Archelon ischyros, the largest species of turtle that ever existed.
Turtles are reptiles of the order Testudines /tɛˈstjuːdɪniːz/. They are characterized by a special bony or cartilaginous shell developed from their ribs that acts as a shield. Colloquially, the word "turtle" is generally restricted to fresh-water and sea-dwelling Testudines. Testudines includes both extant (living) and extinct species. Its earliest known members date from the Middle Jurassic. Turtles are one of the oldest reptile groups, more ancient than snakes or crocodilians.
Turtles are ectotherms—commonly called cold-blooded—meaning that their internal temperature varies according to the ambient environment. 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, birds, and mammals. Like other amniotes, turtles breathe air and do not lay eggs underwater, although many species live in or around water.
Note: Not all reptiles listed here are turtles, some are advanced placodonts, others are stem-turtles(which would be considered turtles, but are more ancient and stand outside of the order Testudines).
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