Dinosaurs' reign on Earth might have already been shaky before that humongous asteroid slammed into the planet some 66 million years ago.
Dinosaur species were going extinct faster than new ones were emerging, according to new research. And that pattern may have begun at least 40 million years before the impact.
"Paleontologists have discussed and argued in length over decades about whether dinosaurs were experiencing a long-term decline even before their final extinction," study lead author Manabu Sakamoto, a paleontologist at the University of Reading in England, tells The Christian Science Monitor in an interview. Or perhaps "they were just unlucky 66 million years ago when the asteroid hit."
So Dr. Sakamoto and colleagues decided to look at the the question using statistics. The team dug into the fossil record and created a statistical model to see how dinosaurs were faring before the asteroid impact. Their results are reported in a paper published Monday in the journal Proceedings of the National Academy of Sciences.
"The study takes a novel, explicitly evolutionary perspective on dinosaur diversity dynamics and their ultimate extinction," David Evans, a vertebrate paleontologist and curator at the Royal Ontario Museum in Toronto who was not part of this study, writes in an email to the Monitor.
And it turns out the dinosaurs' last days were more complex than sheer bad luck. Instead, the fearsome beasts probably experienced a perfect storm of challenges even before the large space rock did them in.
New dinosaur species emerged at staggering rates throughout much of the beasts' evolution, but speciation rates began to slow down. Eventually extinction rates surpassed speciation rates, so that the overall diversity of dinosaurs across the globe started declining. This may have happened as many as 50 million years before the impact, says Sakamoto.
"The results are consistent with a number of other recent studies that also suggest dinosaur diversity was decreasing, particularly in the last 20 million years of the Age of Dinosaurs," Dr. Evans says. "This contributes to an emerging consensus among researchers that dinosaur communities were not as robust to [resist] extinction when the asteroid hit the Earth, as they may have been a few millions of years earlier."
So what happened?
"We're pretty confident that they were in a long-term decline," Sakamoto says. But "we can't really pinpoint what caused it."
Scientists say that the Earth was not exactly idyllic leading up to the end of the Cretaceous period, which ended at the same time that asteroid hit the Earth some 66 million years ago.
There was prolonged volcanism that would have pumped greenhouse gases into the atmosphere, changing the global climate, among other dramatic climatic changes.
Sakamoto thinks the best explanation for this decrease in diversity may lie in another big change. When the dinosaurs first emerged, it was possible for the beasts to roam every corner of the terrestrial world. All the continents were stuck together into one supercontinent, Pangea. But the land had begun to spread apart into separate continents toward the end of the dinosaurs' reign.
"When the landmass is continuous, you can go far," Sakamoto says. When animals settled down far away from their relatives, they would have likely evolved into separate species over time, a process that could keep repeating itself with enough space and time. But once the space is restricted, it will be a more difficult process. Plus, he adds, animals would be competing more for space and resources.
Between the intense climate change and breakup of Pangea, could the dinosaurs have gone extinct without the asteroid?
Probably not, says Steve Brusatte, a paleontologist and evolutionary biologist at the University of Edinburgh in Scotland, who was not involved in the study.
"I think an analogy to the economy may be apt: after a big boom, growth cannot keep occurring at such a pace forever, and even if growth slows down compared to a peak, that doesn't mean the economy is necessarily doomed. It just means things aren't growing as insanely fast anymore," Dr. Brusatte says in an email to the Monitor.
"Maybe that makes the economy a bit weaker if a catastrophe hits, and this could have been what happened with the dinosaurs. The asteroid hit at a time when dinosaurs had already been around for a long time, and had already endured their really prolific periods of evolution."
But ultimately, it comes down to the asteroid, both Brusatte and Sakamoto say.
"I think there's irrefutable evidence to point a finger at the asteroid as the primary cause," Sakamoto says.
Brusatte even says it's the only answer. "No asteroid, no extinction."
"Even if dinosaurs were undergoing a burst of evolution at the time the asteroid hit," he says, "I suspect they still would have had a very tough time dealing with a six-mile-wide rock that struck with the force of several hundreds of millions of nuclear bombs."