Box Tortoise enjoying some strawberries, image by jurvetson (flickr)
Chelonians are one of the oldest reptile groups, they belong to the order Testudines and have about 294 extant species. Chelonia are also the longest living reptiles, some living 150 years or more (the oldest is believed to be a tortoise known as Jonathon who was thought to be 176-178 years old before he passed). Chelonia are found in many parts of the world with temperate and tropical climates. There are three main groups of chelonians, namely, turtles, tortoises and terrapins. Turtles spend most of their time in water, tortoises are land dwellers and terrapins are semi terrestrial, spending both on land and in water. The origin of chelonians is uncertain, but the first recognisable turtles are believed to have existed in the late Triassic period of the Mesozoic era about 220 million years ago (mya). Some of the Triassic turtles lacked characteristic features of those seen today. For example, some of the earliest known turtles, such as Odontochelys semitestacea, had teeth rather than sharp edged jaws and an incomplete shell. The earliest known fully shelled turtle is the late Triassic Proganochelys. By the late Jurassic, turtles were a highly successful group and radiated widely. Chelonians possess an anapsid skull (temporal opening absent) and are suggested to have diverged from other diapsids between 200 and 279 mya. Towards the end of Cretaceous period, about 65 mya, earth underwent some drastic changes which wiped away many groups of organisms from its surface including the dinosaurs. But some groups of turtles survived these changes and the two suborders remain, namely, Cryptodira and Pleurodira. Cryptodira can retract their neck straight into the shell and includes all the marine turtles, tortoises and many freshwater turtles. The Pleurodira are the side-necked turtles that retract their neck in the shell with a sidewise motion and consist primarily of various freshwater turtles.
Chelonians inhabit open sea, tropical reefs and coastlines, saline marshes and estuaries, all sorts of freshwater areas, deserts, rainforests, mountains and prairies. Tortoises are herbivores feeding on grasses, weeds, flowers and some fruits. Some are omnivorous and also consume worms and insects. Turtles eat a wide variety of food including cacti, deadly poisonous sea sponges and jellyfish, squid and other molluscs. African freshwater turtle can also devour shorebirds. Large sharks and killer whales prey on aquatic chelonians. Sadly fish, seabirds and raccoons feed on both their eggs and hatchlings. On sensing danger of any sort, tortoises usually pull the head and other extremities into the shell for protection. The common musk turtle releases a foul smell when it is disturbed and hence often called stinkpot. Snapping turtle is noted for its aggressiveness on land where they will attack large animals, including humans, if molested.
Chelonians vary in size from certain American freshwater turtle, 9 cm long to marine leatherback turtles, 2.4-2.7 m long. The most unique feature of chelonian is its shell which acts as protective armour for the soft vital internal organs. It is divided into upper carapace and lower plastron, joined at the sides by bony bridges. Carapace is slightly domed, low and rounded or flattish with steep sides while the plastron is flat or slightly concave. The outermost layer of the shell is covered by keratinous, epidermal derivatives called scutes. They are decorated with species-specific colours and patterns. Vertebrae and plate-like ribs form the main structural component of the shell. In box turtle, the plastron is hinged and can completely close over the head and leg openings. In pancake tortoise, the bony casing has been reduced to allow free movement and to fit in narrow spaces. Bony plates have given place to small interwoven bones in leatherback sea turtle. Aquatic turtles have lighter, streamlined shells to aid in swimming. Chelonian shells are capable of regeneration when damaged. Tortoises have short sturdy feet, amphibious and freshwater turtles have webbed feet and in marine turtles they are modified into flippers which help them in swimming. Like many reptiles, chelonians carry out shedding or molting of their skin. Turtles molt their skins continuously in small pieces. In tortoises a lot of dead skin accumulates into thick knobs and plates that provide protection to parts of the body outside the shell.
Chelonia have good eyesight and excellent sense of smell but poor sense of hearing. Lungs serve as the main breathing organ even in fully aquatic species. Turtles can remain under water for about 2 hours without breathing, something they do by obtaining oxygen through gaseous exchange in the cloaca and pharynx. The linings of these cavities act as gills. However, they must breathe using their lungs when they are active, in order to have sufficient gas exchange in their tissues. Sea turtles excrete salt absorbed in the sea water from salt glands situated near their eyes. Chelonians are ectotherms and hence depend on external heat sources to control internal body temperature which in turn help to properly perform the natural activities and several metabolic processes. This is why terrapins and pond turtles are fond of gathering in groups and basking in the sun.
Male turtles can often be distinguished from females by their exceptionally long foreclaws, longer, thicker tail and by the lesser distance between the cloaca and the tip of their tail. Chelonians usually mate during spring and summer. They lay soft, leathery eggs in the holes dug into the mud and sand, cover them with sand, soil or organic material and abandon them for incubation. Sex is determined by the temperature. Under 29 degrees they become males and above this temperature they become females. After hatching, baby turtles circle their nest before heading towards the ocean.
Tortoises and turtles are economically important to humans owing to their ecotourism attraction. This has led to a rise in tourism operations that provide jobs and income to communities throughout the tropical and subtropical part of the world. However, their populations are declining because of habitat destruction, road kill and pet trade. Some turtles accumulate pollutants such as chlorinated hydrocarbons in their body fat which gradually poison them or impair their reproduction. Six of the seven species of marine turtles are listed as endangered out of which three are critically endangered. Leatherback turtle, spotted turtle, wood turtle, and eastern spiny soft-shell are some of the endangered species of turtles. The population of Blanding's turtles in Nova Scotia is also considered threatened. This calls for the need to conserve chelonia at all stages of the life cycle.
Tortoise by Mozzer (flickr)