Innominate Bone (Os Coxae)
Irregular bone; part of the appendicular skeleton (limb support)
Also known as: Hip bone, Pelvic bone, or Haunch bone
Classification
Irregular bone; part of the appendicular skeleton (limb support)
Dimensions
Broad, flared, and sturdy. It is characterized by its wide 'blade' at the top and its heavy, cup-like socket in the middle. Typically, its width and height are nearly equal in mammals that walk upright, while being longer and narrower in four-legged animals.
Key Features
Look for a very distinctive, deep, round cup (the socket for the thigh bone). Above this cup is a large, thin, ear-shaped blade with a thickened top rim. Below the cup, there is often a large circular opening or 'window' that makes the bone look a bit like a mask or a pair of glasses when both sides are viewed together.
Similar Bones
The shoulder blade (scapula) is also a large flat bone, but it lacks the deep, circular bowl-shaped socket and the large 'window' opening found in the hip. Some fragments of the skull can appear flat like the hip blade, but skull pieces are usually much thinner and lack the heavy, thick weight-bearing joints found here.
Across Species
In birds, the hip bones are often fused into a long, thin structure called a synsacrum to handle the stresses of landing. In whales, the hip bones have shrunk over millions of years into tiny, internal floating rods that no longer connect to the spine. In heavy land mammals like elephants, the blade is massive and fan-shaped to support immense weight, whereas in fast runners like deer, it is elongated to provide long lever arms for leg muscles.
Evolutionary History
The hip bone began as a simple support for the fins of ancient lobe-finned fish. As vertebrates moved onto land, these bones grew larger and eventually fused with the spine to create a stable platform for walking. Over millions of years, the three separate parts of the bone (the top blade, the seat bone, and the front arc) grew together into the single, solid unit seen in most modern land animals.
Photography Tips
Lay the bone on a flat surface and take a photo looking straight down at the socket. Then, flip it over to show the inner side. Including a common object like a coin or a ruler helps show the size, as hip bones vary wildly from the size of a mouse's thumbnail to the size of a dinner plate.