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Turtles are reptiles of the order Chelonii or Testudines characterised by a special bony or cartilaginous shell developed from their ribs and acting as a shield. "Turtle" may either refer to the order as a whole, or to particular turtles which make up a form taxon that is not monophyletic. The order Chelonii or Testudines includes both extant (living) and extinct species. The earliest known turtles date from 220 million years ago, making turtles one of the oldest reptile groups and a more ancient group than lizards, snakes or crocodiles. Of the many species alive today, some are highly endangered.