Final answer:
The coccyx, or tailbone, is formed by the fusion of the four most distal spiral vertebrae. The sacrum is instead formed by the fusion of five sacral vertebrae. Both contribute to the stability and support of the vertebral column.
Step-by-step explanation:
The most distal four spiral vertebrae, which are fused together, form the coccyx. This structure is commonly referred to as the tailbone and consists of four small coccygeal vertebrae that fuse to form the terminal portion of the vertebral column. In contrast, the sacrum is formed from the fusion of five sacral vertebrae. The sacrum and coccyx are part of the posterior section of the pelvic girdle, providing support and stability as part of the vertebral column. Intervertebral discs and ligaments contribute to the overall support of the vertebral column, whereas the thoracic vertebrae have attachment sites for ribs, and the cervical vertebrae form the neck region.