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 sampling the strata because they provide a rare, undisturbed record of reversals in the earth's magnetic field. Such fluctuations can influence climate, and possibly allow more cosmic radiation to assail the earth's atmosphere. One layer, only a centimeter thick and tracing back 65 million years, showed a sharp excess of iridium, an element 1,000 times more plentiful in otherworldly matter than in the earth's crust. The "spike" in the readings made a sobering point. "It's the first experimental evidence that something quite extraordinary happened then," says Physics Nobel Laureate Luis Alvarez, who gave his son a helping hand. A supernova that could have wiped out the dinosaurs? "A very small probability," says Alvarez pere. Also possible but improbable: a cloud of interstellar gas or a large meteorite. On with the parlor game. qed

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