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Dinosaur is the name given to a group of pre-historic reptiles that first appeared approximately 210 million years ago. They lived during the Triassic, Jurassic, and Cretaceous periods, a time period spanning over 150 million years. By the end of the Cretaceous, about 65 million years ago, most species of dinosaur had become extinct, but a large amount of evidence suggests that modern birds are the descendants of one or more types of dinosaur. They are classed with dinosaurs by those who believe groups should be monophyletic.

The formal name Dinosauria was first proposed by the English scientist Richard Owen in 1842. The term is a combination of the Greek words deinos ("terrible" or "fearfully great") and sauros ("lizard").

Although they varied greatly in size, many kinds of dinosaurs were extremely large. The biggest dinosaurs were the Sauropoda[?]; they were the largest land animals ever to live, and overall are second in size only to certain species of whale.

Many other types of reptiles lived at the same time as the dinosaurs. Some of these were very large, and are commonly, but incorrectly, thought of as dinosaurs. See Plesiosaur.

Dinosaurs disappeared 65 million years ago, in the Cretaceous-Tertiary extinction event. The most common explanation of this event is that it was caused by a very large meteorite hitting the earth (a theory first proposed by Walter Alvarez[?]). However, this theory is contested.

Classification of dinosaurs

Dinosaurs are divided into two major groups, the Saurischia[?] and the Ornithischia[?], on the basis of hip structure.

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