Metapodial
Long Bone (Appendicular skeleton)
Also known as: Shank bone, Cannon bone, or Trotter bone
Classification
Long Bone (Appendicular skeleton)
Dimensions
These bones are generally much longer than they are wide. In many hoofed animals, they can range from 4 to 10 inches in length, featuring a straight, sturdy shaft and flared ends where they connect to the toes.
Key Features
The most telling feature is the bottom end, which looks like a double-knuckled pulley. This 'pulley-like' surface allows the toes to hinge smoothly. The shaft is often flat on one side and rounded on the other, frequently featuring a deep vertical channel or groove running down the center.
Similar Bones
At a glance, it might be confused with a humerus (upper arm bone) or femur (thigh bone). To tell them apart, look at the bottom end: the metapodial will have a very symmetrical, twin-bobbin shape, whereas the larger limb bones have much more complex, lopsided knobs and deep pits for muscle attachments.
Across Species
In humans, these are the bones in the palm of the hand or arch of the foot. However, in many grazing animals like deer, cows, and sheep, the multiple bones of the palm have fused into a single, extremely strong pillar to support high-speed running. In horses, this fusion is so complete it forms a single 'cannon bone,' while in clove-hoofed animals like pigs or cattle, you can often see a vertical groove showing where the bones merged over time.
Evolutionary History
The history of the metapodial is a story of simplification for speed. Early land-dwelling vertebrates had many small bones in their paws to allow for flexibility and grasping. As certain lineages moved into open grasslands, their descendants evolved fewer, longer, and sturdier bones to elevate the body and provide leverage for a powerful stride, essentially allowing them to walk on their tiptoes.
Photography Tips
To get a clear identification, photograph the bone from the side to show the length and from the very bottom to show the shape of the knuckle joints. Placing a common object like a coin or a ruler next to the bone helps viewers understand the animal's size.