Ulna and Radius
Long bones, part of the limb (appendicular) skeleton
Also known as: Forearm bones
Classification
Long bones, part of the limb (appendicular) skeleton
Dimensions
Typically long and slender, these bones are generally around 1 to 1.5 times the length of the animal's upper arm bone. In many hoofed mammals, they often appear fused together into a single, sturdy unit.
Key Features
The most recognizable feature is the large, hooked top end of the ulna, which resembles a wrench or a bottle opener. This hook fits into the elbow joint. The shaft is usually slightly curved, and in many wild herbivores, you can see a visible seam or groove where the two bones have joined into one.
Similar Bones
This unit can be confused with the lower leg bones (tibia and fibula). To tell them apart, look for the 'wrench-like' hook at the top; the lower leg bones have flat or slightly cupped tops rather than a protruding hook. It might also be mistaken for a single large rib, but it is much thicker and has more complex joint surfaces at the ends.
Across Species
In humans and primates, these two bones remain separate to allow the wrist to rotate. However, in running animals like deer, sheep, or horses, the radius and ulna are often fused or tightly pressed together to provide a rigid, stable pillar for high-speed locomotion. In these species, the ulna is often reduced to a thin sliver of bone along the back of the much larger radius.
Evolutionary History
These bones trace back to the fins of ancient lobe-finned fish. Over millions of years, as vertebrates moved onto land, these structures transformed from swimming supports into weight-bearing limbs. In cursorial (running) mammals, the trend has been toward the fusion of these bones to save energy and increase the strength of the leg during a gallop.
Photography Tips
Lay the bone on a flat surface and take a top-down photo showing its full length. Be sure to include a clear shot of the hooked 'elbow' end from the side. Always place a common object like a coin or a ruler next to the bone to provide a sense of scale for identification.