Turtle skeletal system — shell, girdles, and neck retraction
Overview of the turtle skeleton — a low, often lightly ossified shell of fused ribs and vertebrae, girdles enclosed inside the ribcage, and two distinct ways of withdrawing the neck.
General educational reference only — not clinical or veterinary advice. Anatomy varies between individual animals; consult a licensed veterinarian for any health concern.
A low, streamlined shell
A turtle's shell, like a tortoise's, is skeletal: a carapace of neural plates fused to the vertebrae and costal plates formed from expanded ribs, joined by bridges to a ventral plastron. In aquatic species the carapace is typically low and streamlined rather than domed, and the bone is often lighter, with unossified gaps between plates. Softshell turtles go furthest, lacking peripheral bones and horny scutes and covering the shell with leathery skin.
Girdles and limbs
As in all chelonians, the pectoral and pelvic girdles sit inside the ribcage. Freshwater turtles have webbed, clawed feet, while marine species carry forelimbs flattened into flippers. Some turtles, such as box turtles, have a hinged plastron with movable lobes that can be drawn up to close the shell.
Two ways to withdraw the neck
Turtles fall into two groups by how they retract the neck. Hidden-necked turtles (Cryptodira), which include most familiar freshwater species, fold the cervical vertebrae into a vertical S and pull the head straight back. Side-necked turtles (Pleurodira) cannot do this and instead lay the neck sideways under the front rim of the shell.
