Metapodial (Metacarpal or Metatarsal)
Long bone of the appendicular skeleton
Also known as: Cannon bone, Shank bone, Lower leg bone
Classification
Long bone of the appendicular skeleton
Dimensions
These are typically elongated, sturdy structures with a straight shaft and flared ends. In medium-to-large hoofed mammals, they often reach lengths of 15 to 30 centimeters and are significantly taller than they are wide.
Key Features
Look for a long, straight shaft with a distinctive groove running down the front or back, which marks where separate bones fused together over evolutionary time. The top end is usually flat to meet the ankle or wrist, while the bottom end features two rounded rollers that connect to the digits.
Similar Bones
Can be confused with the radius or tibia. You can tell the metapodial apart by its symmetrical, blocky ends and the deep longitudinal groove along the shaft, which other leg bones typically lack.
Across Species
The metapodial is a masterpiece of specialized engineering for running. In deer, sheep, and cattle, the third and fourth toe bones have fused into a single massive 'cannon bone' to provide strength and leverage. In contrast, primates like humans retain several separate, smaller bones in the palm or arch of the foot, allowing for flexibility rather than pure speed.
Evolutionary History
The history of the metapodial tracks the shift from five-toed forest dwellers to single or double-toed grassland specialists. As ancestors of modern ungulates moved into open plains, their outer toes shrank and the central bones lengthened and fused, creating a shock-absorbing pillar that allows for high-speed travel over long distances.
Photography Tips
Lay the bone flat and take one photo of the full length of the shaft to show the fusion groove. Include a second close-up shot of the joint surfaces at both the top and bottom ends, and always place a coin or ruler nearby for scale.