Reptiles are cold-blooded, vertebrate animals that have a dry, scaly skin. The skin is made of one continuous sheet, rather than individual scales as in fish. Water cannot pass out through their skin, unlike that of amphibians. This means that reptiles do not need to keep their skin moist. Reptiles moult regularly, shedding their skin when a new one has grown underneath.
Because they are cold-blooded animals, reptiles need to bask in the sun to raise their body temperature before they are able to move about in search of food. However, they do not need to eat as much food as the warm-blooded birds and mammals, so are able to survive more easily in harsh desert environments. Most reptiles lay eggs, either soft and leathery, or hard-shelled. Some give birth to live young. Reptiles do not care for their young after hatching or birth.
Reptiles are descended from the early amphibians. The first reptiles were probably small, lizard-like creatures. Unlike the amphibians, they did not have to stay close to water to keep moist and lay their eggs. They were able to live more easily on land. The lizards, turtles and tortoises and the crocodilians (crocodiles and alligators), appeared during the Triassic Period, about 250 million years ago. Snakes evolved later, about 135 million years ago. Some modern-day reptiles have hardly changed since the time of the dinosaurs.
THE AGE OF DINOSAURS
The period from 250 million to 65 million years ago is known as the Mesozoic Era. During this time, a new group of reptiles, the dinosaurs, emerged. They held their legs straight beneath their bodies, rather than sprawling out to the sides like other reptiles.
For about 150 million years the dinosaurs were the only large land animals. They ranged in size from the chicken-sized Compsognathus to the 14-metre-tall Brachiosaurus. Some were slow, four-legged plant-eaters, while others ran swiftly on two legs. A few kinds were terrifying predators. Many of the plant-eaters lived in herds like modern-day cattle or horses, while the smaller predators hunted in packs, as hyenas and wild dogs do today.
The dinosaurs all died out quite suddenly, but no-one knows why. Perhaps a huge asteroid hit the Earth, and a dust cloud plunged the Earth into cold darkness.
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