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How did dinosaurs become extinct?

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Scientists can't say for sure what caused the extinction of the dinosaurs, but theories suggest that diseases, climatic change, geologic events and changing plant communities may be to blame. Scientists say that dinosaurs, along with 50 percent of all other life on Earth, died out around 65 million years ago.

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Among the most popular extinction theories is the Alvarez Hypothesis. This hypothesis, which was formed by a father-and-son team comprised of geologist Walter Alvarez and physicist Luis Alvarez, postulates that an asteroid measuring 4 to 9 miles in diameter struck the earth. The impact from this asteroid strike penetrated the crust of the earth and scattered debris and dust into the atmosphere; triggered tsunamis, increased volcanic activity and severe storms; and caused acid rain. The earth's atmosphere was transformed and became concentrated with fluoride compounds, nitric acid and sulfuric acid.

The blast created a wave that incinerated every living thing in its path, while the dust and debris that were thrown into the atmosphere blocked sunlight for a period of months, causing the temperature around the globe to drop. Plants died out, followed by the herbivores that ate them. Carnivorous animals at the top of the food chain were left with no prey other than each other.

Other extinction theories hold that gas and ash from volcanoes suffocated the dinosaur population, disease wiped out the dinosaurs or imbalances in the food chain caused their starvation and ultimate extinction.

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