Os Coxae
Irregular bone; part of the appendicular skeleton (limb support system)
Also known as: Hip Bone, Pelvic Bone, Innominate Bone
Classification
Irregular bone; part of the appendicular skeleton (limb support system)
Dimensions
In larger mammals, this is typically a broad, robust structure. It is often as wide as it is tall, featuring expansive flat surfaces and thick, reinforced rings toward the bottom.
Key Features
Look for a large, flat, ear-shaped blade at the top and a deep, cup-like socket on the outer side for the leg bone. The bottom contains a prominent circular opening, and the center is narrower than the top and bottom, giving it a somewhat hourglass silhouette from certain angles.
Similar Bones
The shoulder blade (scapula) is also a large flat bone, but it lacks the deep circular socket and the large holes found in the hip bone. The sacrum could be confused with a fragment of the hip, but the sacrum is symmetrical and has rows of small holes rather than a single large opening.
Across Species
The shape varies wildly based on how an animal moves. In humans, it is bowl-shaped to support internal organs while standing upright. In quadrupeds like dogs or horses, it is elongated and blade-like to accommodate powerful running muscles. Whales have tiny, floating versions of these bones that serve as evolutionary reminders of when their ancestors walked on land.
Evolutionary History
These bones began as simple plates in early fish to support pelvic fins. As vertebrates moved onto land, these plates fused into a solid ring to transfer the force of walking from the legs to the spine. Over millions of years, the upper 'blade' expanded to provide more surface area for the massive muscles needed for terrestrial locomotion.
Photography Tips
Lay the bone flat on a neutral background. Take one photo from the front to show the general 'butterfly' shape and another from the side to capture the depth of the hip socket. Using a ruler for scale is helpful since these bones can range from the size of a thumb to the size of a dinner plate.