How did the Dinosaurs become Extinct?

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The Asteroid Theory 

One of the most well-known and intriguing theories suggested for dinosaur extinction is the asteroid theory. In the 1980, the father-son team of Luis and Walter Alvarez discovered a layer of iridium in the K-T boundary. Iridium is rare on earth, but abundant in meteorites. The Alvarezs' suggested that a huge asteroid or comet, perhaps miles in diameter, hit the Earth at that time. The result of such an impact would be an enormous explosion that would throw dust clouds into the sky, darkening the planet. Massive forest fires, triggered by the hit, would add smoke to the sky. This would cool the planet causing the climatic changes observed.

A crater, now worn down and partly under the ocean, was found along the Mexican Yucatan Peninsula and its creation coincides nicely with the K-T boundary. NASA scientists estimate that the asteroid that made Chicxulub Crater, as it is now known, would have been about 6 to 12 miles in diameter. The crater is about 130 miles across.

Scientists at the Southwest Research Institute in Boulder, Colorado, have even been able to trace the path of that asteroid back into space. According to their calculations, 160 million years ago a collision between a 100-mile-wide asteroid, named Baptistina, and a smaller unnamed asteroid out beyond the orbit of Mars, shattered the larger object and sent pieces of it into the inner solar system. Ninety-five million years after that these pieces encountered earth, ending the dinosaurs' reign.

Not all scientists are satisfied with the asteroid theory, however. They point out the fossil record shows the dinosaurs were already in decline before the K-T boundary and the asteroid might just have been the final blow that finished off a population already weakened by some other factor.

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In Montana, at a dig supervised by celebrated paleontologist Jack Horner, Nan Crystal Arens from Hobart and Smith Colleges carefully examined the fossil leaves and pollen from a slice of the Hell Creek formation at many different levels. The Hell Creek formation is set just above the K-T boundary and represents the millions of years just before the extinction of the dinosaurs. Dr. Arens found that the number of different species of plants, especially flowering plants, declined significantly as one got closer to the boundary. Perhaps 90% of all the flowering plants had already died before the asteroid struck. Since ...

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