The vertebrae the start to fuse at puberty and completed at early adulthood is the sacral vertebrae or the sacrum. The sacral vertebrae is composed of 5 vertebral bodies which then fuse to form the sacrum. The sacrum then serves as the posterior wall of the pelvis (along with the ilium, ischium, and pubis).
The vertebrae is composed of (superior to inferior) the 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 sacral vertebrae (fused into sacrum), and 4 coccygeal vertebrae (fused into coccyx; a vestigeal organ).
