Iridium was the key, rare-earth element that tipped geologists off that an asteroid struck the Earth at the end of the Cretaceous. While the element is scarce on Earth, asteroids and other extraterrestrial chunks of rock are rich in the stuff. The spike is a sign of extraterrestrial injection, confirmed by a 110-mile-wide crater found at just the right geologic horizon in the Yucatan peninsula and dubbed the Chicxulub Crater.* If the iridium band among the ammonite fossils is in place, Neil Landman and colleagues propose, then the mass-burial layer may represent ammonites that underwent a brief population boom in the aftermath of the impact, only to die off hundreds or thousands of years later when marine productivity collapsed.
The authors are tentative about this conclusion, but the notion that some extinct lineages survived for centuries after impact is not far-fetched. The concept fits with a phenomenon called the Signor-Lipps Effect, which holds that due to the incomplete nature of the fossil record, we will probably never find the last member of a species. The record may peter out, but that doesn’t mean that the youngest fossil represents the true end of the species. Ammonites may be good candidates for short-term survivors, and it’s entirely possible that some non-avian dinosaur populations survived for a short time after the end of the Cretaceous, too—likely in places distant from the site of impact. If only those ammonites had been a little more resilient. Then we might have been able to see them still jetting through the ocean, rather than being stuck with more mundane long-term marine survivors such as the clamlike brachiopods and frond-shaped crinoids.
An asteroid striking the Earth is a major, devastating event, but we shouldn’t expect that species all over the world were immediately extinguished. How that impact translated into extinction is poorly known. What could have wiped out all dinosaurs except birds, but also taken a heavy toll on critters like mammals, lizards, and snakes? There’s no clear sign of what made the difference between a survivor and a victim, especially since we’re mostly extrapolating from patterns we see in a small part of western North America.
A recent spat in the journal Science underscored how complex the issue has become. In a 2010 position paper marking the 30th anniversary of the 1980 paper (also in Science) that proposed an asteroid strike as the Cretaceous killer, a group of more 40 geologists and paleontologists reaffirmed that the impact was the single most important cause of the extinction. The overall pattern of geologic and fossil evidence led them to conclude that, “the Chicxulub impact triggered the mass extinction.”
Other experts quickly disagreed. Some cited immense outpourings of molten rock at the Deccan Traps, a site of fantastic volcanic activity in what is now central India, as a more important cause. Other researchers pushed the poorly supported notion that the impact preceded the extinction and had no effect, while a cadre of vertebrate paleontologists proposed a more pluralistic view that the extinction was a culmination of dropping sea levels due to climate change, fantastic volcanic activity, and the impact.
The effect of an asteroid strike seems easy enough to understand—a natural cannonball shot into Earth at a massive scale. How the other extinction triggers contributed requires a little more of a stretch of our geological imagination. Inland seas were draining off the continents and global sea level was falling. Marine life followed the changes. Toothy sea reptiles and coiled cephalopods can’t swim over ancient Kansas if there’s no longer a sea there. And fluctuating sea levels will alter global climate patterns. Similarly, 800,000 years of volcanic outpouring could have pumped enough dust and debris into the air that plants might have suffered under dark skies, kicking off an ecological collapse through the food web. When the smog cleared, carbon dioxide might have caused a quick pulse of global warming that baked vulnerable species. Alone, none of these pressures accounts for the entire pattern of extinction, but they could have stressed global habitats enough that an asteroid could have tilted the world’s ecosystems into an exceptional extinction.
We know that at the end of the Cretaceous, seas were dropping, volcanoes were erupting on a grand scale, and the asteroid impact vaporized enough iridium to leave a distinctive layer around the planet. The geological record has provided unambiguous evidence of all these events. What we don’t understand is how any—or all—of these worked in concert to create a mass extinction. Was the impact the sole trigger? Or did it intensify a more moderate extinction that was already in progress? Might it be possible that different species disappeared for different reasons—that there was no single blanket cause than can explain everything?
The documentaries I watched as a kid made it seem as if the puzzle of the end-Cretaceous extinction had been solved. The truth is that we are only just beginning to understand what happened 66 million years ago. Recognizing that an asteroid impact played a part in the massive die-off was an unexpected, magnificent discovery, but all the same, the mystery of the end-Cretaceous extinction remains. From the lineages that vanished to the timing of extinction, the pattern and causes of catastrophe are just barely coming into focus. The more we learn, the stranger the event becomes, and the fact that our ancestors survived the disaster seems all the more lucky.
Correction, Feb. 7, 2013: This article originally misstated the width of the Chicxulub Crater as 10 miles wide. (Return to the corrected sentence.)