Metapodials (Metacarpal or Metatarsal)
Long bone, part of the appendicular (limb) skeleton.
Also known as: Cannon bone, Shank bone, or Shin bone (lower limb)
Classification
Long bone, part of the appendicular (limb) skeleton.
Dimensions
In large ungulates, these bones are typically much longer than they are wide, often appearing as a sturdy, cylindrical shaft with flared ends that accommodate hinges for the toes.
Key Features
The most striking feature is the 'pulley' or 'spool' shape at the lower end, which looks like two rounded knobs separated by a deep central groove. This groove acts as a guide for the ligaments that control the movement of the two toes.
Similar Bones
The Humerus (upper arm bone) also has a pulley-like end, but it is usually a single large spool rather than the distinct double-hinge seen on this specimen. The Tibia (shin bone) has a flatter, more cup-like bottom surface.
Across Species
While humans have five separate palm or foot bones, many hooved mammals like deer, cows, and horses have evolved for speed. In deer and bovine species, the middle two bones fused into a single powerful 'cannon bone' to support weight, while horses fused them down to just one. The double-knuckled hinge seen on the bottom of this fragment specifically identifies it as a 'cloven-hoofed' animal like a cow or elk.
Evolutionary History
Over millions of years, as mammals moved from swampy forested areas to open grasslands, the limb bones elongated to increase stride length. The fusion of the lower limb bones provided a more rigid, spring-like mechanism, allowing for efficient running on hard ground while reducing the weight at the end of the leg.
Photography Tips
Lay the bone on a flat surface with a ruler or common object like a coin for scale. To help with identification, take one photo looking directly down at the 'knuckle' end and another from the side to show the profile of the hinge.