Dinosaurs aren’t dead. Not completely. Birds carry on their Mesozoic legacy. But all the forms that inspire our dreams and nightmares – the tyrannosaurs, ceratopsids, sauropods, and their ilk – are all gone. They were wiped out in a blink of geologic time, roundabout 66 million years ago. How could a lineage as long-lived, diverse, and widespread as the non-avian dinosaur be exterminated so thoroughly?
As a little dinomaniac growing up in the 80s, the answer seemed simple. An asteroid struck the planet and struck down the dinosaurs in a stroke of cosmic bad luck. All the documentaries I begged my parents to tape for me were agreed on this point.
Stephen Brusatte, a paleontologist at the University of Edinburgh, grew up watching many of the same shows and reading the same dinosaur books as I did. “The way dinosaur extinction was presented to me, there was definitely an asteroid for sure,” Brusatte recalls. This annoyed many vertebrate paleontologists who had traditionally preferred terrestrial extinction triggers – like sea level fluctuations, volcanic activity, and climate change – but, in time, asteroid impact became accepted as a real and deadly phenomenon in the end-Cretaceous extinction.
Arguments didn’t end there, though. A new debate emerged, Brusatte says, which hinged on the terms “abrupt” and “gradual.” While some experts suspected that dinosaurs were in an extinction freefall, and all but gone by the time the asteroid struck, other paleontologists hypothesized that the impact struck while dinosaurs were in their prime.
Now, after three decades of argument over the role of the asteroid in the planet’s last major mass extinction, paleontologists are starting to zero-in on what actually happened as the Cretaceous curtain fell. “There’s a lot more evidence than we used to have,” Brusatte says, “And that evidence helps show that the dinosaur extinction isn’t this black or white, abrupt or gradual thing. All the new evidence is showing it’s a little more complicated.”
Brusatte and ten of his paleontological peers summarize the emerging view in a Biological Reviews paper just out this week.
Plenty of ink has been spilled about the Cretaceous-Paleogene (K/Pg) extinction, but the new paper will be an important milestone for dinosaur researchers. “When we were doing background research,” Brusatte says, “it became pretty clear that there were a lot of reviews out there, but a big group of dinosaur workers had never sat down and said ‘What do we all agree on?'” The emerging consensus is a more nuanced extinction scenario than what often reaches the media. “We’re starting to put together a story that the asteroid did it, the asteroid did it quick, but the asteroid struck at a really bad time,” Brusatte says.
By looking at dinosaur evolution at the end of the Cretaceous, as well as previously-proposed extinction triggers, Brusatte and his colleagues were able to rule out some longstanding extinction hypotheses. “The extinction was geologically abrupt,” Brusatte says, and his team “removed some of these ideas about climate change and changes in sea level” as main drivers of devastation. There’s no sign of a long-term dinosaurian decline, nor any strong evidence that volcanic activity at the end of the Cretaceous was driving extinction. The asteroid stands out at the primary cause of cataclysm.
Yet this doesn’t mean that the end of the Cretaceous was a dinotopia. In North America, at least, there were fewer species of dinosaur living in the same habitat at the end of the Cretaceous than there had been 10 million years earlier. And in terms of differences in dinosaur body form – called disparity – there were fewer sorts of large herbivore present just before the asteroid struck. This could reflect changes in climate and sea level that were restructuring dinosaur communities.
Changes in sea level and climate would not have totally wiped out the dinosaurs in the long term – as a whole, dinosaurs had been through such changes before – but they may have left North America’s dinosaurs more vulnerable to the rapid ecological change brought on by the asteroid impact. North America’s dinosaurs were caught in an awkward transition. “Maybe if the asteroid hit a few million years earlier, dinosaurs might have been more robust”, Brusatte says, adding “Maybe if the asteroid hit a few million years later, it’s not as bad, either.”
As Brusatte and his co-authors point out, though, most of what we know about the K/Pg extinction comes from a very small geographical area. Montana acts as a proxy for the rest of the world as it’s one of the few places where there’s a continuous geologic record from the end of the Cretaceous through the beginning of the Paleogene, offering the essential Before and After shots of life on Earth. Those exposures are difficult to find elsewhere, and, where they exist, haven’t been studied nearly in as much detail. “For now, it’s really a North American story”, Brusatte says, “but there is some emerging data from China and Spain that can be used to test some of the ideas we put forward.”
Studying such exposures is just part of what Brusatte and coauthors suggest as a wider-ranging research program to understand the K/Pg catastrophe. The researchers list nine areas of research that could help provide a finer-scale view of what happened, from better sampling of Late Cretaceous dinosaurs outside North America to detailed analysis of dinosaur ecology over their last 10 million years. The tale of dinosaurian demise isn’t all told yet, and, as Brusatte notes, “We’re getting to the point where we can tease out some of that nuance.”
Brusatte, S., Butler, R., Barrett, P., Carrano, M., Evans, D., Lloyd, G., Mannion, P., Norell, M., Pepe, D., Upchurch, P., Williamson, T. 2014. The extinction of the dinosaurs. Biological Reviews. doi: 10.1111/brv.12128