Turtles appeared on earth 245-208 million years ago during the Thriassic period. Sea turtle fossils appear, at the earliest, during the Late Jurassic period. However, ancestors of the modern form of sea turtle are believed to have been inhabitants of the marshes during the Late Triassic period. The largest sea turtle that ever lived is believed to have been the Archelon ischyros, reaching lengths of 9.8 to 13 feet. The largest currently living species is the leatherback turtle, growing to aproxiamately 6.5 feet.
Sea turtles are among the four surviving seawater-adapted reptiles.
The other reptiles that survive are the saltwater crocodile, marine snake, and marine iguana. Adults sea turtles are most often found in more shallow coastal waters, bays, lagoons, and estuaries, sometimes venturing into open sea. Migration habits vary between species and differing populations of the same species.