Animal Forecast

What Really Killed the Dinosaurs?

“An asteroid 65 million years ago” is no longer enough.

Artist Donald E. Davis depicts the Chicxulub asteroid slamming into the Yucatan Peninsula.
Artist Donald E. Davis depicts the Chicxulub asteroid slamming into the Yucatan Peninsula, the aftermath of which is believed to have caused the extinction of the dinosaurs.

Photo by Donald E. Davis/NASA/Wikimedia Commons

Sixty-five million years ago, an asteroid killed the dinosaurs. Not that the ruling reptiles made it easy—Tyrannosaurus, Triceratops, and company didn’t stand in one place, stoically waiting for the inevitable. Rather, the six-mile-wide chunk of extraterrestrial rock struck the Earth with such force that it sparked a global firestorm followed by a thick dust shroud that slowly choked whatever life persisted through the first onslaught. A reign of more than 160 million years ended in just a few days or weeks, leaving behind a charred world open to exploitation by our shrewd mammalian forebears.

That’s a nice fairy tale. But it’s not accurate. It’s more of the “based on a true story” version of what really happened as the curtain fell on the Cretaceous.

I heard the story many times as a child (as far as I’m concerned, it was best told by Christopher Reeve in the 1985 documentary Dinosaur!). The killer asteroid theory was concise, dramatic, and easy to understand. A sudden impact caught dinosaurs off-guard, and the sweeping changes happened too fast for the giant reptiles to adapt. It was only small creatures—those shielded from the cosmic fallout by burrowing underground, living in the water, or otherwise buffering themselves from the suddenly hostile outside world—that survived. For all their terrible complexity, dinosaurs perished while birds, crocodiles, turtles, mammals, and other forms of meeker life persisted.

Over the past few years, paleontologists have been revising their view of what actually happened during one of the five worst extinctions of Earth’s history. “Dinosaurs were killed by an asteroid 65 million years ago” is now an indicator of outdated understanding. For one thing, geologists have recalibrated the end of the Cretaceous Period (the final stage of the Mesozoic Era) to 66 million years ago. Granted, from the perspective of deep geologic time, a million years might not seem like much, but given that this was a world-changing event, we might as well get the date right.

More importantly, though, the Dinosauria didn’t totally expire in the catastrophe. Birds are a surviving lineage of dinosaur. This idea was punted around for a long time and gained momentum during the “dinosaur renaissance” of the 1970s. By 1996, paleontologists had begun to find fuzzy, fluffy, feathery dinosaurs that confirmed what had been proposed on skeletal grounds—birds are just an offshoot of the dinosaur family tree. So we can no longer blithely say, “Dinosaurs went extinct 66 million years ago.” All the non-avian dinosaurs perished—an absolute disaster that means I will never have a pet Apatosaurus—but, for reasons that no one has been able to understand, the avian dinosaurs survived and flourished.

To say that mammals survived the extinction doesn’t do justice to the history of life, either. We tend to think of Mesozoic mammals as all alike: small, twitchy, and living in the shadows of the dinosaurs. They were tiny, shrewlike beasts yearning for a chance to evolve if they could just get out from under the thunderous feet of their dinosaurian oppressors. Yet, while mammals did stay relatively small, they actually radiated into a variety of forms that crawled, scampered over tree branches, swam, and even glided through the prehistoric world. Mammals were not locked into a standard body form. They were a diverse bunch, and much like the dinosaurs, they were hit hard by the mass extinction.

Almost everything we know about the end-Cretaceous catastrophe comes from North America, mostly from the West. That’s because the transition between the Cretaceous world and the subsequent Paleogene period is laid out in exquisite detail, in clearly defined layers of rock. Paleontologists can follow the flow of ecological changes through time. (Paleontologists have dropped the Victorian term “Tertiary” for the past 66 million years, so the K/T boundary, with the “K” from the German term for Cretaceous, is now properly called the K/Pg boundary.)

When researchers track mammal species from the last slices of the Cretaceous through the beginning of the Paleogene, they see a dramatic transformation. Last year, Thomas Williamson, Anne Weil, and co-authors looked at the evolution of metatherians, the group that contains modern marsupials as well as their extinct, pouched relatives. At the very end of the Cretaceous, the metatherians were the most diverse mammals. About half of the mammal species discovered in eastern Montana in rock from the final days of the Cretaceous are metatherians. Yet, in the Paleogene, only two major metatherian groups can be found in all of North America. We don’t know how many mammal species passed through the boundary, but it appears to be only about 20 percent. Multiple lineages of archaic mammals disappeared, lopped off entirely from the mammalian family tree, and others suffered major die-backs. The surviving lineages provided the basis for new species that evolved in the absence of titanic dinosaurs. Had things turned out differently and the marsupial lineages continued to dominate, history would have taken a drastically different shape. Perhaps, in such an alternate universe, a marsupial-run Slate would offer articles like “Carrying Your Joey in Your Pouch: You’re Doing It Wrong.”

Lizards and snakes didn’t fare so well, either. A few months ago, Nicholas Longrich and colleagues proposed that there was also a mass extinction of lizards and snakes at the K/Pg boundary. Based on a fossil headcount and some reanalysis of previously-discovered fossils, the paleontologists proposed that 83 percent of lizard and snake species went extinct, wiping out much of the diversity that had been building toward the end of the Cretaceous. The survivors tended to be small, and, more importantly, widespread over the landscape, allowing them more options for survival under the weight of ecological catastrophe. 

As paleontologists and geologists revise dates, boundaries, and fossil identities, the end-Cretaceous extinction becomes all the more confounding. And I personally find it agonizing to think that some groups actually survived the short, sharp extinction pulse but nevertheless perished before reaching the modern era.

Ammonites, coil-shelled cephalopods that bobbed through Mesozoic seas for tens of millions of years, are often mentioned in the litany of lineages that died at the end of the Cretaceous. A section of Cretaceous marine sediment in Monmouth County, N.J., of all places, hints that some survived for centuries or even millennia after the asteroid strike. A rich ammonite burial there is overlain by a second layer with rarer ammonite fossils. Based on the geology and the identity of the fossils, paleontologists would typically call these end-Cretaceous sediments. But there is a weak band of iridium within the lower, mass-burial layer.

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.)