Why doesn't the tail bone decay after we die?

The human tail bone, or coccyx, is actually composed of 4 or 5 small vertebrae. These vertebrae are made of the same material as the other bones of the skeleton, and as far as I know they decay at the same rate (which will depend on factors like temperature, moisture, and whether the body is buried or exposed to air or water). Under the right conditions bone can remain intact for centuries, or even fossilise.

Apparently the individual vertebrae of the coccyx are fused together in some people but not in others, so in a dead body in which they were not fused they might eventually separate from each other as the soft tissues holding them together disintegrated.