Turtles are, evolutionarily and ecologically, a success story. They've survived two major extinction events (the Triassic-Jurassic extinction and the Cretaceous-Tertiary extinction) and many less catastrophic mass extinctions. They've diversified, from terrestrial ancestors, to take advantage of fresh-water and marine environments, and many species (mostly within the Superfamily Testudinoidea) have returned to dry land, and include the modern tortoises and box turtles. Over the course of their evolution, turtles have produced some giants. The largest-known turtle is Archelon ischyros, from the Upper Cretaceous of North America, which more than four meters (13.5 feet) long, and about 4.87 meters (16 feet) wide from flipper to flipper. The largest-known freshwater turtle, the living Asian softshell turtle (Pelochelys cantorii), is only about half that size, but still measures a very respectable six+ feet (about two meters) in length. The largest land species known is the bizarre horned Meiolania of Australia and New Caledonia, which reached lengths of eight and a half feet. And they are among the longest-lived of vertebrates, with some individuals of a few species boasting a longevity in the neighborhood of 200-250 years.
Estimates of the number of living turtle species vary widely, from 250 to 330 (depending of variations in classification schemes, and never mind species as yet discovered). And worldwide, an enormous number of these species are currently endangered or threatened. It has been estimated that about 75% of Asia's ninety tortoise and freshwater turtle species have become threatened.* All marine species are endangered. And even those taxa not officially listed as endangered face vanishing habitat, climate change, human predation, and threats from pollution on such a scale that it's not unreasonable to consider most living turtles in danger of extinction. Numerous species have already become extinct due to the actions of human beings.
Around the globe, turtles figure prominently in our myths, folktales, and religions. In Hindu mythology, the world is believed to rest on the backs of four elephants, who stand on the shell of a turtle. In Hinduism, Akupara is a tortoise who carries the world on his back. It upholds the Earth and the sea. But, in truth, at this point in the history of life of earth, the fate of all turtles (and elephants, for that matter) rests on the back of humanity. Will a single species of primates, and one that only dates back 195,000 years, be the end of a reptilian dynasty stretching back to the earliest days of the "Age of Reptiles"?
*Hilary Hylton, "Keeping U.S. Turtles Out of China," Time Magazine, 2007-05-08.
(Portions of the entry were adapted from relevant Wikipedia articles.)