Sacrum
Irregular Bone; part of the axial (core) skeleton
Also known as: Sacred Bone, Tailpiece, Rump Bone
Classification
Irregular Bone; part of the axial (core) skeleton
Dimensions
Generally shield-shaped or triangular, this bone is typically wider at the top and tapers toward the bottom. In many medium-to-large mammals, it is roughly as wide as the palm of a hand.
Key Features
The most striking feature is the presence of several pairs of symmetrical holes, called foramina, which look like small windows arranged in two vertical rows. The bone has a distinct wedge or butterfly-like shape, with a relatively flat or slightly curved front surface and a bumpy, ridged back surface.
Similar Bones
It is most often confused with the pelvis or individual lumbar (lower back) vertebrae. You can tell it apart by the 'fused' appearance; unlike a single vertebra which has one main central hole, the sacrum has multiple small holes and lacks the long, wing-like side spokes of a typical lower back bone.
Across Species
In mammals, the sacrum is formed by several vertebrae joined together to provide a sturdy anchor for the hips. In birds, this structure is even more fused into a long, rigid piece called a synsacrum, which helps them withstand the forces of landing. In contrast, many fish and ancient amphibians lacked a fused sacrum because they did not need to support their weight on land.
Evolutionary History
The sacrum represents a major milestone in evolutionary history: the stabilization of the spine for land-based movement. As our ancestors moved from water to land, the weight-bearing demands on the back increased. Over millions of years, individual vertebrae at the base of the spine fused into this single, solid unit to efficiently transfer power from the hind legs to the rest of the body.
Photography Tips
Lay the bone on a flat, neutral surface with a ruler next to it for scale. Take one photo looking straight down at the smooth front side, and another showing the bumpy back side. Side-view photos are also helpful to show the curve of the bone.