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Turtles are reptiles of the order Testudines (the crown group of the superorder Chelonia), characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield. "Turtle" may either refer to the Testudines as a whole, or to particular Testudines which make up a form taxon that is not monophyletic, terrapin, tortoise.

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, snakes and crocodiles. Of the many species alive today, some are highly endangered.

Like other reptiles, turtles are ectotherms—their internal temperature varies according to the ambient environment, commonly called Poikilotherm or cold-blooded. However, Leatherback sea turtles (Dermochelys coriacea) have noticeably higher body temperature than surrounding water because of their high metabolic rate.

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.

Neck folding Edit

Turtles are divided into two groups, according to how they evolved a solution to the problem of withdrawing their necks into their shells (something the ancestral Proganochelys could not do): the Cryptodira, which can draw their necks in while contracting it under their spine; and the Pleurodira, which contract their necks to the side.

Systematics Edit

Living taxa only.

Order Testudines

Suborder Cryptodira

  • Family Chelydridae
  • Superfamily Chelonioidea
    • Family Cheloniidae
    • Family Dermochelyidae
  • Superfamily Testudinoidea
    • Family Platysternidae
    • Family Emydidae
    • Family Geoemydidae
    • Family Testudinidae
  • Superfamily Trionychoidea
    • Family Carettochelyidae
    • Family Dermatemydidae
    • Family Kinosternidae
    • Family Trionychidae

Suborder Pleurodira

  • Family Chelidae
  • Family Pelomedusidae
  • Family Podocnemididae

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