Some Dinosaurs Were Already Dying Out Before Asteroid Impact
May 2, 2012

Some Dinosaurs Were Already Dying Out Before Asteroid Impact

Lawrence LeBlond for

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An asteroid impact may have ended the reign of the dinosaurs here on Earth about 65 million years ago, but new evidence suggests many of the large, plant-eating dinosaurs were already dying out during the last 12 million years of the Cretaceous period.

The findings, by an international team of US and German scientists, do not dispute the mass extinction that killed off all the dinosaurs at the end of the Cretaceous era. They do, however, suggest that at least some of the great lizard species were on their way out beforehand.

The study, led by the American Museum of Natural History and published Tuesday May 1 in the journal Nature Communications, suggests that large-bodied herbivores, like triceratops, were in slow decline well before the Cretaceous extinction event, while their medium-sized relatives and meat-eating rivals were flourishing.

It was a hotly debated topic for years - whether dinosaurs were struck down in their prime or whether they were already on a steady path toward extinction. Now, after extensive research, scientists think they know what really happened.

“A lot of the time people think of the dinosaurs going extinct: ℠oh, you know, an asteroid did it ... the dinosaurs were doing just fine, an asteroid came along and killed them all off´,” lead author Steve Brusatte, a Columbia University graduate student affiliated with the Museum of Natural History, told the AFP news agency.

“We found that it was probably much more complex than that, and maybe not the sudden catastrophe that is often portrayed,” he said. “I think our study highlights the fact that we still have a long way to go until we fully understand the extinction of the dinosaurs,” Brusatte told Jennifer Viegas of Discovery News.

“There are a couple of things we know for sure,” he added. “We know a large asteroid or comet hit the planet about 65.5 million years ago, right when the dinosaurs completely disappeared from the fossil record. We also know there was massive volcanism and major sea level changes at this time. We now also know that at least some groups of dinosaurs were undergoing long-term declines in biodiversity during the final 12 million years of the Cretaceous, at least in North America.”

The research is the first of its kind to look at dinosaur extinction based on “morphological disparity” -- the variability of body structure within particular groups of dinosaurs.

The team looked at the changes in biodiversity within different dinosaur groups (150 different species) over time to get a picture of how each group was faring. An increase in one group´s variability could indicate that it was evolving into more species, improving its odds of survival. Meanwhile a decrease could be the first signs of extinction.

The duck-billed hadrosaurs and frilled ceratopsids may have been a few of the dinosaurs experiencing decline in biodiversity over the last 12 million years of the Cretaceous, according to the researchers. But it℠s also more complicated than just saying all hadrosaurs were declining; they only seemed to be declining in North America, while increasing in biodiversity in Asia. And others, including the heavily armored ankylosaurs, the terrifying T. rex, and the enormous sauropods, remained stable and even some slightly increased in biodiversity.

The large herbivores were “becoming more similar to each other, they were losing variability,” Brusatte told AFP. “Usually when you see these big decreases in the anatomy like this, that means that a group is in trouble.”

We know for certain that an asteroid hit the Earth sometime 65 million years ago, but we also now know that it “didn´t hit a world that was totally OK, [it] didn´t hit a static world,” Brusatte added. The reason for the decline is unclear but “was probably something ecological,” he said.

Previous studies were based almost exclusively on estimates of changes in the number of dinosaur species over time. Brusatte said basing research on that alone can cause some inaccuracies.

“By looking just at trends in taxonomic diversity, you get conflicting answers about the state of dinosaurs prior to extinction,” he said in a prepared statement. “This is because the results can be biased by uneven sampling of the fossil record. In places where more rock and fossils were formed, like in America´s Great Plains, you´ll find more species. We wanted to go beyond a simple species count for this study.”

“People often think of dinosaurs as being monolithic–we say ℠The dinosaurs did this, and the dinosaurs did that´,” said study coauthor Richard Butler, a research fellow at Ludwig Maximilian University in Munich. “But dinosaurs were hugely diverse. There were hundreds of species living in the Late Cretaceous, and these differed enormously in diet, shape, and size. Different groups were probably evolving in different ways and the results of our study show that very clearly.”

“Something was going on with large herbivores in the late Cretaceous, at least in North America. Maybe it was the fact that the local environments were in flux due to drastic sea level changes and mountain building at the time,” Brusatte pondered.

Maybe the plant-eaters felt the effects of a changing landscape first since they sat at the bottom of the food chain. “Given a few more million years we would have seen declines in other dinosaur groups higher up in the food chain,” he said.

Brusatte and colleagues note that their findings show a fairly reasonable picture of what happened in North America, where extreme fluctuations of the inland Western Interior Sea and mountain building may have affected dinosaur evolution in distinct ways. They acknowledge that the North American record may not be representative of a global pattern, and stop short of saying biodiversity played a strong role in large herbivoran dinosaur declines elsewhere in the world.

The researchers note that there is no way to tell whether declining dinosaur groups would have survived even if the asteroid had not hit Earth.

“Even if the disparity of some dinosaur clades or regional faunas were in decline, this does not automatically mean that dinosaurs were doomed to extinction,” said study coauthor Mark Norell, chair of the Division of Paleontology at the Museum of Natural History.

“Dinosaur diversity fluctuated throughout the Mesozoic, and small increases or decreases between two or three time intervals may not be noteworthy within the context of the entire 150-million-year history of the group,” he added.

University College London paleobiologist Paul Upchurch, however, doesn´t see any concrete evidence in the findings and stands by his strong belief in the idea that an asteroid wiped out the dinosaurs.

“First, only some dinosaur groups show reduced disparity in the final 12 million years, while other groups continue to do well. So this study could actually be taken as evidence in favor of a sudden extinction,” Upchurch told Discovery News. “We need a mechanism that explains why the smaller dinosaurs and large sauropods died out suddenly at the end of the Cretaceous.”

Secondly, he argued that a more extensive look at all dinosaur history is needed to see if such population declines happened more than once over the dinosaurs´ 165-million-year reign. “The decline in disparity during the final 12 million years might merely be ℠evolutionary business as usual´ and have little to do with the true final extinction,” he added.

Brusatte agrees that the findings are debatable, “but at the very least we can't envision the latest Cretaceous as a static, idyllic lost world that was suddenly exterminated by an asteroid impact. Instead, the dinosaurs living during this time were undergoing major changes before the asteroid hit.”


Image Caption: Tyrannosaurus rex is part of the carnivorous groups of dinosaurs that, according to new research, maintained a stable level of biodiversity leading up to the mass extinction at the end of the Cretaceous. © AMNH/J. Brougham