Turtles are reptiles of the order Testudines, also known as Chelonia. They are characterized by a bony or cartilaginous shell, developed from their ribs, that acts as a shield. Testudines include both extant (living) and extinct species. Its earliest known members date from the Middle Jurassic, around 170 million years ago. Among the turtles are included tortoises and terrapins.
Turtles are the only vertebrates with a complete shell. It is formed mainly of bone; the upper part is the domed carapace, while the underside is the flatter plastron or belly-plate. Its outer surface is covered in scales made of keratin, the material of hair and fingernails. The carapace bones develop from ribs which grow sideways and develop into broad flat plates that join up to cover the body. Many turtles migrate short distances seasonally; the sea turtles are the only reptiles that migrate long distances to lay their eggs on a favored beach, sometimes traveling thousands of kilometers to feed before returning to the beach where they were born. It is not known how they navigate, though they do have a magnetic sense. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden neck turtles) which differ in the way the head is retracted.
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 and adaptations to conserve heat, leatherback sea turtles have a body temperature 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.
Turtles have appeared in myths and folktales around the world. Some terrestrial and freshwater species are widely kept as pets. Turtles have been hunted for their meat, for use in traditional medicine, and for their carapaces. Marine turtles are often killed accidentally as bycatch in fishing nets. Turtle habitats around the world are being destroyed. As a result of these pressures, many species are threatened with extinction.