The dinosaurs might have been killed off by more than one asteroidS

The theory that a giant asteroid impact killed the dinosaurs 65 million years ago gained acceptance when we discovered the gigantic Chicxulub crater. Now we've found the crater of another asteroid that struck Earth at almost the exact same time.

The Chicxulub crater, buried beneath Mexico's Yucatan peninsula, was recently confirmed by a panel of 41 experts as the main driver of the mass extinction event that occurred at the end of the Cretaceous Period. It measures over 110 miles in diameter, and the asteroid that created it would have been more than big enough to trigger the atmospheric and geological turmoil that started the extinction event.

Of course, this theory doesn't enjoy unanimous support. Recent research suggested it might have actually been the Deccan Traps volcanoes in India that triggered the mass extinction, and these findings were compelling enough to call together that panel of experts to review the evidence. Although they ultimately stuck with the current consensus, they left open the possibility that the Chixculub asteroid created geological disturbances that ultimately started the Deccan Traps eruptions.

But even that might not be the whole story. A team of British geologists have turned their attention to Ukraine's Boltysch crater, first discovered in 2002. The crater is relatively small - only about 15 miles in diameter - but the researchers believe it was created by an asteroid that hit Earth just before the Chicxulub asteroid. Previous dating had suggested the Boltysch impact occurred around 65.5 million years ago, putting it at roughly the same geological time as the Chixculub impact. Of course, going by that data, it might have occurred half a million years earlier, which would suggest the two had no effect on each other.

To get a better sense of when the Boltysch impact occurred, the researchers studied fossilized plant pollen and spores around the crater, discovering ferns had colonized the area immediately after the impact. That makes sense, because ferns tend to spring up around recently devastated regions. This is known as "fern spikes." But they also found evidence of a second fern spike about a meter above the first one, which means it must have happened about two to five thousand years later. They believe this second spike was caused by Chicxulub, placing the two impacts at pretty much the exact same geological moment.

So what does this mean? If nothing else, it means the Cretaceous extinction event was more complex than we thought, and it likely doesn't have a single cause so much as a mix of major (Chicxulub, the Deccan Traps) and minor (Boltysch) ones. Researcher Simon Kelley believes this could just be the beginning of a wave of impact crater discoveries, revealing the extinction event was actually caused by a massive asteroid shower that lasted thousands of years.

[Geology; artist's rendering of Chicxulub impact by Don Davis.]