Sacrum
Irregular bone, part of the axial skeleton
Also known as: Tailbone foundation, lower back wedge, keystone of the pelvis
Classification
Irregular bone, part of the axial skeleton
Dimensions
A sturdy, wedge-shaped bone that typically measures around four to six inches in height and width, tapering smoothly from a broad top down to a pointed lower tip.
Key Features
Look for a triangular or keystone shape featuring rows of prominent holes on both the front and back faces, which once allowed nerve bundles to pass through. It has a broad top rim, rough wing-like surfaces on the outer sides for joining the hip bones, and a bumpy central ridge running down the middle.
Similar Bones
Beginners might occasionally confuse a sacrum with a hip bone or large segments of the lower spine. However, unlike the hollow ring of the hip bone or the singular spool shape of a standard backbone segment, the sacrum is immediately recognizable by its solid triangular wedge shape, bilateral symmetry, and neat rows of prominent drainage-like holes.
Across Species
In four-legged running mammals, the sacrum is relatively narrow and tightly locked to the backbone to transmit powerful hind-leg thrusts. In humans and other bipedal walkers, it is broader and curved like a shallow bowl to support the weight of the upper body. Birds have a highly fused, elongated structure that blends many backbone segments together to absorb the heavy impacts of landing and taking off.
Evolutionary History
The sacrum evolved as separate, individual vertebrae fused together into a single, robust anchor. This evolutionary union was crucial for weight-bearing land animals, transitioning from flexible swimming spines of ancient fish ancestors into a rigid brace that could connect the main spinal column securely to the hip girdles.
Photography Tips
Place the bone on a solid, neutral-colored background with soft, overhead lighting to highlight the deep shadows of the surface holes and bumpy central ridges. Take one straight-on shot of the front face and another angled view from the side to capture the gentle, sweeping curve of the bone.