Metapodial (Metacarpal or Metatarsal)
Long bone; part of the limb (appendicular) skeleton.
Also known as: Cannon bone, Shin bone, Lower leg bone
Classification
Long bone; part of the limb (appendicular) skeleton.
Dimensions
Likened to a sturdy marble column, these bones are typically longer than they are wide. In larger mammals like deer or cattle, they range from six to twelve inches in length, featuring a thick, dense shaft and flared ends for attachment and movement.
Key Features
Look for a long, straight shaft with a very distinctive 'pulley-like' lower end. This lower end looks like two or more rounded rollers separated by a deep groove, which allows the toes to hinge smoothly. The top surface is usually flat and blocky where it meets the ankle or wrist bones. Small circular holes, which allow for blood flow in life, are often visible near the ends of the shaft.
Similar Bones
At a glance, it might be confused with a radius or humerus (upper leg bones). However, the metapodial is much more symmetrical and usually has a straighter shaft. The dead giveaway is the bottom end—if it looks like a double pulley for attaching toes, it is almost certainly a metapodial from a hooved animal.
Across Species
In the world of evolution, this bone tells a story of adaptation for speed. While humans have five separate bones in the palm of their hand or the arch of their foot, grazing animals like deer, sheep, and cattle have fused these into a single, powerful 'cannon bone.' In a horse, it is one massive central bone, while in a dog, several smaller versions sit side-by-side. This fusion provides bird-like lightness and incredible strength for sprinting across open plains.
Evolutionary History
Tracing back millions of years, the ancestors of modern hooved mammals had multi-toed feet suitable for soft forest floors. As the world’s climates changed and grasslands expanded, these ancestors transitioned to running on their tiptoes. Over deep time, the outer toes shrank away, and the middle bones grew longer and fused together, creating the specialized, high-leverage limb structures we see in modern ungulates today.
Photography Tips
To get a clear identification, lay the bone on a plain, neutral background and place a ruler or a common object like a coin next to it for size reference. Take one photo of the full length, and then take close-up 'portrait' shots of both the top and bottom ends, as the shapes of these joint surfaces are the best clues for telling species apart.