Three Main Types of Reptiles
Reptiles first appeared about 360 million years ago; the earliest reptiles were the ancestors from which birds and mammals evolved. Characteristically "cold-blooded"---a common term for "ectotherms"---and hatched from eggs, all reptiles--with the exception of two rare species--fall under three main orders.
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"Squamata"---Largest Branch of the Reptilian Family
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With more than a thousand species of snakes and lizards all over the world, the order "Squamata" is by far the largest branch of the reptilian family. Only a few snakes are venomous and potentially dangerous to humans. Some lizards are also venomous, but the majority prey only on insects and defend themselves by maintaining the ability to cut off and leave their tails when escaping predators. With their unique "Jacobson's organs," most snakes and lizards assess their environment for danger and presence of prey by analyzing air samples collected by flicking their tongues.
"Crocodilia" -- Last of the Ruling Reptiles
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Typically large, carnivorous and having powerful jaws and thick skins, the "Crocodilians" are the last of the "ruling" reptiles which---together with the long-extinct dinosaurs---make up the sub-class "Archosauria." Consisting of more than 20 species of crocodiles, alligators, caimans and gharials, crocodilians are found in the swamps and rivers of Africa, Australia, Asia, as well as North and South America.
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"Testudines"---The Oldest Reptiles
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Easily identified by their hard, bony shells and remaining relatively unchanged since the Triassic period, an estimated 300 species of aquatic turtles and land-dwelling tortoises make up the "Testudines." Lacking teeth and means for vocalization, testudines are compensated with an acute sense of smell and sharp vision that make for exceptional navigating skills. Sea turtles are known to swim thousands of miles, returning to the same beach where they were hatched to lay their eggs. The longest-living of the reptiles, large testudines can live well after 100 years.
"Sphenodontia"---The Rarest and Least Known Reptiles
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With only two species of "Tuataras" constituting the order, "Sphenodontia" is the rarest, smallest and least known branch of the reptilian family. Found only in New Zealand and related to the "Squamatas," tuataras are the last living members of this order which thrived almost 200 million years ago.
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References
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