Femur (fragmentary)
Long Bone of the limb (appendicular) skeleton
Also known as: Thigh Bone, Upper Leg Bone
Classification
Long Bone of the limb (appendicular) skeleton
Dimensions
In mammals, these bones are typically the longest and strongest in the body, with a cylindrical shaft and bulbous ends. The proportions vary by species, with large herbivores possessing thick, heavy shafts to support significant weight.
Key Features
Look for a thick, dense outer wall surrounding a hollow or spongy center. Even in fragments, the distinct 'C' shape of the shaft's cross-section is a major clue. The presence of large, rounded knobs where the bone once joined a hip or knee is a classic identification feature of long bones.
Similar Bones
This can be confused with the humerus (upper arm bone). However, the thigh bone is usually much thicker and has more massive, flared attachment points. In small fragments, it may also look like a piece of a tibia (shin bone), but the thigh bone generally lacks the sharp vertical ridge found on the front of the shin.
Across Species
The thigh bone is a masterpiece of natural engineering. In cursorial mammals like horses, it is relatively short and thick to provide massive leverage for running. In contrast, in aquatic mammals like whales, the bone has become tiny and disconnected from the rest of the skeleton. In birds, the bone is often hollowed out to save weight for flight while remaining incredibly sturdy.
Evolutionary History
This bone first appeared as a significant weight-bearing structure when the ancestors of modern land animals transitioned from water to land. Over millions of years, it evolved from a sprawling, side-oriented support in early tetrapods to a vertical, weight-bearing pillar situated directly under the body in mammals and dinosaurs, allowing for more efficient movement.
Photography Tips
To help with identification, place the fragment next to a ruler or a common object like a coin for scale. Take photos from several angles, including a top-down view and a 'cross-section' view of the broken end to show the internal structure. Avoid using a flash, as it can wash out the subtle textures and bumps on the surface.