Professor – Turtles are reptiles of the order Chelonii or Testudines characterized by a special bony or cartilaginous shell developed from their ribs that act as a shield. The word turtle may either refer to the order as a whole or to a particular turtle which make up a form taxon that is not monophyletic. The order Chelonii or Testudines includes both extant – living – and extinct species. The earliest known turtles date back from two hundred and fifteen million years ago, making turtles one of the oldest reptile groups, and a more ancient group than lizards, snakes, and crocodiles. Of the many species alive today, some are highly endangered. Like other reptiles, turtles are ectotherms. Their internal temperature varies according to the ambient environment, commonly called cold-blooded. However, leatherback sea turtles have noticeably higher body temperature than surrounding water because of their high metabolic rate. Like other amniotes, reptiles, dinosaurs, birds, and mammals, they breathe air and do not lay eggs underwater, although those species live in or around water. The largest turtles are aquatic. The largest living Chelonian is the leatherback turtle, which reaches a shell length of 200 centimeters and can reach a weight of over 900 kilograms. Freshwater turtles are generally smaller; but with the largest species, the Asian softshell turtle, a few individuals have been reported up to 200 centimeters. This dwarfs even the better-known alligator snapping turtle, the largest Chelonian in North America, which attains a shell length of up to 80 centimeters and weighs as much as 113.4 kilograms.
Student – And Professor, what is the largest turtle? I thought it was the giant turtle.
Professor – No! The largest ever Chelonian was Archelon Ischyros; a late cretaceous sea turtle known to have been up to 4.6 meters long. Giant tortoises of the Genera, Geochelone, Meiolania and others were relatively widely distributed around the world into prehistoric times and are known to have existed in North and South America, Australia, and Africa. They became extinct at the same time as the appearance of man, and it is assumed that humans hunted them for food. The only surviving giant tortoises are on the Seychelles and Galapagos Islands and can grow to over 130 centimeters in length and weigh about 300 kilograms. The smallest turtle is the speckled padloper tortoise of South Africa. It measures no more than 8 centimeters in length and weighs about 140 grams. Two other species of small turtles are the American mud turtles and musk turtles that live in the area that ranges from Canada to South America. The shell length of many species in this group is less than 13 centimeters in length. Although many turtles spend large amounts of time in the water, all turtles and tortoises breathe air and must surface at regular intervals to refill their lungs. They can also spend much of their lives on dry lands. Aquatic respiration in Australian freshwater turtles is currently being studied. Some species have large cloacal cavities that are lined with many finger-like projections. These projections, called papillae, have a rich blood supply and increase the supply area of a cloaca. The turtles can take up dissolved oxygen from the water using these papillae in much the same way that fish use gills to respire. Turtles lay eggs like other reptiles, which are slightly soft and leathery. The eggs of the largest species are spherical while the eggs of the rest are elongated. Turtle eggs prepared to eat consist mainly of yolk. In some species, temperature determines whether an egg develops into a male or female. A higher temperature causes a female; a lower temperature causes a male. Large numbers of eggs are deposited in holes dug into mud or sand. They are covered and left to incubate by themselves. And when the turtles hatch, they squirm their way to the surface and head towards the water. And there are no known species in which the mothers care for the young. Researchers have recently discovered a turtle’s organs do not gradually break down or become less efficient over time. Unlike most animals and unlike men, it was found that the liver, lungs, and kidneys of a centenarian turtle are virtually indistinguishable from those of its immature counterpart. This has inspired genetic researchers to begin examining the turtle genome for longevity genes.
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