Out of a total of 15 orders of reptiles, 5 remain today:
Squamata (lizards and snakes)
Lizards and snakes have adapted to many life styles -- in trees, in burrows, on land and in the water from the tropics, to deserts, to temperate zones and close to the Arctic Circle. Snakes are carnivorous but lizards eat a variety of foods including plants and insects.
Snakes have descended from lizards and there are many similarities between them. Some characteristics that distinguish snakes from lizards are:
Chelonia (turtles, tortoises)
Turtles, too, occupy a variety of habitats from dry land to fresh water to salt water -- in tropical and temperate zones. Turtles living on dry land are called tortoises; those living in the marine environment are sea turtles; freshwater species are sometimes called terrapins.
Turtles do not have teeth. Horny beaks help them grasp and bite food. Tortoises are usually herbivorous while sea and fresh water turtles are omnivorous.
The turtle's shell consists of two parts -- the carapace (upper shell) and the plastron (under shell). The two parts are connected by bridges between front and hind legs. The ribs and backbone are part of the upper shell. The shell is made of bone covered by a thin layer of horny plates (scutes). The leatherback turtle does not have a complete bony shell nor is there a covering of horny scutes. Soft-shelled turtles have a much reduced shell covered with leathery skin.
Crocodilia (alligators including caimans, crocodiles and gharials)
Crocodilia are found in and near water in warm areas of the world. Most swim in fresh water, but some, like the saltwater crocodile swim in saltwater seas. Crocodillians eat fish, birds, turtles, small (and sometimes medium-sized) mammals. The crocodilians are known for "nest tending" and providing some parental care for their young.
Members of the crocodile group have legs and feet designed for walking on land and a strong tail used for swimming. The three groups may be distinguished from one another by looking at their heads. Alligators have a broad, rounded snout; the crocodiles have a triangle-shaped head with a more pointed snout and gharials have a very thin, skinny snout.
These endangered species live on some islands off the coast of New Zealand. There are two recognized species. While looking like a lizard, there are differences, such as the structure of the skull, that set the tuatara apart from lizards. The tuatara spends daytimes in burrows. It comes out in the evening to feed -- mostly on insects. Tuatara are able to survive at colder temperatures than most reptiles.
Dinosauria (birds -- formerly class Aves)
Birds are the living descendents of dinosaurs. This concept has, generally, been accepted for some time. More recently, the class Aves has been re-evaluated and birds are now considered to be living members of the order Dinosauria and are, therefore, reptiles.