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 sea turtles. All of the seven species listed above, except for the flatback, are present in US waters, and are listed as endangered and/or threatened under the Endangered Species Act. The flatback itself exists 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.
For each of the seven types of sea turtles, females and males are the same size; there is no sexual dimorphism.
In general, sea turtles have a more fusiform body plan than their terrestrial or freshwater counterparts. This tapering at both ends reduces volume and means that sea turtles cannot retract their head and limbs into their shells for protection, unlike many other turtles and tortoises. However, the streamlined body plan reduces friction and drag in the water and allows sea turtles to swim more easily and swiftly.
The leatherback sea turtle is the largest sea turtle, measuring 2–3 m (6–9 ft) in length, 1–1.5 m (3–5 ft) in width, and weighing up to 700 kg (1500 lb). Other sea turtle species are smaller, being mostly 60–120 cm (2–4 ft) long and proportionally narrower.
The skulls of sea turtles have cheek regions that are enclosed in bone. Although this condition appears to resemble that found in the earliest known fossil reptiles (anapsids), it is possible it is a more recently evolved trait in sea turtles, placing them outside the anapsids.