Nature remodels the coastline. An artist's depiction of the asteroid impact 65 million years ago that many scientists say is the most direct cause of the dinosaurs' disappearance. Credit: Don Davis, NASA

Many theories have been proposed for the mystery behind the dinosaurs' ultimate displacement. Perhaps an asteroid impact, perhaps an upsurge in volcanic activity caused their extinction. How about a little (or a lot) of both? NSF has supported many researchers in their efforts to collect evidence that shines light on what may be the most hotly debated question about these monster lizards.

The leading theory holds that an asteroid impact 65 million years ago is the most direct cause of the dinosaurs' disappearance. In 1996 and 1997, two teams of scientists supported in part by NSF's Ocean Drilling Program found evidence to tie the asteroid impact, which left the Chicxulub crater off the coast of Mexico's Yucatan peninsula, to the catastrophic extinction.  

One team, led by Kenneth Miller of Rutgers University and also supported by NSF's Continental Dynamics Program, identified layers of dissimilar plankton fossils separated by a layer of debris produced by the impact. In another area of the Atlantic, the second team, led by Richard Norris of Woods Hole Oceanographic Institution, found a layer of impact debris almost 7 inches thick.

Answering the question of whether an asteroid or comet struck the earth, UCLA's Frank Kyte found fossilized bits of the guilty asteroid itself. The tiny rock fragment showed the original meteorite had a rocky composition like asteroids, and not like comets.

But was a single asteroid enough? Other research suggests that dinosaurs were having a really bad geological era.

The fossil record of clams and ammonites, according to one research team, shows the Chicxulub impact accounts for only 50 to 75 percent of the extinctions at the time. Other factors, including a precipitous drop in sea level before the asteroid struck, caused the rest of the extinctions. 

Painting an even bleaker picture with an alternate theory, Gerta Keller of Princeton University has collected, with NSF support, evidence for volcanic eruptions over half a million years, and perhaps repeated asteroid impacts, as the real dinosaur killers.

But aside from the dinosaurs unlucky enough to be caught in the impact zones of massive asteroids or trapped in the path of volcanic lava flows, there remains the question of the "kill mechanism" that caused worldwide extinctions, even while mammals and other species survived.

Climate change is the leading suspect. However, a team of NSF-supported climate scientists has argued that neither asteroids nor eruptions could inject enough long-lasting debris into the stratosphere to affect global climate on the scale needed for mass extinction.

But perhaps "hypercanes" could. A computer model by Richard Rotunno of the National Center for Atmospheric Research, Kerry Emanuel of the Massachusetts Institute of Technology, and three other scientists showed how hypercanes—theoretical superstrong hurricanes—could have arisen after asteroid impacts or undersea eruptions heated swaths of ocean, generating storms with wind speeds approaching the speed of sound and vast sheets of stratospheric clouds that could alter radiation patterns and perhaps trigger ozone depletion.