Turtles 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 cartilaginous shell developed from their ribs. The Order Testudines includes both extant (living) and extinct species. The earliest known turtles date from 215 million years ago,^[1] making turtles one of the oldest reptile groups and a more ancient group than lizards and snakes. About 300 species are alive today, and some are highly endangered.

Turtles cannot breathe in water, but they can hold their breath for various periods of time.

Like other reptiles, turtles are poikilothermic (or "of varying temperature"^[2]). 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.

he largest chelonian is the great leatherback sea turtle, which reaches a shell length of 200 cm (80 inches) and can reach a weight of over 900 kg (2,000 lb, or 1 short ton). 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 or 80 in (Das, 1991). This dwarfs even the better-known Alligator Snapping Turtle, the largest chelonian in North America, which attains a shell length of up to 80 cm (31½ in) and a weight of about 60 kg (170 lb).

Giant tortoises of the genera Geochelone, Meiolania, and others were relatively widely distributed around the world into prehistoric times, and are known to have existed in North and South America, Australia, and Africa. They became extinct at the same time as the appearance of man, and it is assumed that humans hunted them for food. The only surviving giant tortoises are on the Seychelles and Galápagos Islands and can grow to over 130 cm (50 in) in length, and weigh about 300 kg (670 lb).^[3]

The largest ever chelonian was Archelon ischyros, a Late Cretaceous sea turtle known to have been up to 4.6 m (15 ft) long.^[4]

The smallest turtle is the Speckled Padloper Tortoise of South Africa. It measures no more than 8 cm (3 in) in length and weighs about 140 g (5 oz). Two other species of small turtles are the American mud turtles and musk turtles that live in an area that ranges from Canada to South America. The shell length of many species in this group is less than 13 cm (5 in) in length.