The age of the dinosaurs was growing stale long before that infamous impact.
A new study claims that dinosaurs were doomed to extinction before a city-sized space rock abruptly ended their reign some 66 million years ago. The analysis, published online Monday in Proceedings of the National Academy of Sciences, shows many species had already been dwindling for tens of millions of years.
The team’s analysis shows some of these shriveling branches on the dinosaur tree of life extend back more than 100 million years before final the extinction. And, starting around 40 million years before the impact, as existing species disappeared, fewer new species sprung up to replace them.
With diversity in decline, the overall dinosaur population was primed for a mass extinction event, the authors say. So when the asteroid hit, there were fewer species to adapt to the new conditions.
The team used a phylogenetic approach — you can think of it as an evolutionary tree containing hundreds of dinosaur species. This allowed them to examine the statistics of speciation events, which happen when certain dinosaur species appear or go extinct. It’s the first time researchers have taken such an approach. The authors do concede that sampling bias might have skewed their results. However, logic would imply that the fossil record should have fewer older specimens; instead, they found significantly fewer younger specimens exist.
The die-offs weren’t isolated to one part of the world either.
“We looked at trends across dinosaurs as a whole, in terms of their whole evolutionary history and across the entire globe,” says the study’s lead author Manabu Sakamoto of the University of Reading in the United Kingdom. “We did run a version of the analysis separating the data according to different parts of the world, but this did not have a major effect on the outcomes, meaning that the long-term demise was global.”
Sauropods, a long-lived family of massive herbivores that includes Brontosaurus, struggled disproportionately. The number of sauropod species dying off exceeded new species evolving as far back as 114 million years ago.
But in a surprise finding, their analysis shows that populations of ceratopsids like triceratops and hadrosaurs — an extremely common kind of duck-billed dinosaur — actually increased as other species disappeared.
“These two groups acquired specialized jaw structures that allowed them to process food efficiently” says Sakamoto. “They were also numerous yet very similar to their close relatives — slight variations on the same theme — and their ability to differentiate amongst themselves rather than having a single cosmopolitan species resulted in numerous new species arising.”
The Rise of the Mammal
Another surprise benefactor: mammals. Dinosaurs dominated the day, but as more and more Mesozoic megafauna disappeared, our ancestors were able to flourish in these ecological niches. Until recently, paleontologists didn’t recognize this explosion of mammals happened at the feet of the dinosaurs. It was more commonly thought that small mammals were simply the eventual benefactors of the asteroid impact.
But what could have caused this earlier die off? The researchers’ statistical method doesn’t point to a cause, but the team highlights a collection of possible calamities that would make an Egyptian pharaoh blush.
Supercontinents broke up and limiting free movement. Intense volcanism persisted for millions of years. The climate changed and sea levels rose.
Despite these factors, the team says the asteroid still delivered the coup de grâce
“The asteroid impact did indeed wipe out the dinosaurs (except birds), and while there are other alternative causal explanations, for instance, prolonged volcanism, it is not clear whether these would have triggered the mass extinction event 66 million years ago had the asteroid not impacted,” says Sakamoto.