Reptiles are cold blooded vertebrates with epidermal scales. Most reptiles lay eggs (oviparous reproduction) but as exception some reptiles like boa constrictor give birth to young ones (viviparous reproduction). Egg and live birthing methods are combined too in some rare reptile species. Asexual reproduction among reptiles is also rare but it is still common compared to egg-live birth method. Reptile eggs are fertilized internally unlike amphibians. Scales of reptiles are rough and made up of keratin (substance of which nails, hair and claws are made of). Reptiles keep shedding their skin throughout their life. Reptiles breathe only through lungs unlike amphibians. 10000 species of reptiles are present on the earth. Reptiles have considerable representation in fossil record. Origin of the word reptile is Latin. It comes from Latin word reptilis meaning creeping. So, it is not at all surprising that reptiles are creepy crawly creatures in general.

Four orders of reptiles

Reptiles are divided into four different orders based on their distinct characteristics. All turtle species comprise the order testudines. Cartilage-based shell that acts as protective shield is distinguishing characteristic of this order of reptiles. Squamata is the order that comprises most reptile species. This youngest order of reptiles comprises all known species of snakes, lizards, geckos and skinks. This order includes many of the smallest reptiles in the world. Some members of this order are venomous. Crocodalia, as the name suggests, is the order that comprises all crocodiles and other similar creatures. All alligators, caimans, gharials and crocodiles are members of this order of reptiles. These are among the largest carnivores on the planet. Rows of large teeth, long tail and tough skin are common characteristics of reptiles of this order. Members of Crocodalia order are closely related to birds as their ancestors were closely related to ancestors of birds, dinosaurs. Rhynchocephalia has been reduced to a single genus from being very diverse. Tuatara is the single living genus of this order of reptiles.          

Origins of reptiles

Reptiles originated from amphibians approximately 300 million years ago in carboniferous period but living orders can be traced back to Triassic period. Casineria, one of the oldest known amniotes, had both reptilian and amphibian characteristics. Earliest undisputed reptile fossils is of Hylonomus. In Permian period amniotes diverged into three groups – diapsids, anapsids and synapsids. In the same Permian period diapsid reptiles diverged into archosaurs and lepidosaurs. Archosaurs were predecessors of dinosaurs, crocodilians and birds while lepidosaurs were predecessors of snakes and lizards.