Metatarsus (Metatarsal)
Long bone; part of the limb (appendicular) skeleton
Also known as: Cannon bone, shank bone, or mid-foot bone
Classification
Long bone; part of the limb (appendicular) skeleton
Dimensions
Broad and sturdy with a long, columnar shaft. In grazing animals, this bone is often much longer than it is wide, providing significant height and leverage.
Key Features
Look for a long, straight shaft with a deep vertical groove running down the front or back. The bottom end usually features two distinct, rounded pulleys (condyles) that resemble a double-knuckle, which connect to the toes.
Similar Bones
This is often confused with the metacarpal (front leg cannon bone). The metatarsal (back leg) typically has a more square or D-shaped cross-section compared to the more rounded or oval cross-section of the front leg version. It might also be mistaken for a tibia, but the tibia lack the deep, full-length vertical groove and the 'double-pulley' bottom.
Across Species
In many hooved mammals like deer, sheep, and cattle, the mid-foot bones have fused into a single 'cannon bone' to provide strength for running. While humans have five separate bones in the arch of the foot, these animals have consolidated them for efficiency. Birds also possess a similar fusion called a tarsometatarsus, which forms the lower part of their visible leg.
Evolutionary History
The history of this bone tracks the transition from five-toed ancestors to specialized runners. Evolution favored the fusion and elongation of the mid-foot bones in many lineages to move the muscles higher up the leg, allowing for a lighter, faster limb swing—a perfect example of adaptation for life on open plains.
Photography Tips
Lay the bone flat on a neutral background and take one photo from directly above. Include a second photo of the end with the double-knuckles to show their shape. Using a ruler or a common object like a coin for scale is very helpful for identification.