Environment: Doomed Dino

Puzzling over its demise

When dinosaurs vanished abruptly 65 million years ago, they left an enduring mystery—and created a scientific parlor game. Hypotheses abound to explain the extinction. Brains too small in bodies too large? Emerging mammals feasting on dinosaur eggs? Now comes evidence for another possibility. Geologist Walter Alvarez, probing an ocean canyon near Gubbio, Italy, discovered an abrupt increase in iridium in a limestone layer dating back to the dinosaurs' demise. Probable cause: some mysterious, still unfathomable extraterrestrial event.

Alvarez and his team from the University of California at Berkeley were...

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