A turtle is any reptile of the order Testudines characteristically having a shell composed of upper and lower parts, each with its own set of hinges. Turtles are found throughout the world and typically live long lives; some species are said to have lifespans in excess of 150 years. The largest turtles are sea turtles; the largest of these, the leatherback sea turtle, can reach lengths of nearly 7 feet (2 meters) and weigh as much as 1,500 pounds (680 kilograms).
There are two main groups of turtles: Cryptodira and Pleurodira. Cryptodira includes all living turtles except for those in the family Trionychidae (soft-shelled turtles), whereas Pleurodira excludes this family. Both groups share a common ancestor that lived during the Late Jurassic period (roughly 150 million years ago).
Turtles are ectothermic animals, meaning that they rely on external sources of heat to regulate their body temperature. This makes them particularly vulnerable to changes in temperature and climate. As a result, many species of turtles are threatened or endangered due to habitat loss and other environmental factors.