Turtles are [[reptiles of the order Testudines (the crown group of the superorder Chelonia), most of whose body is shielded by a special bony or cartilaginous shell developed from their ribs. "Turtle" may either refer to the Testudines as a whole, or to particular Testudines which make up a form taxon that is not monophyletic—see also sea turtle, terrapin, tortoise, and the discussion below.
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Turtles are [[reptiles of the order Testudines (the crown group of the superorder Chelonia), most of whose body is shielded by a special bony or cartilaginous shell developed from their ribs. "Turtle" may either refer to the Testudines as a whole, or to particular Testudines which make up a form taxon that is not monophyletic—see also sea turtle, terrapin, tortoise, and the discussion below.
The order Testudines includes both extant (living) and extinct species. The earliest known turtles date from 215 million years ago, 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.
Like other reptiles, turtles are ectotherms—varying their internal temperature according to the ambient environment, commonly called cold-blooded. 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.
Anatomy and morphology
The largest chelonian is the great leatherback sea turtle, which reaches a shell length of and can reach a weight of over . Freshwater turtles are generally smaller, but with the largest species, the Asian softshell turtle Pelochelys cantorii, a few individuals have been reported up to . This dwarfs even the better-known Alligator Snapping Turtle, the largest chelonian in North America, which attains a shell length of up to and a weight of about .
The largest ever chelonian was Archelon ischyros, a Late Cretaceous sea turtle known to have been up to long.
The smallest turtle is the Speckled Padloper Tortoise of South Africa. It measures no more than in length and weighs about . 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 in length.
Neck folding
Turtles are broken down into two groups, according to how they evolved a solution to the problem of withdrawing their neck into their shell (something the ancestral Proganochelys could not do): the Cryptodira, which can draw their neck in while contracting it under their spine; and the Pleurodira, which contract their neck to the side.
Head
