There are more than 8,200 living species of reptiles, with the majority being snakes or lizards. They are commonly placed in four different orders.

OrderCharacteristics
Crocodilia: crocodiles, alligators, caimans, gharialsThey have four sprawling legs that can be used to gallop; they replace their teeth throughout life; they have strong jaws and a powerful bite; they have a more advanced brain and greater intelligence than other reptiles; they have a four-chambered heart.
Sphenodontia: tuatarasThey are the least specialized of all living reptiles; their brain is very similar to the amphibian brain; they have a three-chambered heart, but it is more primitive than the heart of other reptiles.
Squamata: lizards, snakesLizards: most have four legs for running or climbing, and they can also swim; many change color when threatened; they have a three-chambered heart.Snakes: they do not have legs, although they evolved from a tetrapod ancestor; they have a very flexible jaw for swallowing large prey whole; some inject venom into their prey through fangs; they have a three-chambered heart.
Testudines: turtles, tortoises, terrapinsThey have four legs for walking; they have a hard shell covering most of their body; they have a three-chambered heart.

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