Some 66 million years ago, about 75 percent of species on Earth disappeared. It wasn't just dinosaurs but most large mammals, fish, birds and plankton. Scientists have known this for a long time just from looking at the fossil record. If you dig deep enough, you find lots of dinosaur bones. And then a few layers up, they're gone.
But scientists couldn't figure out exactly what had caused this phenomenon. Of course, there were lots of theories.
"Some of them are pretty wacky," says J. David Archibald, an evolutionary biologist at San Diego State University who wrote the book Dinosaur Extinction and the End of an Era. "The really weird ones, of course, are that space hunters came and killed them all off, they died of constipation, mammals ate their eggs."
Then, in 1980, a new theory surfaced.
"It's the one that everybody hears about all the time because it's most dramatic," Archibald says.
Near what is now the town of Chicxulub in the Yucatan Peninsula, an asteroid more than 5 miles across slammed into the Earth. It caused tsunamis and earthquakes, and threw up a cloud of dust that smothered the world.
It sounds like a movie premise, but the Chicxulub impact left behind evidence. It threw up small blobs of black glass that were later found in Haiti. It dusted the world with iridium, an element that is rare on Earth but common in meterorites. It left a barely detectable imprint on the Yucatan Peninsula. Many scientists came to believe that the Chicxulub asteroid alone killed off the dinosaurs — and the public ate it up.
"We have this thing for big glitz and dramatic things," Archibald says. "Instantaneous is better."
But Princeton professor Gerta Keller wasn't convinced. She has her own theories about the mass extinction.
"Vulcanism has played a major role," Keller says.
In the hundreds of thousands of years before the Chicxulub impact, volcanoes in a region of India known as the Deccan Traps erupted repeatedly. They spewed sulfur and carbon dioxide, poisoning the atmosphere and destabilizing ecosystems. Keller says the dinosaurs were already on death's door by the time the asteroid hit.
And there is confusion about when that actually happened.
"If [the impact] is the cause, it had to be precisely at the time of the mass extinction," Keller says. "It can't be before and it can't be afterwards."
Keller's data suggest that the impact happened about 100,000 years before the mass extinction. Previous studies, on the other hand, put it 180,000 years after the dinosaurs died off.
Enter Paul Renne, a geologist from the University of California, Berkeley. To pin down the date, he headed out to the badlands of northeastern Montana.
"It's a region that has yielded a huge number of dinosaur fossils over the years," Renne says. "It's very famous for that."
Renne collected samples of ash that were deposited at the time of the mass extinction just above that treasure trove of fossils. He also obtained some of the glass blobs left by the Chicxulub impact. Measuring the rate of decay of radioactive potassium from these two samples, Renne was able to estimate the age of the impact and the age of the extinction.
"And lo and behold they are exactly the same," Renne says. "The impact clearly occurred right at the extinction level."
His results are published in the journal Science. They reinforce an idea that many scientists have held for years: The Chicxulub asteroid was the straw that broke the dinosaurs' back.
Gerta Keller thinks Renne's method was admirably precise, but she doesn't agree with some of his conclusions. She says his data are contradicted by other samples from Texas where a similar age date shows the Chicxulub impact predates the KT boundary — the point in time between the Cretaceous and Tertiary periods when the dinosaurs are believed to have gone extinct.
Still, there is one thing that Keller and Renne agree on: The asteroid isn't the whole story.
"There were significant extinctions and ecological perturbations going on a million or 2 million years before the impact, so we think that something else was already happening," Renne says. "What caused those things? There is an outstanding candidate — the early eruptions of the Deccan Traps."
The next step will be to find the age of these eruptions.
"We need to be able to place that set of eruptions into a time framework," Renne says.
Then they can better piece together what happened to the dinosaurs — and the rest of the species that went extinct. Renne and Keller will join Archibald and dozens of their colleagues at the Natural History Museum in London at the end of March to talk over their ideas.
"I'm looking forward to rather spirited discussions," Keller says.