The pelvis was connected to a sacrum formed by seven sacral vertebrae. The sacrum consisted of three to ten sacral vertebrae. The dural sac ends at the vertebral level of the second sacral vertebra. Major plexuses include the cervical, brachial, lumbar, and sacral plexuses. The straight sacrum was long and adult individuals show a fusion of all sacral vertebrae. It is interconnected with the lower part of Sacral plexus. The underside of the second sacral vertebra has an angular longitudinal keel. The number of sacral vertebrae varies slightly. The primary curves (thoracic and sacral curvatures) form during fetal development. Less commonly ectasia of the sacral nerve root sheaths may occur. Several other ligaments complete the foramen of the last sacral nerve. However, the pelvis was still connected to one of the sacral vertebrae. Another congenital disorder is that of caudal regression syndrome also known as "sacral agenesis". Only four out of the five sacral vertebrae are preserved. Those of the five sacral vertebrae were the longest.