How the dinosaurs went extinct is a contentious topic of endless scientific debate. Were they killed by a giant asteroid, a rash of volcanic eruptions or some deadly combination of the two? Or, perhaps, we've been thinking about the problem all wrong. Here's a different take. It wasn't just cataclysmic events that did in the dinosaurs — these were the final nail in the coffin. The lineage had already been crumbling for millions of years. The idea isn't new, but a study published in the Proceedings of the National Academies of Sciences today offers the strongest evidence yet that the extinction of the dinosaurs was less like a healthy tree getting toppled by a chainsaw, and more like a sickly one blowing over in a gust of wind.
"Throughout the '80s and '90s, there were all these claims that the dinosaurs lived happily up to the impact and then they were wiped out," Gerta Keller, a palaeontologist at Princeton University, who was not involved with the study, told Gizmodo. "I think this [study] kills that notion."
A dispute over what ended the reign of the biggest and baddest reptiles in Earth's history has been raging in the halls of bones for decades, ever since the discovery of the Chicxulub crater off Mexico's Yucatan Peninsula in 1980. The crater, which dates to exactly the same moment in time as the disappearance of dinosaurs in the fossil record, was lauded by some as smoking gun evidence that a giant impact triggered the KT-extinction 66 million years ago.
But other palaeontologists, including Keller, disagree. Around the same time that the Chicxulub crater was unearthed, other digs were turning up evidence of widespread volcanic activity 66 million years ago, in a series of geologic formations known as the Deccan traps. "The impact was a one-hit wonder, whereas the volcanism happened over 250,000 years," Keller said. "During that time, the dinosaurs disappeared just like that."
Most of us have heard of the giant asteroid impact, and the possibility that volcanoes played a role is also well-known. But a complementary school of thought contends that neither of these two events gives us the full story. Rather, some researchers say the dinosaur lineage was slowly pruning itself for many millions of years prior to the KT-boundary. Until now, this idea has seen limited scientific support.
"Previous studies were quite simple," Manabu Sakamoto, a paleontologist at the University of Reading and lead author on the new study told Gizmodo. "They counted the number of [dinosaur] species around at each age or time interval to see which ones were peaking or troughing when. To be honest, it's not a very statistical approach."
Sakamoto and his colleagues used a more rigorous procedure, measuring the number of times new dinosaur species emerged (so-called "speciation events") throughout geologic history. Over the Triassic and Jurassic, dinosaur diversity was on the rise, but by the early Cretaceous, speciation had begun to plateau. By the mid to late-Cretaceous, the rate of dinosaur evolution had taken a sharp downturn. It would continue to fall for millions of years before the Chixculub impact.
"New species weren't being produced as fast as species were going extinct," Sakamoto explained. "That made the dinosaurs vulnerable to drastic environmental changes — especially something like an apocalypse."
While Sakamoto's study does not delve into the reasons behind the dinosaurs' slow slide into oblivion, he said that many factors may have contributed. "The things we can model are quite limited," he said. "But we can say that, during the time when we observed a switch from slowdown to an actual decline, the world was going through some quite drastic changes." These include prolonged volcanism, the breakup of the supercontinent Pangea, and a major global cooling episode.
"They could have made a really strong case with climate," Keller said. "At the end, where the strong decline happens, is really when climate starts terminally cooling. And that's where the disappearance is very rapid."
We may never have the full story on the collapse of the dinosaurs — too much of the evidence has been lost to time. But the more we refine our tools for peering into the past, the more obvious it becomes that these impressive beasts weren't killed off in a single shot. It was death by a thousand cuts, ranging from environmental change to extraterrestrial impacts to massive eruptions. And that trajectory is disturbingly relevant when considering our planet's present course.
"We live in a world where we face unprecedented levels of extinction almost daily," Sakamoto said. "If we are going by the example of dinosaurs, it might mean were are priming our world for a mass of extinction, given some kind of a catastrophic event. By inferring things that happened in the past, we can say something about our own future."
Top: The Deccan traps, a large volcanic formation located in west-central India. Image: Mark Richards