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About 140 million years before the asteroid impact that is generally thought to have wiped out the dinosaurs, there may have been other collisions of extraterrestrial objects with Earth that allowed the great reptiles to flourish in the first place.

Geologists have found evidence of three closely spaced impacts that may have caused the mass extinction that closed the Triassic period, an event that may have cleared ecological niches for dinosaurs, until then a relatively minor form of life.The new evidence consists of quartz sand grains that have sharp, angular edges indicating that they were produced by a shattering blow and deposited without being worn smooth again.

David M. Bice and colleagues at Minnesota's Carleton College and at Syracuse University report in the current Science that they found the "shocked quartz" in Italy's Appenine Mountains in the boundary layer between Triassic deposits and those of the ensuing Jurassic.

In fact, they found three layers, suggesting three impacts within perhaps 150,000 years. Such a scenario fits with the prevalent idea that many comets were dislodged simultaneously from stable orbits in the Oort Cloud, which surrounds the solar system, and fell toward the sun.