Metapodials (Metacarpal or Metatarsal)
Long bone, part of the limb (appendicular) skeleton
Also known as: Cannon bone, shank bone, or lower leg bone
Classification
Long bone, part of the limb (appendicular) skeleton
Dimensions
These are typically elongated, sturdy cylinders that are much longer than they are wide. In hoofed animals, they usually represent the longest segment of the lower leg, characterized by a straight shaft with flared ends.
Key Features
Look for a long, straight shaft with a very distinct vertical groove running down the front or back. One end usually features a hinge-like joint with two rounded 'pulleys' separated by a deep notch, which looks somewhat like a double knuckle. The other end is flatter with a more squared-off or slightly rounded surface for meeting the upper leg joints.
Similar Bones
The radius or tibia (upper leg bones) can look similar but are generally thicker and lack the deep, full-length vertical groove found in a cannon bone. You can tell a cannon bone apart because its cross-section is often more D-shaped or squared compared to the more triangular or circular shape of the upper limb bones.
Across Species
In the natural world, these bones showcase incredible adaptations for speed. In deer, elk, and cattle, two bones that are separate in many other mammals have fused together into a single, powerful beam. While humans have five separate bones in the palm of the hand or sole of the foot, many grazing animals have reduced these to a single 'cannon bone' to provide a rigid, spring-like lever for running across open plains. The deep groove running down the center is a classic hallmark of this fused structure in ruminants.
Evolutionary History
The history of the cannon bone is a story of simplification for efficiency. Early ancestors of modern hoofed mammals had multiple toes and several foot bones. As these lineages moved from soft forest floors to hard grasslands, their side toes shrank and the central foot bones lengthened and fused. This process created a lighter, stronger limb that allows modern ungulates to cover vast distances with minimal energy expenditure.
Photography Tips
To get the best identification, lay the bone flat and take a clear shot looking straight down at the vertical groove. Be sure to include a second photo of the 'pulley' end (the bottom joint) looking directly into the hinge, as this helps identify the specific animal group. Including a ruler or a common object like a coin helps viewers understand the scale of the animal.