Humerus
Long bone of the appendicular (limb) skeleton
Also known as: Upper Arm Bone
Classification
Long bone of the appendicular (limb) skeleton
Dimensions
A long, cylindrical shaft generally measuring between 11 to 14 inches in length, characterized by a rounded top and a flared, complex bottom end.
Key Features
Look for a smooth, marble-like ball at the very top and a distinctive 'spool' shape at the bottom that fits into the elbow. The shaft usually has a slight spiral twist to it, and the bottom end features deep hollows on the front and back where other bones tuck in during movement.
Similar Bones
Can be confused with the femur (thigh bone). To tell them apart, check the top: the humerus has a rounded head that sits directly on the shaft or has a very short 'neck,' whereas the femur has a very prominent, distinct neck that angles the head away from the main shaft.
Across Species
The humerus is a cornerstone of the vertebrate limb. In birds, it is often hollow and light to allow for flight, while in digging animals like moles, it is extremely short, thick, and powerful. In marine mammals like whales, it has evolved into a short, flattened part of the flipper, losing much of its length but gaining surface area for steering through water.
Evolutionary History
The humerus emerged as a distinct skeletal element over 350 million years ago when the first lobe-finned fish began transitioning to life on land. As tetrapods evolved from dragging themselves across marshlands to walking upright, the humerus rotated and straightened to support the lifting of the trunk off the ground, eventually allowing for the wide range of motion seen in primates.
Photography Tips
Lay the bone flat on a neutral surface with a ruler next to it for scale. Provide one photo from the front showing the full length, one close-up of the rounded top ball, and another of the complex flared bottom end to capture the unique pits and grooves.