Femur
Long bone; part of the limb (appendicular) skeleton.
Also known as: Thigh bone, upper leg bone
Classification
Long bone; part of the limb (appendicular) skeleton.
Dimensions
The long bone of the upper leg, typically the largest and strongest bone in a mammal's body. It is characterized by a long shaft with complex, rounded ends that fit into the hip and knee.
Key Features
The most recognizable part is the lower end, which resembles a pair of smooth, rounded 'rollers' or knuckles separated by a deep groove. These rollers allow the knee to track smoothly. The upper end usually features a distinct ball-shaped head that fits into the hip socket, often set at an angle on a short 'neck'.
Similar Bones
The humerus (upper arm bone) is the most frequent look-alike. However, the humerus usually has a shallower groove at the bottom and a less defined 'neck' at the top. The tibia (shin bone) can also look similar, but its upper end is generally flatter and T-shaped rather than having rounded, knuckle-like rollers.
Across Species
Across species, the femur's shape reflects how an animal moves. In heavy animals like elephants, the femur is thick and pillar-like to support immense weight. In fast-running jumpers like deer or rabbits, it is slender and lightweight. The specific specimen shown features a heavy, double-knobbed lower end, typical of large grazing mammals or ungulates where the bone must withstand significant pressure during walking.
Evolutionary History
The femur has its origins in the lobe-finned fish of the Devonian period, over 350 million years ago. As vertebrates moved onto land, this bone evolved from a small fin-support into a powerful lever capable of hoisting the body off the ground. Over millions of years, it rotated and straightened to allow mammals to walk with their legs tucked directly under their bodies rather than sprawling out to the sides like lizards.
Photography Tips
To help with identification, place a common object like a coin or a ruler next to the bone for size reference. Take one photo from directly above to show the overall length, and another looking directly at the 'knuckled' end to show the shape of the joint surface. Use soft, natural light to avoid harsh shadows that might hide the bone's texture.