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Dinosaurs were real animals that once lived on the earth, but they died out about 65 million years ago. Since the very first kind of human did not appear on the earth until about 3.5 million years ago, it is clear that no one has ever seen a living dinosaur.
Fossil remains can be in the form of bones, found either in skeletons or separately; footprints in rock; impressions of skin, also in rock; and eggs. Most of the dinosaur specimens that have been collected are in museums. But at Dinosaur National Monument, near Vernal, Utah, a museum has been built around rocks holding dinosaur bones. (Weishampel, p.1)
The disappearance of the dinosaurs is in some ways a great mystery. During the time the dinosaurs lived, many new kinds developed and other older kinds died out. Not all kinds died out at once, but by the end of the Cretaceous period, the last of the dinosaurs had disappeared. Scientists have proposed several theories to explain the disappearance of the dinosaurs. There are two main theories of Dinosaur extinction as follows:
In 1980 the asteroid theory was proposed by Luis and Walter Alvarez. This theory is based on the discovery of unusually high concentrations of the rare metal iridium at rock levels close to the level of the last dinosaur remains. Iridium is more common in meteorites than in rocks found in the earth's crust.
According to this theory, a large asteroid crashed into the earth about the time the dinosaurs disappeared. The force of the collision caused a huge cloud of dust to circle the earth. The dust blocked the sun's light for as long as five years. This killed many plants, and without food, the dinosaurs died too.
High concentrations of the rare-e ...
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