The turtle is one of the few animals that have both an internal and an external skeleton. The endoskeleton is comprised of bones that are similar to other mammals', while the exoskeleton is unique in the animal kingdom. One striking difference is that the shoulder girdles of turtles reside inside their rib cages.
The shells of most turtles are formed by the fusion of skeletal bones and dermal bones, which arise from within the skin. In most species, the spine fuses to the shell. One exception is the world’s largest species, the leatherback sea turtle. The leatherback's spine remains unattached because this turtle does not have a hard shell. Instead, its back has leathery skin that contains numerous tiny bones. This adaptation allows the leatherback sea turtle to safely dive to depths of 3,000 feet.
Scientists categorize the sections of the turtle's endoskeleton into an axial skeleton, which consists of the spine and skull, and an appendicular skeleton, which is comprised of the pelvic and pectoral girdles, as well as the humerus, radius, ulna, carpals, femur, metacarpals, tibia, tarsals, metatarsals, phalanges and fibula.
Unlike amphibians with skeletons containing large amounts of cartilage, turtle skeletons are mostly bone. Some turtle bones contain marrow that produces both red and white blood cells.