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 huge 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 much older group than lizards, snakes or crocodiles. There are many species of turtles that are alive today, some are highly endangered.
Like all other extant reptiles, turtles are ectotherms—their internal temperature varies according to the ambient environment, commonly called cold-blooded. However, because of their high metabolic rate, leatherback sea turtles have a body temperature that is noticeably higher than that of the surrounding water.
Like other amniotes (reptiles, dinosaurs, birds, and mammals), they breathe air and do not lay eggs underwater, although many species live in or around water. The largest turtles are aquatic.
The largest living chelonian is the leatherback sea turtle (Dermochelys coriacea), which reaches a shell length of 200 cm (6.6 ft) and can reach a weight of over 900 kg (2,000 lb). Freshwater turtles are generally smaller, but with the largest species, the Asian softshell turtle Pelochelys cantorii, a few individuals have been reported up to 200 cm (6.6 ft).