According to recent studies we know that the first reptiles evolved in the Permian (300 million years ago) from certain amphibians. While amphibians were the first vertebrates to rule on the mainland, yet its existence was closely linked to the liquid element. The amphibians need water to breed, and lay their eggs in it and went through an aquatic larval stage before metamorphosing into adults and capable of leaving the water (there are exceptions, but in general the development cycle of almost any amphibious works this way) Furthermore, they could not spend long periods of time on land because their permeable skin that dries easily and have to keep frequently moistened or otherwise perish. However reptiles lack these impediments. Reptiles lay hard-shelled eggs that do not report directly to the aqueous medium to which developed and small reptile babies hatch miniature replicas of their parents, so that a water resource is not necessary for its development. Also reptiles can not dry out as easily as amphibians, since their skin is waterproof and this allows them to survive without constantly hydrated.
So amphibians were relegated to an amphibious existence (pun intended) to develop its life cycle in damp and often near a water resource to be played, while reptiles are able to adapt more quickly to varied climates and even ends. Today we find reptiles in all the earth: in jungles, forests, meadows, mountains to heights around 5000 meters, barren deserts and even a few species have adapted to life at sea. The only places where we find reptiles is in the polar areas.
This first adaptive radiation of reptiles and their subsequent evolution over millions of years produced the first reptiles that arise countless species and different groups. As determining morphological characters to classify the various groups of reptiles temporary holes of the skull (ie after the orbits) are taken. The arrangement and number (or absence) of these holes gives us the key determinants to determine the evolutionary origin of reptiles and classify basal broad groups:
His skull has no openings in the temporal region. Today only survive the current turtles, which have fossils that date back to the Triassic, but an even more ancient origin for this group theorizes. Traditionally it has been included in Anapsida to all current and extinct reptiles that lack fenestrations, although today the origin and consistency of this group is discussed and it is possible that different lineages of reptiles alcanzasen the Anapsida condition independently. In fact some scientists hold the theory that the turtles or turtle current diapsid descended from reptiles and later lost these openings cranial features.
Diapsids appeared in the Permian and were a group of reptiles of great evolutionary success. They are diapsid lepidosaurs (lizards, snakes and sphenodontians current and extinct plesiosaurs), ichthyosaurs and archosaurs (crocodilians, dinosaurs, pterosaurs and birds). Diapsids skull characterized by the presence of two fenestrae, or holes behind the orbits.
The other kind of amniotes includes mammals and mammal-like reptiles called (also called reptiles with mammalian characters). His skull has an opening under the skull bone postorbital, and were also the first tetrapods to develop a specialized with various kinds of teeth teething. They made their early appearance more than 300 million years ago in the Carboniferous, with its peak in the Permian. After that, they were overshadowed by the archosaurs and reduced to small animals that could not otherwise thrive in the shadow of the great reptiles. It was not until the Paleocene, after the Cretaceous mass extinction and disappearance of dinosaurs fauna are similar to current mammals.
Therapsids is characterized by a single fenestra on postorbital skull bone. They are a polyphyletic group, this means that several lineages of not closely related condition euriápsida reptiles reached independently. Such is the case of plesiosaurs, ichthyosaurs, placodonto and nothosaurs.