Go digging with dinosaur hunters and they will show you that the last of the Cretaceous beasts died out suddenly, 66 million years ago. For 30 years the prevailing theory has been that T. rex and its brethren were wiped out by a comet or asteroid crashing in Central America, kicking up so much dust and ash that the Earth cooled for years afterward and made survival impossible.

Or was it really that simple? Some scientists, digging into well-preserved layers of the earth, said the timing was off. Yes, there is a 110-mile-wide crater, called Chicxulub, in the Caribbean off the coast of southern Mexico — but radioactive dating suggested it was made 180,000 years after the last dinosaur fossils. Something was wrong.

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