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What Are Four Groups of Reptiles?

Reptiles are a class of vertebrate animals distinguished by a combination of cold-bloodedness --- the absence of internal means of maintaining body temperature --- and scaly skin. The majority of reptiles lay eggs, the hatchlings of which are miniature versions of adults. Reptiles are found in virtually all habitats with the exception of polar ice and tundra. Taxonomists further classify the more than 8,700 known species of reptiles into four groups known as orders.

Crocodilia
- Fewer than 1,300 gharials remain in the wild in India and Nepal.
  
  The order crocodilia comprises the 23 living species of alligators, crocodiles, caimans and gharials. A four-chambered heart makes crocodilians unique among reptiles, most of which have three-chambered hearts. All crocodilians are carnivores that swallow their prey whole or in large pieces. Crocodiles have V-shaped snouts, while the snouts of alligators are U-shaped. The critically endangered gharial, found exclusively in the Indian subcontinent, has the longest and thinnest snout of any crocodilian, which limits its diet to fish. Alligators occur in the southern United States and eastern China; caimans are found in Central and South America. Crocodiles have the most extensive range, which encompasses parts of the Americas, Africa, Southeast Asia and Australia. Crocodiles are common in saltwater habitats, whereas alligators are usually found in fresh waters.
Squamata
- The squamata order, the most diverse, includes almost 4,700 species of lizards and close to 3,000 snake species. All shed their scaly skin. For the most part, lizards, such as geckos and iguanas, have small heads and short necks, but long bodies and tails. The Komodo dragon is the largest known lizard. Snakes, believed to have descended from lizards, lack legs and eyelids. The jaw bones of the snake, a carnivore, disengage, allowing it to swallow prey larger than itself. Unlike most other snakes, boas give birth to live young.
Chelonia
- The herbivorous tortoise occurs mostly in warm, arid habitats.
  
  The chelonia order includes turtles, tortoises and terrapins, of which there are approximately 250 species. Their distinguishing feature is a protective shell attached to the spine. Turtles rarely leave the water and have webbed feet that enable them to swim, whereas tortoises are terrestrial and occur in arid habitats. Terrapins, usually found in swampy areas, spend time on land and in water. Most chelonians consume both plant and animal matter, though tortoises tend to be herbivorous. The leatherback turtle is the largest and heaviest of the group. Tortoises, which can live more than 150 years, have the longest life span among reptiles.
Sphenodontia
- Two species of tuatara, a reptile endemic to New Zealand, make up the sphenodontia order. Tuataras are considered "living fossils" because their skeletons are almost identical to those preserved in million-year-old fossils. Easily mistaken for lizards, tuataras differ because they are nocturnal, lack external ears and prefer cool weather. Tuataras have spiky scales along the center of their backs and the top of their tails, and a blind but light-sensitive eye on top of their heads that helps them determine the time of day. Tuatara feed mostly on insects. They reach maturity between the ages of 13 and 20 and can live up to 80 years.

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