Sixty-five million years ago, an asteroid six miles in diameter cannoned into Earth. The collision triggered the catastrophic downfall of a vast and thriving dinosaurian ecosystem, ultimately ending with their extinction. Only that's not what happened. Not
at all exactly. (Cue Sam Neill's voice.)
Despite decades of investigation, much remains unanswered about the true nature of the dinosaurs' downfall. Now, research published in this week's issue of Nature Communications adds to an increasingly detailed (and increasingly complex) picture of their true demise, including evidence that some dinosaur populations were declining long before the asteroid hit.
Cause and Effect
"It's easy to fall into the habit of painting a simplistic story about the extinction of the dinosaurs," explains paleontologist Steve Brusatte, lead author of the study, in an interview with io9. The result, he says, is "a widely adopted misconception that [the Chicxulub asteroid] collided with a static, idyllic 'lost world.'"
It's this oversimplified conception of a stable, unvarying ecosystem that paleontologists take issue with. "We know there was an asteroid, we know there were volcanos, we know there were changes in sea level — but we also know that there were significant changes going on among dinosaurs as well."
The nature of that change has been the subject of debate for decades: were dinosaurs thriving before the asteroid hit, or were they already disappearing? The distinction is an important one, explains study co-author Mark Norrell — chair of the American Museum of Natural History's paleontology division — because while nobody doubts that a massive asteroid hit Earth at the end of the Cretaceous, people do tend to butt heads over questions of cause and effect; was this asteroid the primary culprit in the demise of the Cretaceous ecosystem, or were these biological communities eroding long before the impact?
A New Approach to a (Very) Old Problem
For decades, paleontologists have addressed this question of cause v. effect by looking at the diversity of dinosaurs in the years leading up to the end of the Cretaceous. Historically, this is has been done by estimating changes in the total number of dinosaur species. But this method of assessment has a number of shortcomings. For one thing, the results can be heavily biased by an uneven fossil record. "In places where more rock and fossils were formed, like in America's Great Plains, you'll find more species," explains Brusatte.
Total species counts also lack specificity. "People often think of dinosaurs as being monolithic," explains study co-author Richard Butler. "We say: 'the dinosaurs did this, and the dinosaurs did that'... But dinosaurs were hugely diverse. There were hundreds of species living in the Late Cretaceous, and these differed enormously in diet, shape, and size." It stands to reason, therefore, that different groups were evolving in different ways.
So Brusatte and his colleagues decided to examine dinosaur biodiversity from an entirely new angle. For the first time ever, the researchers looked at dinosaur extinction based on something called morphological disparity — the amount of anatomical diversity within particular groups of dinosaurs.
Anatomical diversity is closely tied to variations in diet, ecology, and behavior. Generally speaking, the greater a group's diversity, the better off it is. By tracking which groups were increasing or decreasing in variability in the last 12 million years of the Cretaceous, Brusatte's team could gain unprecedented insight into which dinosaurs might have been evolving and thriving, and which ones were disappearing.
A More Nuanced Picture of Dinosaur History
Using powerful databases, Brusatte and his colleagues calculated morphological disparity for seven major dinosaur groups. This allowed them to compare hundreds of skeletal features across nearly 150 different species.
When all was said and done, the researchers found that large, North American herbivores — like hadrosaurs (aka "duck-billed" dinosaurs) and ceratopsids (horned dinosaurs, including triceratops) — were declining in variability throughout the 12 million years leading up to the end of the Cretaceous.
On the other hand, small herbivores (ankylosaurs and pachycephalosaurs), carnivorous dinosaurs (tyrannosaurs and coelurosaurs), and more massive herbivores like sauropods, remained more or less stable over the same period of time. Some of these dinosaur groups even increased in biodiversity.
Interestingly, the decline in duck-billed hadrosaurs was not the same worldwide. In Asia, measurements indicate these dinosaurs were actually increasing in disparity, underscoring the importance of biogeography in the study of dinosaur extinction.
"The world is a big place," explains Brusette. "If you look today at ecosystems worldwide, you'll see that some are changing dramatically and a lot of species are being lost, while other areas aren't changing very much at all.
"During the Cretaceous, the continents were pretty much where they were today. So it's not unreasonable to expect dinosaurs in North America and dinosaurs in Asia to have been evolving in different ways."
According to Brusatte, the most striking thing his team found was that different dinosaurs were doing different things throughout the same time period, and that some groups were in fact pretty stable over those last 10—12 million years; but at the same time, two independent groups (the hadrosaurs and the ceratopsids) were also undergoing a dramatic decline:
The duck billed dinosaurs and the horned dinosaurs... were both large, both plant eaters, and both bulk-eaters. So although they were distant relatives, they were very similar ecologically. It's interesting when you see two very different groups with similar ecologies undergoing similar declines in biodiversity — it really supports that your data is real.
So were the dinosaurs destined to go extinct, asteroid or not? That's very difficult to say. Brusatte explains:
What we do know is that there were a lot of ecological changes going on during the Late Cretaceous. We also know that the two types of herbivores that were declining were among the most common species during that time period, and that they were at the base of the food chain — so it's not necessarily unexpected that these animals would have felt these environmental effects first.
Given a few more million years, Brusatte says it's likely we would have seen declines in other dinosaurs farther up the food chain. But would large, bulk-eating herbivores like hadrosaurs and ceratopsids (let alone dinosaurs as a whole) have actually died out in the long run? That's impossible to say.
"We see lots of evidence in the fossil record of animal groups going through a boom and bust pattern," explains Norrell, "where groups of organisms go from being very diverse to being very rare, only to come back in the long run."
Brusatte echoes Norrell's sentiments:
It's pretty clear that at least in North America these plant eaters were undergoing a decline, but that doesn't mean that they would have gone extinct, especially globally. If you look at dinosaur diversity over time, there are ups and downs. There's no reason to believe that the declines we see would have had to lead to an extinction; there's not anywhere near enough evidence to make that kind of conclusion. All we can say is that some dinosaurs were experiencing a decline and others were not. Whether catastrophes like an impact event or volcanism were the primary cause of the extinction remains equally unclear.
That might sound like an unsatisfying answer, but consider this: what studies like this one do is add pieces to an increasingly detailed puzzle that depicts a remarkably complex period in our planet's history — a period which, by the way, took place over sixty millon years ago. Teasing out the details of what really happened to the dinosaurs is an effort requiring multiple approaches, multiple results, and multiple re-evaluations.
It's also important to bear in mind that new ways of looking at events like the extinction of the dinosaurs are not always readily available to researchers. According to Brusatte, a study like this one (i.e. a study examining morphological disparity on a global scale) would have been impossible even as recently as five years ago. They wouldn't have had enough data to draw significant conclusions on how biodiversity was changing in these groups. Today, the fossil record that makes studies like this one possible is improving on a weekly basis.
"The more and more we're finding, the bigger databases we're building," says Brusatte, "and the better perspective we can gain on all of this."
The researchers' findings are published in the latest issue of Nature Communications.