Pelvis
Commonly categorized as a collection of irregular and flat bones forming part of the limb-attachment (appendicular) and lower core (axial) skeleton.
Also known as: Hip bone, pelvic girdle, or basin
Classification
Commonly categorized as a collection of irregular and flat bones forming part of the limb-attachment (appendicular) and lower core (axial) skeleton.
Dimensions
In many large mammals, it forms a broad, bowl-like structure. It is typically wider than it is deep, featuring expansive wings on the top and a central opening in the middle.
Key Features
Look for the large, flared 'wings' at the top and the deep, cup-shaped sockets on the outer sides where the leg bones attach. The most recognizable feature is the large central hole and the wedge-shaped tailbone at the back that locks the two halves together into a sturdy ring.
Similar Bones
The shoulder blade (scapula) might be confused with the upper 'wing' of the pelvis because both are flat and broad. However, the pelvis is much thicker and forms a complete loop or 'girdle,' whereas the shoulder blade is a single, free-floating triangular bone.
Across Species
The pelvic structure varies wildly across the animal kingdom depending on how a creature moves. In birds, the pelvis is often fused and elongated to provide a sturdy anchor for flight muscles and to withstand the force of landing. In aquatic mammals like whales, the pelvis has shrunk over millions of years into small, internal 'floating' bones because they no longer need to support their weight on land. Fast-running mammals like deer have narrow, long pelvises to keep their legs moving in a straight, efficient line.
Evolutionary History
The pelvis tells the story of the transition from sea to land. Early fish-like ancestors had simple plates to support fins. As vertebrates moved onto land, these plates grew larger and fused with the spine to support the weight of the body against gravity. For those that eventually walked on two legs, the pelvis shortened and became more bowl-shaped to support the internal organs in an upright position.
Photography Tips
To get the best identification photos, take one shot looking straight down into the 'bowl' to show the overall symmetry. Then, take a side-view photo to capture the depth of the leg socket. Including a common object like a coin or a ruler nearby helps researchers understand the scale.