article : The Group Animal of Reptiles

By Regina Bailey, About.com
Reptiles have been around for a long time. They are believed to have evolved from amphibians and have developed several adaptations for life out of water. Reptiles range in size from the green anole (5 to 8 inches) to the crocodile (23 feet). Reptiles can also live in habitats ranging from swamps to deserts. This great diversity among reptiles is one of the reasons they have managed to survive.

One major characteristic of reptiles is the presence of scales composed of a protein called keratin. These scales form a waterproof barrier of skin, which allows reptiles to exist away from water without the threat of dehydration. Another characteristic is the regulation of internal body temperature by the external absorption of heat. Thus, reptiles are ectothermic. Unlike endothermic creatures, which must use calories from food to regulate body temperature, reptiles don't require much food to maintain body temperature and survive.

Reproductively, they can lay eggs on land due to the development of a protective shell around the egg. However, these eggs must be fertilized inside the female before the shell forms.

The reptilia class can be divided into three main orders: squamata, chelonia, and crocodilia.

Squamata (includes lizards and snakes)

• Lizards outnumber all other reptiles. They are generally small and very diverse.
  
  
• Snakes are closely related to lizards. They can detect vibrations, and some can sense changes in temperature.
  
  

Chelonia

• Turtles have a hard protective shell. Most live on land or must return to land to lay their eggs.
  
  

Crocodilia (alligators and crocodiles)

• These creatures live mostly in the water and are generally the largest of the reptiles.
  
  

The crocodilians are thought to be the closest living relative of the dinosaurs among reptiles. Along with birds, dinosaurs and crocodilians are descendants of a group of ancient reptiles known as the thecodonts. Snakes, lizards, and turtles appear to have evolved separately from the thecodonts.