Femur
Long bone belonging to the appendicular (limb) skeleton.
Also known as: Thigh bone
Classification
Long bone belonging to the appendicular (limb) skeleton.
Dimensions
Typically recognized as the heaviest and longest bone in a vertebrate's body. It features a long, straight shaft with distinct, rounded knobs at both the top and bottom ends.
Key Features
Look for a very prominent, ball-shaped 'head' at the upper end that fits into a hip socket. This head is usually perched on a short 'neck' that sits at an angle to the main shaft. At the opposite lower end, you will find two smooth, rounded rollers resembling knuckles, which allow for a hinge-like movement.
Similar Bones
The femur is most commonly confused with the humerus (upper arm bone). However, the humerus usually has a shallower, less globular head at the top and a more spool-shaped, pulley-like surface at the bottom rather than the two distinct rounded rollers found on a femur.
Across Species
Across the animal kingdom, the femur is a master of adaptation. In large mammals like elephants, it is thick and pillar-like to support immense weight, while in fast runners like deer, it is slender and lightweight. Birds have femurs that are often quite short and tucked close to the body, whereas in whales, the femur has shrunk over millions of years into a tiny, vestigial remnant buried deep within the body wall.
Evolutionary History
The femur traces its roots back over 350 million years to the first lobe-finned fish that began to crawl onto land. These early 'sturdy fins' contained a single upper bone that eventually evolved into the primary weight-bearing pillar of the hind limb in tetrapods. Over deep time, its shape changed to allow for different ways of moving—from the sprawling gait of lizards to the upright, efficient stride seen in modern mammals.
Photography Tips
To help with identification, place a common object like a coin or ruler next to the bone for scale. Take one photo from a 'front' view to show the full length, and then take close-up photos of both the top rounded head and the bottom knuckle-like joints from multiple sides.