Turtles are reptiles of the order Chelonii or Testudines qualified by an unique bony or cartilaginous shell created from their ribs and working as a shield. Turtle might refer to the chelonian order all at once (American English) or to fresh-water and sea-dwelling chelonians (British English).
The order Chelonii or Testudines includes both extant (living) and extinct species. The earliest known turtles day from 220 million years ago, making turtles one of the oldest reptile groups and a much more old group than reptiles, crocodiles or snakes. Of the 327 known species alive today, some are very jeopardized.
Turtles are ectotherms-- their internal temperature level varies according to the ambient setting, commonly called cold-blooded. However, due to their high metabolic rate, leatherback sea turtles have a body temperature that is noticeably higher than that of the concerning water.
Turtles are classified as amniotes, in addition to various other reptiles, birds, and mammals. Like other amniotes, turtles breathe air and do not lay eggs underwater, although lots of species live in or around water.
The biggest living chelonian is the leatherback sea turtle (Dermochelys coriacea), which gets to a shell length of 200 cm (6.6 ft) and could reach a weight of over 900 kg (2,000 pound). 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 centimeters (6.6 feet). This overshadows even the better-known alligator snapping turtle, the biggest chelonian in North America, which acquires a shell length of up to 80 cm (2.6 ft) and weighs as much as 113.4 kg (250 lb).
The biggest ever chelonian was Archelon ischyros, a Late Cretaceous sea turtle known to have been up to 4.6 m (15 feet) long.
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