One of the great debates in paleontology has been updated. According to a new paper in Nature Communications the diversity of dinosaur species was declining in the last 10 million years of the Cretaceous Era. However, that doesn't mean that the old theory the dinosaurs were on their way out, asteroid or no asteroid, has necessarily been vindicated.
Fossil records show fewer dinosaur species in the immediate lead-up to the asteroid impact than millions of years earlier, particularly in North America. This observation has set off furious debate. According to one interpretation, dinosaurs were teetering on the edge of extinction 66 million years ago, and the asteroid provided nothing more than the final push. Others have challenged this idea, arguing the apparent lack of species reflects a decline in fossil-preserving geology on the western side of North America's inland seaway.
Broader sampling indicates the species decline was real, the authors of the new study claim, with more extinctions than new species formation over the period 76-66 million years ago. "We looked at the six most abundant dinosaur families through the whole of the Cretaceous, spanning from 150 to 66 million years ago, and found that they were all evolving and expanding and clearly being successful,” lead author Dr Fabien Condamine of the Université de Montpellier said in a statement. "Then, 76 million years ago, they show a sudden downturn. Their rates of extinction rose and in some cases, the rate of origin of new species dropped off."
Condamine and co-authors claim this decline holds up even once one allows for variations in the quality of the fossil record available from different times, as well as other possible sources of error.
"We also looked at how these dinosaur ecosystems functioned, and it became clear that the plant-eating species tended to disappear first, and this made the latest dinosaur ecosystems unstable and liable to collapse if environmental conditions became damaging." said co-author Dr Guillaume Guinot.
The authors think the major cause of this decline was climate change. Although there were plenty of fluctuations, the very long-term trend in the Late Cretaceous took place about 50,000 times more slowly than it is doing today, and in the opposite direction. Slow as this was, warmth-adapted dinosaurs apparently couldn't cope. The cooler temperatures towards the end also allowed for a modest uptick in mammalian abundance, possibly squeezing smaller dinosaurs from some niches.
A reduction in species doesn't necessarily mean fewer individual animals. Condamine and Guinot found the hadrosaurs were flourishing until very close to the end, and may have pushed other species to extinction. Perhaps the planet was teeming with them to the point there was little food for others. A more diverse ecosystem is healthier and more resilient, but an abundance of one family doesn't really suggest the dinosaurs were in grave danger, barring one very unpredictable event.
Meanwhile, the Atlantic Ocean at the time supported more large predator species than any ecosystem we know of before or since. Since these all went extinct at the same time as the dinosaurs (birds aside) did on land, it's certainly not the case that only the animals already in decline were pushed over the edge by the asteroid's impact.