Most paleontologists have accepted [that dinosaurs were killed off by] an asteroid more than six miles wide that socked Earth 65 million years ago. It gouged a crater more than 100 miles wide on what is now the Yucatán Peninsula in Mexico. According to the leading scenario, the impact threw huge amounts of dust and other debris into the atmosphere, blocking sunlight and sinking the Earth into darkness for weeks or even months. A global disaster certainly struck at the time, according to overwhelming fossil and geological evidence. As Fastovsky and Weishampel write in The Evolution and Extinction of the Dinosaurs, "the world's oceans were virtually 'dead'" as photosynthesis by plankton ceased and marine food webs unraveled. The dinosaurs died, while the ancestors of today's mammals, birds and reptiles hung on.
Paleontologists disagree about what the existence of polar dinosaurs says about the asteroid-winter scenario. Fiorillo says he is skeptical of it because "dinosaurs in Alaska were doing just fine in conditions just like that." He argues that climate changes caused by shifts in circulation of the atmosphere and oceans probably did in the dinosaurs.
But Rich says that the lives of polar dinosaurs can help researchers understand why dinosaurs went extinct after the impact. The catastrophe had to have been long and severe enough to kill off the dark- and cold-adapted animals. "You can't just have it [darkness] for a month and do the job," he says.
But Fastovsky says that polar dinosaurs tell us nothing about the animals' demise because we don't know whether these particular species were even alive at the end of the Cretaceous period. Rich's Australian dinosaurs were long extinct by the time the asteroid hit. Whether the dinosaurs on the North Slope of Alaska were alive is uncertain, he says; researchers have found no fossil layers there from the very end of the Cretaceous period.
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