Sacrum
Irregular bone, part of the core (axial) skeleton
Also known as: Sacral bone or tailbone base
Classification
Irregular bone, part of the core (axial) skeleton
Dimensions
Wedge-shaped structure, typically wider at the top and tapering to a blunt point at the bottom, featuring a curved profile from front to back.
Key Features
Characterized by a broad top base, a curved triangular shape, smooth wing-like side areas, and neat rows of prominent holes running down the front and back surfaces for nerve pathways.
Similar Bones
Can sometimes be confused with the shoulder blade due to the flat, wing-like side portions, but the sacrum is much thicker, curved, and features symmetrical rows of holes.
Across Species
In four-legged walking mammals, the sacrum anchors the lower limbs to the spine, transmitting the pushing force of the hind legs into the main body. In humans, who walk upright, it bears the weight of the entire upper torso and distributes it evenly into the hip girdles. Birds have a much more fused and extensive structure combining the lower backbone and hip bones into a rigid unit to handle the stress of flight and landing.
Evolutionary History
The sacrum evolved as vertebrate animals transitioned from swimming in water to walking on land. Early fishlike ancestors did not need a specialized hip attachment, but as limbs developed to support body weight against gravity, separate backbone segments fused together to form a sturdy platform anchoring the hind appendages.
Photography Tips
Photograph with strong side lighting to highlight the curved contours and the depth of the symmetrical holes. Place a ruler next to the bone for scale and shoot directly from the front and back views against a neutral background.