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Debate continues over dinosaur demise

Many paleontologists consider the cause of the extinction of most of the dinosaurs to be a closed case: About 65 million years ago, an enormous extraterrestrial object struck the Yucatán area in Mexico, creating the infamous 200-kilometer-wide Chicxulub impact crater. The energy released on impact, which equaled about 100,000 gigatons of TNT, changed Earth’s ecosystem and led to the worldwide mass extinction event at the boundary between the Cretaceous and Tertiary periods (K/T).

Not so, according to Gerta Keller, a geologist at Princeton University in Princeton, N.J. Analysis of new core samples from Texas support her previous research from 2004 that the Chicxulub meteor struck about 300,000 years prior to the K/T extinction event and, therefore, did not cause the event, said Keller in October at the Geological Society of America’s (GSA) annual meeting in Philadelphia, Pa.

Supposing that Keller is correct, and the Chicxulub impact did not kill the dinosaurs, the finding begs the question as to what caused the mass extinction. Keller suggests that a yet-undiscovered impact on the scale of Chicxulub must have occurred, to explain the anomalous presence of iridium — an extremely rare element associated with certain types of meteorites — at the K/T boundary.

The odds, however, of more than one Chicxulub-sized impact are “beyond astronomical,” according to Christian Koeberl, a geochemist at the University of Vienna in Austria who studies impacts. “America loves the underdog,” he said in an interview at the GSA meeting. “That person doesn’t have to be right.”

Koeberl said that he, along with about 99 percent of other impact crater specialists, do not think that any debate remains about whether the Chicxulub impact led to the K/T extinction.

Kathryn Hansen

This story was shortened and modified from a Web Extra published online on Oct. 26. Read the original story in the Web Extra archive at

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