Turtles, Tortoises, and Terrapins are reptiles of the Order Testudines (all living turtles belong to the crown group Chelonia), most of whose body is shielded by a special bony or cartilagenous shell developed from their ribs. The Order Testudines includes both extant (living) and extinct species, the earliest known turtles being from around 215 million years ago, making turtles one of the oldest reptile groups, and a much more ancient group than lizards and snakes. About 300 species are alive today; some are highly endangered. Like other reptiles, turtles are ectothermic (or cold-blooded ). Like other amniotes (reptiles, dinosaurs, birds, and mammals), they breathe air and don t lay eggs underwater, though many species live in or around water. The largest turtles are aquatic. Turtles vary widely in size, although marine turtles tend to be relatively larger animals than their land and freshwater relatives.