Metatarsal (specifically a Cannon Bone)
Long bone; part of the appendicular skeleton (limbs).
Also known as: Shank bone, lower leg bone, or cannon bone in hooved animals.
Classification
Long bone; part of the appendicular skeleton (limbs).
Dimensions
In many medium-to-large mammals, this bone is significantly longer than it is wide, acting as a sturdy, vertical pillar. Its length varies greatly depending on the species' overall height, ranging from a few inches in small deer to over a foot in horses.
Key Features
Look for a long, straight shaft with a very distinct deep vertical channel or 'vascular groove' running down the front. The top end is usually flat and blocky for meeting the ankle, while the bottom end often features two distinct rounded pulleys or 'knuckles' where the toes would attach.
Similar Bones
It can be confused with the Metacarpal (from the front leg). Metacarpals are usually shorter and have a more rounded or 'D-shaped' cross-section, whereas the Metatarsal from the back leg is typically longer and more square or angular in cross-section.
Across Species
This bone showcases amazing diversity in nature. In humans, we have five separate metatarsals forming the mid-foot. However, in 'cloven-hoofed' animals like deer, sheep, and cattle, two of these bones have fused together into a single, powerful 'cannon bone' to provide strength for running. You can often see a deep groove running down the center where the two bones joined. In horses, this fusion is even more extreme, resulting in one primary weight-bearing bone.
Evolutionary History
The history of this bone tracks the transition from five-toed ancestors walking on flat feet to modern runners walking on their tiptoes. Over millions of years, as animals moved from forests to open grasslands, the middle foot bones elongated and fused together. This 'stilt-like' adaptation allowed for a longer stride and faster speeds to escape predators in open terrain.
Photography Tips
To get the best identification, place the bone on a plain, non-reflective surface. Take one photo looking straight down at the long groove, and take two additional photos showing the shapes of the top and bottom ends. Including a common object like a coin or a ruler helps show the size of the animal.