigid, three-part basin allowed for the development of powerful walking and running gaits in land animals.
Photography Tips
Place the bone on a neutral background and include a common object like a coin or ruler for scale. Capture one shot directly through the large hole, one of the deep socket, and a profile view to show the overall curvature and thickness of the bone.