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Did impacts help dinosaurs thrive?

The plot could come right out of a Greek tragedy: A ruler from long ago suffers defeat from the very forces that brought him to power years earlier. Only this time, the main character isn't Oedipus but the dinosaurs. Many scientists believe a meteorite or comet ended these reptiles' reign on Earth 65 million years ago, at the close of the Cretaceous period. Now, evidence from Italy raises the possibility that earlier impacts some 200 million years ago -- at the end of the Triassic period -- may have helped the dinosaurs rise to dominate their world.

In the Jan. 24 SCIENCE, David M. Bice of Carleton College in Northfield, Minn., and his colleagues report discovering pieces of shocked quartz in sediments dating to the boundary between the Triassic and Jurassic periods. Shocked quartz has microscopic fractures, believed to result from intense shock waves created by impacts. Geologists have found similar bits of shocked quartz in sediments from the end of the Cretaceous.

Bice and his co-workers found three layers of shocked quartz spaced close together in the Italian rocks, leading them to suggest that a shower of comets struck Earth during a period lasting a few hundred thousand years. If one or more impacts did occur at that time, they may have caused some of the well-known extinctions at the end of the Triassic, the researchers suggest. The boundary between the Triassic and Jurassic periods marks one of the most severe die-offs in Earth's history. Many of the dinosaurs' competitors disappeared, paving the way for the reptiles' evolutionary success.
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Publication:Science News
Date:Feb 8, 1992
Words:258
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