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).[4]

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 many species alive today, some are highly endangered.[6]

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.

Turtles are classified as amniotes, along with other reptiles (including birds) and mammals. Like other amniotes, turtles breathe air and do not lay eggs underwater, although many species live in or around water.

Turtle, tortoise, or terrapin

The word chelonian is popular among veterinarians, scientists, and conservationists working with these animals as a catch-all name for any member of the superorder Chelonia, which includes all turtles living and extinct, as well as their immediate ancestors.[citation needed] Chelonia is based on the Greek word kelone,[7] for armour or interlocking shields.[8] "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 meaning of the word turtle differs from region to region. In North America, all chelonians are commonly called turtles, including terrapins and tortoises.[9][10] In Great Britain, the word turtle is used for sea-dwelling species, but not for tortoises.

The term tortoise usually refers to any land-dwelling, non-swimming chelonian.[10] Most land-dwelling chelonians are in the Testudinidae family, only one of the 14 extant turtle families.[11]

Terrapin is used to describe several species of small, edible, hard-shell turtles, typically those found in brackish waters, and is an Algonquian word for turtle.[9]

Some languages do not have this problem, as all of these are referred to by the same name. For example, in Spanish, the word tortuga is used for turtles, tortoises, and terrapins. A sea-dwelling turtle is tortuga marina, a freshwater species tortuga de rio, and a tortoise tortuga terrestre.[12]
Anatomy and morphology
Chelonia mydas in Kona, Hawaii

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). 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 (2.6 ft) and weighs as much as 113.4 kg (250 lb).[13] 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 humans hunted them for food. The only surviving giant tortoises are on the Seychelles and Galápagos Islands, and can grow to over 130 cm (51 in) in length, and weigh about 300 kg (660 lb).[14]

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

The smallest turtle is the speckled padloper tortoise of South Africa. It measures no more than 8 cm (3.1 in) in length and weighs about 140 g (4.9 oz). Two other species