Reptiles were the first animals to leave water and come on land. Now that was nearly 300 million years ago. Of the existing lot of animals, only reptiles can boast of having walked the planet when it was ruled by dinosaurs. You may have studied different reptiles in school, or maybe you have even seen some of them, such as snakes, lizards, turtles, etc. On the basis of their characteristic traits, these reptiles are grouped into four types. Crocodilia This group of reptiles includes crocodiles, alligators, gharials, and caymans. There are at least 23 different species of crocodilians in this world. They appeared about 84 million years ago. That was during the late cretaceous period. The closest living relatives of the crocodilians are aves, i.e., birds. These reptiles have a semi-erect posture. Their teeth are like human teeth, i.e., they are set in bony sockets. Unlike other mammals, they can replace their teeth all through their lifetime. They have a secondary bony palate, which helps them to breathe even when their mouth is full of water. Squamata Also known as scaled reptiles, this reptilian group consists of snakes and lizards. It has over 7000 species to its credit. Lizards, which belong to the Lacertilis category, come in various shapes and sizes. There is the miniature dwarf Gecko, and then there is the huge Komodo dragon. Snakes, on the other hand, belong to the Serpentes category. There are small species, like the 87 odd species of thread snakes, and there are gigantic ones, like the anacondas and reticulated pythons. Contrary to the popular belief, not all snakes are venomous. Similarly, some are terrestrial, while others are aquatic. Note: People do keep snakes as pets, but then one has to be a little careful whilst choosing the species. It's not just about your safety, but also about the safety of the snake. Testudines Tortoise and turtles are the ones that belong to this category of reptiles. These include box turtles, green sea turtles, alligator snapping turtles, African spurred tortoises, big headed tortoises, and so on. Most turtles and tortoises live for many years. In fact, some of these species live for more than 300 years. Additionally, a whole lot of testudines are also quite popular as pets. Rhynchocephalia This category of reptiles has only one extant genus to its credit, the tuatara, endemic to New Zealand. The two extant species of tuatara are the Northern tuatara and the rare Brothers Island tuatara. In terms of appearance, they are almost similar to lizards, which explains why they were originally classified as lizards in 1831. Next time you see any of these species, do not be afraid. They are just misunderstood creatures. If you see them closely, you will find that just like cats and dogs, even they can be very cute and adorable.