Calcaneus
Short Bone/Irregular Bone, part of the limb (appendicular) skeleton
Also known as: Heel Bone, hock (in four-legged animals)
Classification
Short Bone/Irregular Bone, part of the limb (appendicular) skeleton
Dimensions
This bone is typically the largest and longest of the ankle bones. In humans, it is more blocky, while in many larger mammals, it features a long, lever-like extension that gives the ankle its characteristic power.
Key Features
Look for a prominent, elongated 'neck' or handle-like section that leads to a complex, knobby joint surface. The joint end usually has smooth, bowl-like scoops where it fits against other ankle bones, while the long end provides a rugged surface where the strong tendons of the lower leg attach. It often resembles a small hammer or a chunky pistol grip.
Similar Bones
It is most often confused with the Astragalus (talus), which is the other large ankle bone. However, the calcaneus is usually longer and has a distinct 'tail' or lever arm, whereas the astragalus is rounded and resembles a spool or a pulley for hinge-like movement.
Across Species
Across the animal kingdom, the shape of the heel bone tells a story of movement. In humans and bears, which walk with their heels touching the ground (plantigrade), the bone is robust for weight-bearing. In horses and deer, this bone is elongated upward to form the 'hock' high on the leg, acting as a powerful lever for sprinting. In leaping animals like kangaroos, it is exceptionally long to maximize the spring-like snap of their jump.
Evolutionary History
The heel bone emerged as land-dwelling vertebrates needed a way to push off the ground. Over millions of years, as mammals specialized for speed, the bone moved upward away from the ground, evolving from a simple weight-bearing block into a dynamic mechanical lever that allows for the rapid, efficient striding seen in modern hoofed animals.
Photography Tips
To help with identification, place a common object like a coin or ruler next to the bone for scale. Take one photo from the side to show the full length of the lever arm, and another looking directly at the complex joint surfaces on the top end. Soft, natural light helps reveal the subtle textures on the bone's surface.