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Mind the gap! Yale University graduate student Stephen Chester admires a dinosaur bone he helped find just below fallout from an asteroid impact (dark layers).

Tim Webster

Case Closed for Dino Killer?

What happened to the dinosaurs? For more than 100 million years, they ruled the world. Then, suddenly—poof—about 65 million years ago, they were gone. At least that's the way it looks to most scientists, who blame an asteroid hitting Earth for the ancient beasts' dramatic demise. Some researchers are still skeptical about the asteroid hypothesis, but a new fossil discovery in Montana may lend it new impact.

Back in 1980, when the late Nobel laureate physicist Luis Alvarez and his son, geologist Walter Alvarez, first tried to pin dinosaur extinction on an errant asteroid, they faced a major credibility gap. At the time, there was little firm evidence for such a catastrophic event. But then they and other researchers found an overabundance of iridium in geological formations at the 65-million-year transition line between the Cretaceous and Tertiary periods, known as the K-T boundary. Iridium is common in asteroids but rare in Earth's crust.

Still, the Alvarezes' hypothesis faced another hurdle: No dinosaur fossils had been found any higher than 3 meters below the K-T boundary, a gap which equates to about 100,000 years. Most researchers concluded that the dinosaurs went extinct before the asteroid impact and that they died off gradually. Alternative hypotheses for their demise included an increase in Earth's volcanic activity around the same time which threw ash into the atmosphere, diminished available sunlight, and affected the growth of plants that herbivorous dinosaurs ate, or a draining away of the shallow inland seas that dinosaurs relied upon for their vegetation-rich habitats.

Then, in 1991, paleontologist Peter Sheehan of the Milwaukee Public Museum in Wisconsin and his colleagues reported on a systematic search for dinosaur fossils in North Dakota and Montana, narrowing the fossil gap to only 60 centimeters below the K-T boundary. And right around the same time, dinosaur track expert Martin Lockley of the University of Colorado, Denver, found dinosaur footprints just 37 centimeters below the K-T boundary in Colorado. Thanks to these finds—and the discovery of a huge, 180-kilometer-wide crater in Chicxulub, Mexico, that is believed to be the mark of the K-T killer—most scientists now accept the Alvarez hypothesis. Nevertheless, a vocal minority still insists that other, more gradual causes of dinosaur extinction, such as volcanic eruptions or the retreat of inland seas, are more likely.

Now a team led by Tyler Lyson, a paleontologist at Yale University, claims to have found a dinosaur bone in Montana that narrows the fossil gap to just 13 centimeters. Last year, Lyson and his colleagues were working at the Hell Creek Formation in the southeastern part of the state, looking for mammals that had lived after the asteroid impact. Two members of the team, Yale anthropologist Eric Sargis and his graduate student Stephen Chester, stepped around the corner for a break and came upon a large piece of horn from the brow of a ceratopsian, an armored dinosaur (the team was not able to identify the species of the 45 centimeter-long fossil, but ceratopsians include well-known beasts such as Triceratops.)

To pin down the exact location of the K-T boundary in relation to the fossil, the team excavated a section of the Hell Creek Formation adjacent to the horn and analyzed the samples for evidence of the asteroid's impact. These indicators include a thin deposit of iridium as well as a marked change in the species of plant pollen found below and above the boundary, such as a sharp increase in fern species.

The data narrow the fossil gap to just 13 centimeters, the team concludes in its online report today in Biology Letters, meaning that dinosaurs were still alive and kicking pretty much right up until the asteroid struck. "Here we have a specimen that basically goes right up to the boundary, indicating that at least some dinosaurs were doing fine," Lyson says.

Sheehan agrees, saying that the paper is "very well done" and supports the team's claim that it has found "the closest bone to the boundary to this point." He adds that the apparent 3-meter gap was probably due to the relative rarity of dinosaur bones throughout the Hell Creek Formation rather than a gradual extinction of these mighty animals before the asteroid struck.

But Gregory Retallack, a soil scientist at the University of Oregon, Eugene, who had earlier proposed that the gap is due to acid rain dissolving fossils after the asteroid impact, says that the team has not made a strong case. "Sadly, it is only one bone," Retallack says, adding that the find of a rare fossil 13 centimeters below the K-T boundary could be used in support of both the asteroid impact hypothesis and more gradual extinction scenarios. The reason, Retallack says, is that if gradual extinction had occurred, researchers would expect to find fewer fossils as they got closer to the boundary.

And J. David Archibald, a biologist at San Diego State University in California who thinks that receding inland seas were a key factor in dinosaur extinction, insists that the new paper "is not really news at all"; "finding one fragment of dinosaur [does not] suddenly make this gap go away; ... the gap is real."

Lyson agrees that the new study does not entirely rule out a more gradual extinction process, but he insists that it does argue strongly against one of that hypothesis's main tenets: that dinosaurs had gone extinct before the asteroid hit. Only more fieldwork in the remaining small gap, Lyson says, will resolve the issue definitively.

*An earlier version of this article stated in error that T. rex tracks had been found 37 centimeters below the K-T boundary in New Mexico. The tracks, from an undetermined dinosaur species, were found 37 centimeters below the K-T boundary in Colorado.