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 “Dinosaurs were killed by an asteroid 65 million years ago” is now an indicator of outdated understanding.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.