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Dinosaur, (clade Dinosauria), the common name given to a group of reptiles, often very large, that first appeared roughly 245 million years ago (near the beginning of the Middle Triassic Epoch) and thrived worldwide for nearly 180 million years. Most died out by the end of the Cretaceous Period, about 66 million years ago, but many lines of evidence now show that one lineage evolved into birds about 155 million years ago.
The name dinosaur comes from the Greek words deinos (“terrible” or “fearfully great”) and sauros (“reptile” or “lizard”). The English anatomist Richard Owen proposed the formal term Dinosauria in 1842 to include three giant extinct animals (Megalosaurus, Iguanodon, and Hylaeosaurus) represented by large fossilized bones that had been unearthed at several locations in southern England during the early part of the 19th century. Owen recognized that these reptiles were far different from other known reptiles of the present and the past for three reasons: they were large yet obviously terrestrial, unlike the aquatic ichthyosaurs and plesiosaurs that were already known; they had five vertebrae in their hips, whereas most known reptiles have only two; and, rather than holding their limbs sprawled out to the side in the manner of lizards, dinosaurs held their limbs under the body in columnar fashion, like elephants and other large mammals.
Although estimates vary, one of the contenders for the largest dinosaur is the Argentinosaurus. Discovered in Argentina in 1987, the Argentinosaurus is estimated to have been 37 to 40 metres (121 to 131 feet) long and weighed up to 100 metric tons.The decline of dinosaurs was most likely caused by many different factors. A popular theory is that an asteroid crashed into the Yucatán Peninsula 66 million years ago, causing enough terrestrial and asteroid material to be ejected into the atmosphere that sunlight was possibly blocked for several years. This would have severely impacted plant life and subsequently brought about the end of most dinosaurs.
Dinosaur fossils and tracks have been discovered throughout human history. The first description of dinosaurs as a separate group of life forms (known as “Dinosauria”) was published in 1842 by British paleontologist Richard Owen, who based his findings on fossil fragments of the Cladeiodon and the Cetiosaurus.
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