Metapodial (Metacarpal or Metatarsal)
Long bone; part of the limb (appendicular) skeleton.
Also known as: Cannon bone (in ungulates), shank bone, or foot bone.
Classification
Long bone; part of the limb (appendicular) skeleton.
Dimensions
In deer-sized animals, these are typically long, slender, and straight. The length is usually several times the width, following a column-like structure to support weight efficiently.
Key Features
Look for a long, straight shaft with a very distinct 'pulley' shape at the bottom end. In many hoofed mammals, this bottom joint is split into two identical rounded sections with a deep notch or groove in the middle, resembling a double-ended spool. The shaft often has a visible vertical groove running down its length, which marks where two separate bones fused together during the ancestor's evolution.
Similar Bones
This can be confused with a radius (forearm bone) or tibia (shin bone). However, the metapodial is usually more perfectly symmetrical and straight. The most telling difference is the 'double pulley' at the bottom; forearm and shin bones usually have more complex, asymmetrical joints that look like hooks or shallow cups rather than gears.
Across Species
The metapodial varies wildly across the animal kingdom. In humans, these are the five individual bones inside the palm or the arch of the foot. In 'cloven-hoofed' mammals like deer, sheep, and cattle, two of these bones have fused together over time into a single, sturdy shaft. This fusion ends at the bottom in two distinct 'knuckles' or pulleys, which allows the animal to walk on two toes. In horses, this fusion has gone even further, resulting in a single massive central bone to support high-speed running on one toe.
Evolutionary History
The history of the metapodial is a story of specialization for locomotion. Early land-dwelling vertebrates had multiple separate metapodials for flexible, sprawling movement. As lineages like artiodactyls (even-toed ungulates) adapted for life on open grasslands, their feet became more rigid and elongated. By fusing these bones together, they created a longer lever for the leg, allowing for faster speeds and greater energy efficiency when fleeing predators or migrating long distances.
Photography Tips
To help with identification, place a common object like a coin or a ruler next to the bone for scale. Take one photo looking therapy straight down at the shaft, and a second 'end-on' photo looking directly at the double-knuckle joint, as the shape of that joint is the best clue for identifying the species.