Sacrum
Irregular bone; core (axial) skeleton
Also known as: Sacrum, Rump-bone, Holy Bone
Classification
Irregular bone; core (axial) skeleton
Dimensions
This bone is typically wedge-shaped or triangular, often being wider at the top and narrowing toward the bottom. Its proportions usually correlate with the size and weight-bearing needs of the animal's lower trunk.
Key Features
Look for a large, butterfly-like or shield-shaped bone with several pairs of holes (foramina) running down the sides. It generally has a flattened front surface and a more rugged back surface with a central ridge. The top part often has broad, wing-like flares that would connect to the hip bones.
Similar Bones
It can be confused with the individual lower back bones (lumbar vertebrae) or the tailbone (coccyx). You can tell it apart by its fused nature; while vertebrae are single units, the sacrum looks like several vertebrae melted together into one solid piece. It is also much larger and flatter than a single tailbone segment.
Across Species
In birds, several bones fuse together to form a long, rigid 'synsacrum' which provides a stable platform for flight and landing. In whales, the sacrum is less distinctive because they lack hind limbs, while in heavy land mammals like elephants or horses, the sacrum is massive and fused to support the immense weight of the body over the legs.
Evolutionary History
The sacrum evolved as the bridge between the spine and the hind limbs. As early vertebrates moved onto land, this area strengthened and fused to better transmit the force of walking from the legs to the rest of the body, eventually forming the solid, multi-boned structure seen in modern mammals.
Photography Tips
To help with identification, place the bone on a plain, neutral background and include a ruler or common object like a coin for scale. Take one photo looking straight down at the flat front, one of the rugged back, and one from the top looking down into the spinal channel.