Theories of the Extinction of the Dinosaurs

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Theories of the Extinction of the Dinosaurs

Controversy surrounds the extinction of the dinosaurs. According to one theory, dinosaurs were slowly driven to extinction by environmental changes linked to the creeping withdrawal of shallow seas from the continents at the end of the dinosaur era. Proponents of this theory propose that dinosaurs dwindled in number and variety over several million years.

An opposing theory proposes that the impact of asteroid or comet caused catastrophic destruction of the environment, leading to the extinction of the dinosaurs. Evidence to support this theory includes the discovery of a buried impact crater (thought to be the result of a large comet striking the earth) that is 200 km (124 mi.) in diameter in the Yucatan Peninsula of Mexico. The spray of debris was blown from the edge of the crater. The debris has been found over large regions of North America. Comet enriched material from the impact's fiery explosion was spread all over the world. With radiometric dating, scientists have used the decay rates of certain atoms to date the crater, spray of debris, and fireball layer. Using similar techniques to date the dramatic changes in the record of microscopic fossils, they have found that the impact and the dinosaur extinction occurred nearly simultaneously.

Although great amounts of ash suggest that most of North and South America was devastated by fire from the impact, the longer-term environmental effects of the impact were ultimately more deadly to life than the fire. Dust blocked sunlight from the earth's surface for many months. Burned sulfur from the impact site, water vapor and chlorine from the oceans, and nitrogen from the air combined to produce a worldwide fallout of intensely acidic rain. Scientists postulate that darkness and acid rain caused plant growth to die. As a result, both the herbivorous dinosaurs, which were dependent on plants for food, as well as the carnivorous dinosaurs, which fed on the herbivores, died out. On the other hand, animals such as frogs, lizards, and small insect-eating turtles and mammals, which were dependent on organisms that fed on decaying plant material, were more likely to live.

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