Reptiles evolved from early amphibians and rose to dominate the world for hundreds of millions of years. The dinosaurs became extinct at the end of the Cretaceous period but they live on in a sense through the birds. Mammals are also the descendants of one group of reptiles, the therapsids.
This may be the age of mammals but the reptiles have survived, although in fewer numbers than during their heyday. Modern reptiles belong to four groups: the crocodiles and their relatives (order Crocodilia); the turtles and tortoises (order Testudines); the snakes and lizards (order Squamata) and the order Rhynchocephalia, which is represented by only one species, the tuatara of New Zealand.
There are 21 living species of Crocodilia, including crocs, alligators, caimans and the gavial. They are found in the equatorial regions of the world and all are predators.
Turtles and tortoises are all characterised by a hard shell or carapace. When they are in danger, most pull their heads and feet into their shells. It must work for them, because they have existed for some 200 million years and survived the extinction of the dinosaurs. Some are carnivorous while others eat plants.
Land dwelling testudines are called tortoises. The Galapagos tortoises are huge, attaining lengths of one and a half meters and are extremely long-lived. A tortoise brought to Australia by Darwin on the Voyage of the Beagle lived in a zoo until just a few years ago. She was probably close to 200 years old when she died.
Turtles can be either fresh water or marine. There are seven species of sea turtles and their legs have evolved into flippers. They still breathe air and must return to land to lay their eggs. Fresh water turtles have webbed feet and also breathe air, although they can stay underwater for lengthy periods of time.
About 95 percent of the 9,000 odd species of living reptiles are snakes and lizards. If limbs are present, then it is a lizard. Some lizards, however, are limbless and can be hard to distinguish from snakes. Snakes never have an external ear opening but lizards do, so this is one way to tell them apart. In lizards, the tail is longer than the body whereas in snakes, the tail is shorter than the body. However, since you have to turn the snake upside down to see where its body ends and its tail begins, this is not always a very useful difference. Other than the American Gila Monster, lizards are never poisonous. The same cannot be said of snakes, but the fact is, most snakes are harmless. Even in Australia, most of the snakes are non-venomous.
Reptiles are ectothermic or cold-blooded. They have four five-fingered (pentadactyl) limbs and have lungs. Most live on land although turtles and sea-snakes cope quite well for most of their lives in marine and fresh-water environments. However both breathe air and so must surface regularly and they must return to land to lay their eggs.
Reptiles have shelled eggs and this is the feature that detached them from aquatic environments in the first place and allowed them to leave their amphibian ancestors behind. They also have thick, non-porous and scaly skin that retains their body moisture. This, in common with efficient kidneys that remove water from wastes so they excrete dry uric acid instead of urine, means that reptiles can live in even the driest of desert environments.
Some reptiles bear live young. The eggs are retained in the body and do not develop a hard shell. The young develop in the mother and then are born. This is known as ovoviviparity. Slow worms, adders and garter snakes are ovoviviparous.
A few reptiles form a kind of placenta so the young are linked to the mother and can receive food from her. This is true viviparity and occurs in some skinks. Because they are ectothermic, reptiles are more common in tropical regions of the world. They are missing from the Antarctic and rare in the Arctic. In temperate and tropical regions though, they are common, numerous and important components of many food chains. They are also beautiful, interesting animals, even the snakes, if people would leave their prejudices behind and take the time to get to know them.
For more information: http://animal-world.com/encyclo/reptiles/reptiles.htm