Metapodial
Long bone, part of the limb (appendicular) skeleton.
Also known as: Shin or Shank bone (ungulates), Cannon bone (horses/deer), Hand/Foot bone
Classification
Long bone, part of the limb (appendicular) skeleton.
Dimensions
These are characteristically long and straight bones. In many animals, they are significantly elongated to provide extra height and stride length. They are usually much longer than they are wide, with a square or semi-circular cross-section.
Key Features
The most defining feature is the long, straight shaft with a distinct groove or 'fused line' running down the length in many grazing animals. The ends are often flared. One end typically has a smooth, spool-like surface for a hinge joint, while the other end may have rounded knobs or flat surfaces depending on the species.
Similar Bones
From a distance, these can be confused with the humerus (upper arm) or femur (thigh), but metapodials are much straighter and lack the large, ball-shaped 'head' found on the upper limb bones. They also lack the twisting or curving shape seen in the radius or tibia of the mid-limb.
Across Species
In the animal kingdom, these bones show incredible variety. In humans, they are the small bones inside our palms and mid-foot. In contrast, hoofed animals like deer and cattle have 'cannon bones' where multiple bones have fused into one strong, pillar-like structure to support high-speed running. A horse's lower leg is essentially a massive single metapodial. In birds, the same structure is fused with ankle bones to form the long, thin leg section above the toes.
Evolutionary History
Metapodials began as a series of five separate rod-like bones in early land-dwelling vertebrates. As mammals evolved for different lifestyles, these bones changed drastically. Speed-specialists, like antelopes, saw their side metatarsals shrink and the central ones fuse to create a lightweight but incredibly strong lever. This allows for a 'digitigrade' or 'unguligrade' stance, where the animal effectively walks on its tiptoes or nails.
Photography Tips
To get a good identification, lay the bone flat next to a ruler or common object like a coin for size. Take one photo from directly above to show the shaft's width, and most importantly, take close-up photos of both ends. The shape of the joint surfaces at the top and bottom are the best clues for determining which animal the bone came from.