Humerus
Long bone; part of the appendicular skeleton (limb)
Also known as: Upper arm bone
Classification
Long bone; part of the appendicular skeleton (limb)
Dimensions
In mammals, it is typically a sturdy, cylinder-shaped bone. Its length usually exceeds its width significantly, featuring a rounded top that fits into a socket and an flared lower end with complex ridges.
Key Features
Look for a long, straight shaft with a distinctive flare at one end. The surface often features small ridges or rough patches where muscles once attached. The lower end usually has smooth, hourglass-shaped surfaces that look like a spool of thread, which serve as hinges for the lower limb bones.
Similar Bones
It can be mistaken for the femur (thigh bone). To tell them apart, look at the top: the femur has a very distinct, long 'neck' and a ball-shaped head that sticks out to the side, whereas this bone has a broader, more integrated rounded top. The lower end of this bone also tends to have more flattened, spool-like surfaces compared to the large, rounded knobs of a femur.
Across Species
Across the animal kingdom, this bone's shape tells a story of movement. In fast-running hoofed animals like deer, it is short and powerful for rapid sprinting. In birds, it is often hollow and light to support flight, while in whales, it has evolved into a short, flattened block that provides a rigid foundation for the front flipper.
Evolutionary History
This bone can be traced back over 350 million years to the first lobe-finned fish that began hauling themselves onto muddy shores. Over eons, this single bone became the essential anchor for the forelimb, adapting from a paddle-support into the complex lever used by land-dwelling vertebrates for climbing, digging, flying, and walking.
Photography Tips
To help with identification, place the bone next to a ruler and take photos from multiple angles. Capture a straight-down view of the shaft, but most importantly, take close-up shots of the joint ends (the top and bottom), as these contain the most helpful visual clues for naming the bone and the species it belongs to.