Calcaneus
Short bone, part of the appendicular skeleton (hind foot/lower limb)
Also known as: Heel bone
Classification
Short bone, part of the appendicular skeleton (hind foot/lower limb)
Dimensions
Varies widely by species, ranging from a few inches in medium-sized mammals to over a foot in large ungulates and marine mammals. It is generally blocky, elongated, and forms a distinct projecting lever at the back of the foot.
Key Features
Look for a sturdy, somewhat chunky block of bone featuring a prominent, rugged extension or hook at one end, which serves as a handle-like attachment point for tendons. The opposite end usually has smooth, saddle-shaped or faceted surfaces where it connects with neighboring ankle bones.
Similar Bones
Beginners might confuse the calcaneus with other large foot or wrist bones, such as the talus. However, the calcaneus is easily distinguished by its long, projecting heel extension, whereas the talus is more rounded and topped with a smooth dome-like pulley surface.
Across Species
In digitigrade runners like dogs and cats, the calcaneus is dramatically elongated to lift the heel high off the ground, maximizing running speed and stride length. In plantigrade animals like humans and bears, it sits flat on the ground to bear full body weight. In ungulates like deer and horses, it forms a prominent point of the hock joint, acting as a powerful anchor point for leaping and running.
Evolutionary History
The calcaneus evolved from early tetrapod ankle bones as vertebrates transitioned from sprawling postures to upright, weight-bearing limbs. Over millions of years, its shape shifted to meet the mechanical demands of specialized locomotion, whether that means acting as a shock absorber for walking or a high-powered lever for springing forward.
Photography Tips
Place the bone on a neutral, non-glare background and use side lighting to cast shadows across its bumpy surfaces and joint facets. Take one shot showing the full profile of the projecting heel lever, and a second shot focusing squarely on the smooth connection facets at the opposite end.