Turtle

Turtles are the reptile order Testudines. They have a special bony or cartilaginous shell developed from their ribs that acts as a shield.The order Testudines includes both living and extinct species. The earliest fossil turtle comes from the early Upper Triassic of China, about 220 million years ago. So turtles are one of the oldest surviving reptile groups and a more ancient group than lizards, snakes and crocodiles.

Molecular evidence shows they are definitely archosauromorphs, the group of diapsid reptiles which also includes the dinosaurs. Turtles have been very successful, and have almost world-wide distribution. But, of the many species alive today, some are highly endangered.

Turtle

Although the word turtle is widely used to describe all members of the order Testudines, it is also common to see certain members described as terrapins, tortoises or sea turtles, as well. How these names are used, if at all, depends on the type of English.

British English describes these reptiles as turtles if they live in the sea; terrapins if they live in fresh or brackish water; or tortoises if they live on land.

American English tends to use the word turtle as a general term for all species. "Tortoise" is used for most land-dwelling species, and oceanic species are usually referred to as sea turtles. The name "terrapin" is typically reserved only for the brackish water diamondback terrapin, Malaclemys terrapin.

Australian English uses turtle for both the marine and freshwater species, but tortoise for the terrestrial species.

To avoid confusion, the word "chelonian" is popular among some who work with these animals, as a catch-all name. Unfortunately, Chelonia is also the name of a particular genus of turtles, so this conflicts with its use for the entire order Testudines.