Sea turtles are modern marine reptiles.
Sea turtles, along with other turtles and tortoises, are part of the order Testudines.
The seven living species of sea turtles are: flatback sea turtle, green sea turtle, hawksbill sea turtle, Kemp's ridley sea turtle, leatherback sea turtle, loggerhead sea turtle and olive ridley sea turtle.
All species except the leatherback are in the family Cheloniidae. The leatherback belongs to the family Dermochelyidae and is its only member.
The species are primarily distinguished by their anatomy: for instance, the prefrontal scales on the head, the number of and shape of scutes on the carapace, and the type of inframarginal scutes on the plastron. The leatherback is the only sea turtle that does not have a hard shell; instead, it bears a mosaic of bony plates beneath its leathery skin.
Sea turtles constitute a single radiation that became distinct from all other turtles at least 110 MYA. They included the giant Archelon turtle, which died out 65 MYA alongside the dinosaurs, other giant marine reptiles, and the last of the pterosaurs. Other, smaller species of sea turtles, however, had survived the K/T extinction and they continue to exist in modern times.
Archelon, featured in the last episode of Sea Monsters, was a sea turtle.
In the second episode of Walking with Beasts, a sea turtle is seen on a beach, later killed by an Andrewsarchus.