Reptiles are a group of vertebrate animals amniotas tetrapods. The oldest fossils that date back to humanity has Permian period, a period in which it is known that evolved from amphibians, more than 300 million years ago. Of these primitive reptiles in turn they evolved existing reptiles, dinosaurs, some now extinct reptiles, mammals and birds.
Currently nearly 10,000 known species of reptiles living on earth, and are classified as:
-Crocodylomorphos Or crocodilians
-Tuataras Or Rhynchocephalos, a kind of very primitive lizards characters
-Tortugas Or turtles
-Serpientes And scaly lizards or (Squamata)
Although there are many specific differences that distinguish these groups of reptiles each other, they all have one common feature is that reptiles are cold-blooded animals or ectotherms. This means you do not have a constant temperature (as mammals and birds for instance) but that their body temperature is directly dependent on the environment and how to use it to thermoregulate. In practice this means that if for example a reptile want to refresh, will be put in the shade and temperature descend, on the contrary need to be exposed to hot sun and it will raise its temperature. But one mammal will keep basically the same internal temperature is the sun or the shade and does not depend on the ambient temperature to regulate their own. This means that although a reptile has certain disadvantages compared to a bird or a mammal, as that can not maintain a steady pace of activity and heat needs to stay active, but it has its advantages: by not generating heat from the chemical energy from food, needed a reptile eating a much smaller amount of food that other warm-blooded animal and the same size needs to survive. Another common feature of all reptiles is that his heart is divided into three chambers (two atria and one ventricle) and have 2 arteries. Thus, the separation of oxygenated and deoxygenated blood is not very efficient. An exception to this are the crocodilians, despite having also 2 arteries, your heart basically has 4 cameras (like birds).