Some dinosaur populations were already dying out during the last 12 million years of the Cretaceous — long before a big asteroid smashed into Earth, a new study claims.
The asteroid that hit 65.5 million years ago may have been just one factor among many that led to the demise of the world's non-flying dinosaurs, the research says.
"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," lead author Stephen Brusatte, a Columbia University graduate student affiliated with the American Museum of Natural History's Division of Paleontology, told Discovery News. The study was published in the latest issue of Nature Communications.
"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 study presents the first look at dinosaur extinction based on morphological disparity, meaning the variability of body structure within particular groups of dinosaurs. The more the variability in a species, generally, the healthier the population was.
Earlier research was based almost always on estimates of change in the number of dinosaur species over time — but that can be affected by uneven sampling within the fossil record.
Some geological formations, for example, tend to preserve dinosaur remains better than others.
Brusatte and his team calculated morphological disparity for seven major dinosaur groups using databases that include wide-ranging characteristics about the intricate skeletal structure of nearly 150 different species.
They discovered that large-bodied, bulk-feeding plant-eaters were dying out long before the natural disasters of 65.5 million years ago. These animals included hadrosaurs and ceratopsids.
On the other hand, small plant-eaters (ankylosaurs and pachycephalosaurs), carnivorous dinosaurs (tyrannosaurs and coelurosaurs) and huge plant-eaters without advanced chewing abilities (sauropods) remained fairly stable over the same period of time.
While the die-off of the larger species remains a mystery, Brusatte said, "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."
He explained that plant-eaters may have felt the effects of a changing land area 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.
Paul Upchurch, a University College London paleobiologist doesn't buy it and stands by the idea that a big 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 added. "We need a mechanism that explains why the smaller dinosaurs and large sauropods died out suddenly at the end of the Cretaceous."
Second, he argues that a more extensive look at all dinosaur history is needed to see if such population declines happened more than once over the 165 million years that dinosaurs were in existence.
"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 said.
Brusatte agrees that his team's 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."