Metapodial (Metatarsal or Metacarpal)
Long bone, Appendicular skeleton (limb)
Also known as: Cannon Bone, Shank Bone, Lower Leg Bone
Classification
Long bone, Appendicular skeleton (limb)
Dimensions
These are typically long, slender, and rod-like. Depending on the animal, they can range from 3 to 12 inches in length. They are much longer than they are wide, with a straight central shaft and slightly flared ends.
Key Features
Look for a long, straight shaft with a very distinctive pulley-like shape at the lower end. This lower end features two rounded lobes separated by a central groove, similar to the knuckle of a finger. The cross-section of the shaft is often roughly D-shaped or oval rather than perfectly circular.
Similar Bones
It can be confused with the humerus or femur. However, the metapodial is much straighter and lacks the complex ball-and-socket hinge found at the top of a thigh or arm bone. Its double-lobed bottom end is also much more symmetrical than the bottom of a humerus.
Across Species
In hoofed animals like deer and sheep, these bones are highly specialized for running. While humans have five separate bones in the palm or foot, many grazing animals have evolved to have these bones fused into a single, sturdy 'cannon bone.' In birds, this bone is fused with the ankle bones to form the long lower leg seen in species like herons or ostriches. In whales, these bones remain short and flat within the paddle-like flipper.
Evolutionary History
The history of the metapodial is one of consolidation. Ancestral mammals had five digits, but as lineages like horses and deer adapted for high-speed travel across open plains, they lost their outer toes to reduce weight. The central bones elongated and thickened to provide a more efficient lever for sprinting away from predators.
Photography Tips
Lay the bone flat on a neutral surface and place a common object like a coin or a ruler next to it for size comparison. Take one photo from directly above to show the full length and another close-up photo of the bumpy, jointed end to capture the unique pulley-shaped features.