Reptiles are tetrapod animals and include well-known specimen like crocodiles, snakes, turtles and lizards. They are present on all continents, except Antartica. They appeared on earth some 321 million years ago during the Carboniferous period. Reptiles are divided in several categories, including testudines (turtles and tortoises), squamata (lizards, snakes, etc), crocodilia (crocodiles, alligators, etc.), and aves (birds). Altogether there are over 20’000 different species of reptiles.