The basic mammalian skeleton has undergone a number of specialized changes. The forelimbs have been modified into flippers, and the bones of the hind limbs have disappeared altogether. There is a pelvic girdle, which remains as a mere vestige buried in the ventral musculature. A large number of the dolphin's ribs are "floating", not connected to the sternum. The ribs that are attached are usually joined, enabling the rib-cage to collapse under the pressure of a deep dive without becoming damaged. The skull is tilted upwards and lines up with the spinal column, and the cervical (or neck) vertebrae are fused together in almost all species (one exception is the bottlenose dolphin, who can bend his neck).
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