Metapodial (Metacarpal or Metatarsal)
Long bone, part of the limb (appendicular) skeleton
Also known as: Shank bone, Cannon bone, Lower leg bone
Classification
Long bone, part of the limb (appendicular) skeleton
Dimensions
In deer-sized animals, these are typically long and slender, frequently exceeding 20 centimeters in length. They are characterized by a straight, semi-cylindrical shaft that is much longer than it is wide.
Key Features
Look for a long, straight shaft with a distinctive groove running down the front. The most identifying feature is the lower end, which terminates in two symmetrical, pulley-like 'knuckles' shaped like beads. These rounded surfaces allow the toes to hinge smoothly.
Similar Bones
It is most often confused with the radius or tibia (forearm and shin bones). You can tell a metapodial apart by its symmetry; the 'pulley' shape at the bottom is very distinct, whereas the shin and forearm bones have more complex, uneven joints for connecting to the wrist or ankle.
Across Species
In hoofed mammals like deer, sheep, and cattle, the bones of the palm or foot have fused over millions of years into a single, sturdy column called a cannon bone. In contrast, humans have five separate bones in the hand (metacarpals), while dogs and cats have four or five. Whales have flattened, paddle-like versions of these bones shortened within their flippers.
Evolutionary History
This bone represents a masterclass in adaptation for speed. Evolutionary ancestors of hoofed mammals had multiple separate toes. To survive on open grasslands, lineages like deer and horses evolved to run on their tiptoes; as they did, the bones the outer 'fingers' or 'toes' shrank or disappeared, while the central bones fused together to create a lightweight, high-leverage lever for rapid locomotion.
Photography Tips
Lay the bone flat on a plain background. Capture one photo from the front to show the long vertical groove, and a close-up of the bottom end (the 'knuckles') looking straight at the joint surface. Including a ruler or a common object like a coin helps determine the size of the animal.