Testudines in the context of "Terrapin"

Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and amniotic development. Living traditional reptiles comprise four orders: Testudines, Crocodilia, Squamata, and Rhynchocephalia. About 12,000 living species of reptiles are listed in the Reptile Database. The study of the traditional reptile orders, customarily in combination with the study of modern amphibians, is called herpetology.

Reptiles have been subject to several conflicting taxonomic definitions. In evolutionary taxonomy, reptiles are gathered together under the class Reptilia (/rɛpˈtɪliə/ rep-TIL-ee-ə), which corresponds to common usage. Modern cladistic taxonomy regards that group as paraphyletic, since genetic and paleontological evidence has determined that crocodilians are more closely related to birds (class Aves), members of Dinosauria, than to other living reptiles, and thus birds are nested among reptiles from a phylogenetic perspective. Many cladistic systems therefore redefine Reptilia as a clade (monophyletic group) including birds, though the precise definition of this clade varies between authors. A similar concept is clade Sauropsida, which refers to all amniotes more closely related to modern reptiles than to mammals.

Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhead, Kemp's ridley, and olive ridley. 5 of the seven species are listed as threatened with extinction globally on the IUCN Red List of Threatened Species, the remaining two are not considered to be threatened with extinction. One of which, the flatback turtle, is found only in the waters of Australia, Papua New Guinea, and Indonesia.

Sea turtles can be categorized as hard-shelled (cheloniid) or leathery-shelled (dermochelyid). The only dermochelyid species of sea turtle is the leatherback.

The Pleurodira are one of the two living suborders of turtles, the other being the Cryptodira. The division between these two suborders represents a very deep evolutionary divide between two very different types of turtles. The physical differences between them, although anatomical and largely internal, are nonetheless significant, and the zoogeographic implications of them are substantial. The Pleurodira are known more commonly as the side-necked turtles and the name Pleurodira quite literally translates to side neck, whereas the Cryptodira are known as hidden-necked turtles. The Pleurodira turtles are currently restricted to freshwater habitats in the Southern Hemisphere, largely to Australia, South America, and Africa. Within the Pleurodira, three living families are represented: Chelidae, also known as the Austro-South American side-necked turtles, the Pelomedusidae, also known as the African mud terrapins, and the Podocnemididae, also known as the American side-neck river turtles. However, they were a cosmopolitan clade during the Cretaceous and most of the Cenozoic, and even occurred in marine environments around the world.

The Cryptodira (Greek: hidden neck) are a suborder of Testudines that includes most living tortoises and turtles. Cryptodira is commonly called the "Hidden-Neck Turtles" or the "Inside-Neck Turtles". Cryptodira differ from Pleurodira (side-necked turtles) in that they lower their necks and pull the heads straight back into the shells, instead of folding their necks sideways along the body under the shells' marginals. They include among their species freshwater turtles, snapping turtles, tortoises, softshell turtles, and sea turtles.

Tortoises (/ˈtɔːrtəs.ɪz/ TOR-təs-iz) are reptiles of the family Testudinidae of the order Testudines (Latin for "tortoise"). Like other testudines, tortoises have a shell to protect from predation and other threats. The shell in tortoises is generally hard, and like other members of the suborder Cryptodira, they retract their necks and heads directly backward into the shell to protect them.

Tortoises can vary in size with some species, such as the Galápagos giant tortoise, growing to more than 1.2 metres (3.9 ft) in length, whereas others like the speckled Cape tortoise have shells that measure only 6.8 centimetres (2.7 in) long. Several lineages of tortoises have independently evolved very large body sizes in excess of 100 kilograms (220 lb), including the Galapagos giant tortoise and the Aldabra giant tortoise. They are usually diurnal animals with tendencies to be crepuscular depending on the ambient temperatures. They are generally reclusive animals. Tortoises are the longest-living land animals in the world, although the longest-living species of tortoise is a matter of debate. Galápagos tortoises are noted to live over 150 years, but an Aldabra giant tortoise named Adwaita may have lived an estimated 255 years. In general, most tortoise species can live 80–150 years. Tortoises are typically slow-moving.