Humerus
Long bone of the appendicular skeleton
Also known as: Upper Arm Bone
Classification
Long bone of the appendicular skeleton
Dimensions
Proportions feature a slender central shaft flaring out into wider ends. Dimensions vary widely by species, ranging from a few inches in small mammals and birds to several feet in large quadrupeds.
Key Features
Look for a long, smooth central shaft with a rounded, ball-like top designed to fit into a shoulder socket, and a wider, grooved bottom end that forms a hinge joint for the lower arm.
Similar Bones
Can be easily confused with the femur, the main bone of the upper leg. The humerus can usually be distinguished by the specific rounded shape of its upper ball and the pulley-like grooves at the lower end tailored for elbow movement rather than hip rotation.
Across Species
Across the animal kingdom, the humerus adapts to vastly different lifestyles. In swimming mammals like whales and dolphins, it becomes remarkably short and stout. In birds, it is lightweight and often hollowed out to save weight for flight, while in powerful diggers like moles, it is exceptionally broad and rugged to anchor massive arm muscles.
Evolutionary History
The humerus traces its ancestry back to the paired fins of ancient lobe-finned fish, where sturdy internal bones first evolved to help creatures navigate shallow waters and eventually step onto land. Over millions of years, this single upper-limb bone became a cornerstone of tetrapod locomotion.
Photography Tips
Place the bone on a neutral, solid-colored background with good side-lighting to highlight the surface contours and textures. Include a common coin or ruler next to the bone to provide an instant sense of scale.