TIME | April 27, 2009
By Jeffrey Kluger

When a scientific principle is common knowledge even in grammar schools, you know it's long since crossed the line from theory to established fact. That's the case with dinosaur extinction. Some 65 million years ago — as we've all come to know — an asteroid struck the Earth, sending up a cloud that blocked the sun and cooled the planet. That, in turn, wiped out the dinosaurs and made way for the rise of the mammals. The suddenness with which so many species vanished after the 65-million-year mark always suggested a single cataclysmic event, and the 1978 discovery of a 112-mi., 65-million-year-old crater off the Yucatán peninsula near the town of Chicxulub seemed to seal the deal.

Now, however, a new study in the Journal of the Geological Society throws all of that into question. The asteroid impact and the dinosaur extinction, argue the authors, may not have been simultaneous, but rather may have occurred 300,000 years apart. That's an eye-blink in geological time, but it's a relevant eye-blink all the same, one that occurred at just the right moment in ancient history to have sent the extinction theory entirely awry.

The controversial new paper was written by geoscientists Gerta Keller of Princeton University and Thierry Addate of the University of Lausanne, in Switzerland — and both researchers knew that challenging the impact doctrine would not be easy. The asteroid charged with killing the dinosaurs, after all, left more than the Chicxulub crater as its calling card. At the same 65-million-year depth, the geological record reveals that a thin layer of iridium was deposited pretty much everywhere in the world. Iridium is an element that's rare on Earth but common in asteroids, and a fine global dusting of the stuff is precisely what you'd expect to find if an asteroid struck the ground, vaporized on impact and eventually rained its remains back down. Below that iridium layer, the fossil record shows that a riot of species was thriving; above it, 65% of them went suddenly missing.

But Keller and Addate worried that we were misreading both the geological and fossil records. They conducted surveys at numerous sites in Mexico, particularly at a spot called El Peñón, very near the impact crater. They were especially interested in a 30-ft. layer of sediment just above the iridium layer. That sediment, they calculate, was laid down at a rate of about 0.8 in. to 1.2 in. per thousand years, meaning that the entire 30 feet took 300,000 years to settle into place.

Analyzing the fossils at this small site, they counted 52 distinct species just below the iridium layer. Then they counted the species above it. The result: the same 52. It wasn't until they sampled 30 feet higher — and 300,000 years later — that they saw the die-offs.

"The mass extinction level can be seen above this interval," Keller says. "Not a single species went extinct as a result of the Chicxulub impact."

Keller's and Addate's species samplings are not, of course, conclusive, and plenty of other surveys since 1978 do tie the extinctions closely to the asteroid. But since the new digs were so close to ground zero, the immediate species loss ought to be have been — if anything — greater there than anywhere else in the world. Instead, the animals seemed to escape unharmed. Other paleontologists, however, believe that the very proximity of El Peñón to the impact site makes the results not more reliable, but less. Earthquakes and tsunamis that resulted from the collision could have wrought havoc on the sedimentary record, causing discrete strata to swirl together and completely scrambling timelines. Keller disagrees, pointing out that the slow accretion of sediment she and Addate recorded is completely inconsistent with a sudden event like a tsunami.

"The sandstone complex was not deposited over hours or days," she says. "Deposition occurred over a very long time period."

So if the Chicxulub asteroid didn't kill the dinosaurs, what did? Paleontologists have advanced all manner of other theories over the years, including the appearance of land bridges that allowed different species to migrate to different continents, bringing with them diseases to which native species hadn't developed immunity. Keller and Addate do not see any reason to stray so far from the prevailing model. Some kind of atmospheric haze might indeed have blocked the sun making the planet too cold for the dinosaurs — it just didn't have to have come from an asteroid. Rather, they say, the source might have been massive volcanos, such as the ones that blew in the Deccan Traps in what is now India at just the right point in history.

For the dinosaurs that perished 65 million years back, extinction was extinction and the precise cause was immaterial. But for the bipedal mammals who were allowed to rise once the big lizards were finally gone, it is a matter of enduring fascination.