Ilium
Flat bone; part of the appendicular skeleton within the pelvic girdle.
Also known as: Elephant-ear bone, Hip blade, Flank bone
Classification
Flat bone; part of the appendicular skeleton within the pelvic girdle.
Dimensions
It is the largest and uppermost part of the three-part hip structure. In wide-hipped mammals, it takes on a broad, flared, fan-like appearance that is significantly wider than it is thick.
Key Features
The most striking feature is the wide, flared 'wing' that resembles a paddle or a large leaf. The top edge follows a long, thickened curve, while the lower portion narrows down to meet the round socket where the leg bone attaches. Its overall texture is relatively smooth on the flat surfaces, with a rugged, thickened ridge along the top rim.
Similar Bones
The ilium is often confused with the Scapula (shoulder blade). Both are large and flat, but the ilium is thicker and usually has a much more complex, cup-shaped base where it joins the rest of the hip. The scapula is generally flatter and has a prominent raised ridge running across its back, which the ilium lacks.
Across Species
In nature, the shape of the ilium tells the story of how an animal moves. In four-legged mammals like horses and dogs, the ilium is long and narrow, oriented straight along the spine to help with powerful forward lunging. In humans, who stand upright, the bone has fanned out into a dish-like shape to support the weight of the torso over the legs. Birds have a very long, fused ilium that provides a sturdy base for the massive muscles used in flight and for landing impact.
Evolutionary History
Tracing back to early tetrapods, the ilium began as a simple rod-like extension that connected the back legs to the spine. As animals moved from water to land, this bone expanded to provide more surface area for the developing leg muscles. Over millions of years, the shape transitioned from a small attachment point in ancient amphibians to the massive, fluttering butterfly wings seen in the pelvises of modern land-dwelling vertebrates.
Photography Tips
To capture the best identifying features, take a photo looking directly at the broad, flat face of the bone. Include a second shot from the side to show the gentle S-curve of the blade. Using a dark, non-reflective background will help highlight the thin edges of the 'wing,' and placing a coin or ruler nearby will help viewers understand the scale.