Here's why:
* Irregular bones have complex shapes that don't fit into the categories of long, short, flat, or sesamoid bones.
* Vertebrae have a unique shape with a central body, a vertebral arch, and various processes (projections) that allow for articulation with other bones, muscle attachment, and the formation of the vertebral canal that houses the spinal cord.
Let me know if you have any other bone-related questions!