Rewriting the Book on Dinosaurs

Forget what you knew: they weren't necessarily cold-blooded or pea-brained, and may not really be extinct

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The evidence for this theory is a thin layer of iridium, an element rare on the earth's surface but relatively abundant in comets and asteroids, found at about the right level in ancient sediment. The iridium layer was discovered in Italy by the late physicist Luis Alvarez and his geologist son Walter, and has since been found all over the world. An impact crater that may be the right age and size was identified two years ago on Mexico's Yucatan Peninsula.

The comet-asteroid theory is much beloved by physicists, astronomers and the general public; but while many paleontologists accept it, others have their doubts -- and some don't even care. "Everyone wants to know why dinosaurs went extinct except me," says Horner, who is far more interested in how they / lived. And Bakker states flat out that the meteorite theory is a crock. "It just doesn't wash ecologically," he says. "The wrong animals die." His point is that a worldwide catastrophe should have wiped out late Cretaceous creatures such as frogs and turtles, which were vulnerable because they cannot adjust easily to environmental changes. Yet those animals survived, while the presumably more adaptable dinosaurs disappeared.

There is evidence that dinosaurs were already on the way out, even if an asteroid did deliver the final blow. The fossil record shows that the number of dinosaur types dropped 70% between 73 million and 65 million years ago. Flying and swimming reptiles, which weren't true dinosaurs, declined too: pterosaurs, ichthyosaurs, plesiosaurs and mosasaurs all died out before the dinosaurs did. "What caused the decline?" the Smithsonian's Brett-Surman asks. "Was it a change in climate? A change in ocean currents? The changing distribution of plants?"

Whatever the reason, Horner insists that the more interesting and surprising question is how the dinosaurs managed to hang on for so long. Humans should be as lucky. "It was only 80 years from the time that Darwin published On the Origin of Species until we detonated the first nuclear bomb," he says. "In the lifetime of one person, we went from figuring out where we came from to figuring out how to get rid of ourselves." When the history of life on earth is complete, Horner suspects, the world's most beloved extinct creatures may have outlived their admirers by some 100 million years.

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