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Case Closed for Dino Killer?

Mind the gap! Yale University graduate student Stephen Chester admires a dinosaur bone he helped find just below fallout from an asteroid impact (dark layers).
Credit: Tim Webster

What happened to the dinosaurs? For more than 100 million years, they ruled the world. Then, suddenly—poof—about 65 million years ago, they were gone. At least that's the way it looks to most scientists, who blame an asteroid hitting Earth for the ancient beasts' dramatic demise. Some researchers are still skeptical about the asteroid hypothesis, but a new fossil discovery in Montana may lend it new impact.

Back in 1980, when the late Nobel laureate physicist Luis Alvarez and his son, geologist Walter Alvarez, first tried to pin dinosaur extinction on an errant asteroid, they faced a major credibility gap. At the time, there was little firm evidence for such a catastrophic event. But then they and other researchers found an overabundance of iridium in geological formations at the 65-million-year transition line between the Cretaceous and Tertiary periods, known as the K-T boundary. Iridium is common in asteroids but rare in Earth's crust.

Still, the Alvarezes' hypothesis faced another hurdle: No dinosaur fossils had been found any higher than 3 meters below the K-T boundary, a gap which equates to about 100,000 years. Most researchers concluded that the dinosaurs went extinct before the asteroid impact and that they died off gradually. Alternative hypotheses for their demise included an increase in Earth's volcanic activity around the same time which threw ash into the atmosphere, diminished available sunlight, and affected the growth of plants that herbivorous dinosaurs ate, or a draining away of the shallow inland seas that dinosaurs relied upon for their vegetation-rich habitats.
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