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Any of various reptiles of the order Chelonia (or Testudines), which includes the turtles and tortoises. Chelonians lack teeth and usually have a hard shell that protects the body and consists of bony plates fused to the vertebrae and ribs. Unlike all other living reptiles, the skulls of chelonians lack temporal openings, which is characteristic of the earliest known reptiles (called anapsids). Chelonians evolved during the late Permian or Triassic Period and have changed little since.