Turtles are reptiles of the order Chelonii[2] or Testudines characterised by a special bony or cartilaginous shell developed from their ribs and acting as a shield.[3] Turtle may refer to the chelonian order as a whole (American English) or to fresh-water and sea-dwelling chelonians (British English).

The order Chelonii or Testudines includes both extant (living) and extinct species. The earliest known turtles date from 220 million years ago,[5] making turtles one of the oldest reptile groups and a more ancient group than lizards, snakes or crocodiles. Of the 327 known species alive today, some are highly endangered.