Metatarsal / Metacarpal (Cannon Bone)
Long Bone, part of the limb (appendicular) skeleton
Also known as: Cannon Bone, Shin Bone, Foot Bone
Classification
Long Bone, part of the limb (appendicular) skeleton
Dimensions
In medium to large ungulates like deer or sheep, these bones typically range from 6 to 12 inches (15 to 30 cm) in length. They are characteristically long, straight, and slender, with a noticeable central groove running down the shaft and a flared lower end.
Key Features
Look for a long, straight shaft with a flattened cross-section and a prominent longitudinal groove or seam running down its length. The lower joint end flares outwards into two distinct pulley-like rounded surfaces (condyles) with a raised central ridge running through each pulley, which locks into the pastern joints of split-hoofed animals.
Similar Bones
It can be easily confused with the main upper leg bones, like the humerus or femur. However, the cannon bone lacks a large, rounded, ball-like joint at the top end. It is also much straighter and flatter than a radius or tibia, and that long, distinct central groove down the shaft is a dead giveaway unique to ungulate foot bones.
Across Species
The cannon bone is a masterclass in evolutionary efficiency seen in hooved mammals (ungulates). In creatures like deer, sheep, cattle, and horses, the individual toes of ancestral mammals fused together over time into a single, strong, lightweight pillar. In deer and cattle, you can still see the dual nature of this fusion—a distinct vertical ridge or groove running down the front or back marks where two separate foot bones merged into one. Horses take this a step further, having a single massive central cannon bone with two tiny remnant splint bones on either side.
Evolutionary History
Early land vertebrates possessed five separate foot bones (metatarsals/metacarpals), similar to a human hand or foot. As mammals adapted to open grasslands and running lifestyle, speed became a primary survival driver. Elongating the lower leg and reducing the weight at the end of the limb allowed animals to take longer, faster strides. Fusion of the remaining digits created the rigid, shock-absorbing structure known today as the cannon bone, perfectly engineered for high-speed running over hard terrain.
Photography Tips
To get the best identification photo, lay the bone flat next to a clear ruler or scale object like a coin. Take one photo straight-on showing the full shaft and the long central groove, another looking directly at the wide flared joint surface on the lower end, and a third from the side to show the profile thickness.