There’s no question that the dinosaurs of the late Cretaceous were dealt one of the crappiest hands in history. After an incredibly successful 170-million-year reign on the planet, the universe just couldn’t resist hucking a massive space rock at the poor animals like a total bully.
But a new study published today in Biological Reviews suggests that the story has another excruciating twist: The asteroid that capped off the Cretaceous struck when the dinosaurs were particularly vulnerable. If it had hit a few million years earlier, when their biodiversity was more robust, birds might not have been the only dinosaurian clade to pull through. And if it had hit a few million years later, when the dinosaurs had had a chance to recover, the descendents of Rexes and Triceratops might still be wandering the Earth.
The study was led by University of Edinburgh paleontologist and author Steve Brusatte, who assembled a dream team of dinosaur specialists to tackle the question. “I'm still smiling, because it's always tough to get 11 scientists to agree on anything let alone on the dinosaur extinction,” he told me over the phone.
The paper’s first major conclusion is that the impact event killed off the dinosaurs swiftly, firmly dispelling the theory that the animals slowly died out over millions of years. That finding alone is important, as it settles a debate that has been raging for decades. But Brusatte’s team decided to take their research a little further by conducting an unparalleled analysis of the fossils left by the very last dinosaurs to roam the planet.
“What we've been able to do in the analytical part of this paper is basically take the most up-to-date database of almost all dinosaur discoveries from around the world, and we applied some new pretty powerful statistical tests to that,” Brusatte explained. “We looked at how the dinosaurs were changing over the last 15 or 20 million years of their evolution, and it shows pretty clearly that there was no universal long-term decline.”
“But there was this one particular type—the big plant-eating dinosaurs, the ‘base-of-the-food-chain" dinosaurs—that actually were in a little bit of a rut,” he continued. “Their diversity had declined quite significantly. That's really what led us to this hypothesis that the asteroid hit at a particularly bad time; that if it had hit a few million years earlier, the ecosystems would have been more robust and better able to withstand the effects of that asteroid impact.”
Dwindling plant-eater populations. Image: Steve Brusatte.
The two main taxa of veggiesauruses suffering at the time were the horned dinosaurs, such as Triceratops, and the hadrosaurs, or duck-billed dinosaurs. The long-necked sauropods are mysteriously absent from the North American fossil record during this period, though they have been found in many other parts of the world.
Brusatte compared the niche that horned and duck-billed dinosaurs occupied to that of gazelles in modern savannahs. When their populations dwindled, it would have kicked off a cascade effect throughout the entire ecosystem. Using advanced simulated food-web models pioneered by fellow paleontologist Jonathan Mitchell, the team was able to see how shaky the ecological Jenga tower of the late Cretaceous really was.
The simulations were one of many interdisciplinary approaches the team applied to the question of why the dinosaurs were brutally eradicated by the impact while other species survived it.
“One of the great things about paleontology is that we're learning new things at an astronomical rate,” explained Brusatte. “People are finding a new dinosaur species somewhere around the world once a week now. So, just the database of dinosaur fossils that we have study is so much bigger than it used to be, and there are also so many new statistical techniques for looking at evolutionary trends, quantifying how groups change over time, and also taking into account the biases of the fossil record, because the fossil record is far from perfect.”
Indeed, Brusatte emphasized the need to find late Cretaceous fossil beds around the world. “Maybe our most important goal of all is to find more fossils, better fossils, and better-dated fossils of those dinosaurs that lived right at the end, but outside North America,” for example, in China and Spain. “Those records are only starting to be explored. The North American records have been explored for 150 years,” he said.
New fossils and analytical methods will help illuminate the Cretaceous-Paleogene extinction event. But perhaps more importantly, they can provide valuable context to the extinction event we’re currently entering.
“An asteroid isn't hitting, but humans are changing the planet very very quickly,” Brusatte told me. “Way quicker than most geological processes out there. I think it's really important that we understand as much as can what happened at these mass extinctions in history—what caused them, what were the effects, which organisms were targeted, which organisms seemed to do well, how quickly the recoveries were, and what the recoveries were like.”
“I think that will help us definitely understand and hopefully predict and cope better with the kind of change that is going on today,” he concluded. “That's probably the biggest reason why fossils are important. The fossil record is the only experiment that's ever been done in planning how organisms change over time, how evolution works over time, and how real organisms respond to real crises.”
In other words, the more we learn about the dinosaurs’ crummy luck, the less we’ll be doomed to share it. We can’t stop an asteroid from hitting the planet (yet) but we can at least prevent ourselves from being the root cause of mass extinction event number six.