Ulna
Long Bone, Appendicular Skeleton
Also known as: Forearm Bone / Elbow Bone
Classification
Long Bone, Appendicular Skeleton
Dimensions
In small mammals such as rodents, rabbits, or small carnivores, this bone is typically slender, measuring anywhere from 1 to 4 inches (2.5 to 10 cm) in length, featuring a distinctly enlarged top end that forms the hinge of the elbow joint tapering down to a delicate lower shaft.
Key Features
The ultimate field mark for an ulna is its wrench-like upper end. Look for a deep, smooth C-shaped notch near the top that cradles the upper arm bone to form the elbow hinge. Just above this notch is a prominent knob or projection that forms the point of the elbow. The shaft is usually slightly curved and tapers toward a small, rounded lower tip.
Similar Bones
The bone most easily confused with the ulna is its forearm partner, the radius. However, while the radius has a round, button-like or spool-like head at its top end designed for twisting, the ulna is instantly recognizable by its classic hook-like or ice-ice-tongs shape at the elbow end. Fibulae (the slender lower leg bones) can also look similar in size, but they lack the distinct deep C-shaped notch found on the ulna.
Across Species
The ulna shows remarkable adaptations across the animal kingdom depending on how a species moves. In diggers like moles and badgers, the ulna is extremely thick with an elongated upper projection that acts as a powerful lever arm for heavy digging muscles. In running animals like horses and deer, the ulna is fused completely to its neighbor, the radius, forming a single light, rigid column that saves energy during sprinting. In birds, the ulna is often the larger of the two forearm bones and features small bumps along its shaft where quill ligaments anchor flight feathers. In humans and primates, the ulna remains separate from the radius, allowing our forearms to rotate freely to turn our hands palm-up or palm-down.
Evolutionary History
The dual-bone arrangement of the forearm—consisting of the radius and ulna—is an ancient design inherited from early lobe-finned fish that first crawled onto land over 375 million years ago. This two-bone architecture provided structural stability while allowing early tetrapods to support their weight on land. Over millions of years, as mammals specialized for digging, running, swimming, or flying, the ulna was modified: lengthened in bats to support wing membranes, reduced or fused in hoofed mammals to stabilize forward movement, and broadened in aquatic mammals like seals to form sturdy paddles.
Photography Tips
To help experts identify small forearm bones, lay the specimen flat on a neutral background alongside a ruler or standard coin for scale. Take one straight-on photo showing the side profile of the deep elbow notch, and a second photo looking down at the top end to show its thickness and shape. Soft, angled light helps bring out subtle ridges along the main shaft.