Order (Animalia): Testudines

Turtles are reptiles of the order Chelonia /kɪˈloʊniə/ or 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. Of the 360 known extant species, some are highly endangered.

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.

Naming and etymology

The common terms "turtle", "tortoise" and "terrapin", depending on the English dialect used, are common names and do not reflect precise biological or taxonomic distinctions. "Turtle" may name either the order as a whole, or to particular turtles that make up a form taxon that is not monophyletic, or may apply only to aquatic species. "Tortoise" usually means any land-dwelling, non-swimming chelonian. "Terrapin" is used for several species of small, edible, hard-shell turtles, typically those found in brackish waters.

In North America, all chelonians are commonly called "turtles", just as in Spanish, they are all called tortuga. "Tortoise" is used only in reference to fully terrestrial turtles or, more narrowly, only those members of Testudinidae, the family of modern land tortoises. Terrapin may refer to small semi-aquatic turtles that live in fresh and brackish water, in particular the diamondback terrapin (Malaclemys terrapin). Although the members of the genus Terrapene dwell mostly on land, they are referred to as box turtles rather than tortoises. The American Society of Ichthyologists and Herpetologists uses "turtle" to describe all species of the order Testudines, regardless of whether they are land-dwelling or sea-dwelling, and uses "tortoise" as a more specific term for slow-moving terrestrial species.

In the United Kingdom, the word turtle is used for water-dwelling species, including ones known in the US as terrapins, but not for terrestrial species, which are known only as tortoises.

The word chelonian is popular among veterinarians, scientists, and conservationists working with these animals as a catch-all name for any member of the superorder Chelonia, which includes all turtles living and extinct, as well as their immediate ancestors. Chelonia is based on the Greek word for turtles, χελώνη chelone; Greek χέλυς chelys "tortoise" is also used in the formation of scientific names of chelonians. Testudines, on the other hand, is based on the Latin word for tortoise, testudo. Terrapin comes from an Algonquian word for turtle.

This article uses "turtle" for the entire order, which is a single clade.

Reference: Wikipedia