Metapodial (Metacarpal or Metatarsal)
Long bone; part of the limb (appendicular) skeleton.
Also known as: Cannon bone, shank bone, or lower foot bone
Classification
Long bone; part of the limb (appendicular) skeleton.
Dimensions
These bones are generally notably longer than they are wide. In many hoofed mammals, they are among the sturdiest bones in the leg, featuring a straight, hollow shaft that widens at the top and bottom ends.
Key Features
Look for a long, straight shaft with a very distinctive bottom end that looks like a double-knob or a spool. This bottom surface often has a central ridge or groove that acts like a track, allowing the toes to hinge forward and backward smoothly.
Similar Bones
It can be confused with a humerus (upper arm bone) or femur (thigh bone). However, the metapodial is much straighter and lacks the ball-and-socket joint found at the top of a femur. The double-knob at the bottom end is a giveaway that it is a foot or hand bone.
Across Species
In nature, the metapodial shows incredible variety. In humans, these are the five bones deep inside the palm of the hand or arch of the foot. In deer and horses, several of these bones have fused together into a single, powerful "cannon bone" to provide height and speed. In birds, the metapodial is fused with ankle bones to form the long, scaly part of the leg we often see above the toes.
Evolutionary History
This bone represents a trek from the five-toed feet of ancient land-dwelling vertebrates to the specialized limbs of modern animals. Over millions of years, creatures that needed to run faster across open plains evolved longer metapodials, often losing or fusing side toes to create a single, efficient lever for movement.
Photography Tips
To help with identification, place a common object like a coin or a ruler next to the bone for scale. Take one photo looking straight down at the shaft, and another specifically showing the bottom end where the knobs and grooves are visible.