Dinosaurs Probably Weren't Declining When The Asteroid Hit

It was almost certainly sunset for sauropods even before the asteroid wiped out the non-avian dinosaurs. However, many other families of dinosaurs were still flourishing, and would probably still dominate the Earth were it not for that fateful event. Catmando/Shutterstock.com

Paleontologists have long believed the number and diversity of dinosaurs were falling before the arrival of the asteroid that caused the Chicxulub crater. The reasons for this have been much debated, and the observation has even been used to cast doubt on the theory the asteroid impact caused dinosaurs' extinction (birds aside). Yet the whole idea may be an illusion.

Before the discovery of a giant crater off Mexico, widespread belief in a long-term dinosaur decline caused many to favor slow explanations for their mass extinction, such as competition from mammals. Even once the evidence for an asteroid impact became overwhelming, many argued this just hastened the end for those beasts that were on the way out anyway.

For his doctorate at the University of Bath, Joe Bonsor studied each major group of dinosaurs and looked at the rate of new species production during the late Cretaceous. He concludes that reports of the slow demise of dinosaurs are greatly exaggerated.

Gaps in the fossil record make it challenging to measure the richness of any family of long-dead animals over time. “The large unavoidable biases in the fossil record and lack of data can often show a decline in species, but this may not reflect the reality at the time,” Bonsor said in a statement.

There are statistical techniques to try to address this, allowing for periods where there were either not a lot of good fossil-preservation sites or those that did exist are now relatively inaccessible. However, Bonsor disputes the conclusions some predecessors have drawn, arguing they have prioritized models that show downturns over those consistent with abundant new species.

“Previous studies have used various methods to draw the conclusion that dinosaurs would have died out anyway, as they were in decline towards the end of the Cretaceous period,” Bonsor said. “However, we show that if you expand the dataset to include more recent dinosaur family trees and a broader set of dinosaur types, the results don’t actually all point to this conclusion — in fact only about half of them do.”

Bonsor and co-authors analyzed the rates of new species formation in the journal Royal Society Open Science, rather than the number of species apparently present at a specific time. Some dinosaur families might have been doomed even without the asteroid, the sauropods being the most notable example. Others, including hadrosaurs that were recently discovered to have reached Africa just before the impact, were on the rise and might well be still dominant today had things gone differently.

Bonsor is not the first to challenge the idea of a late-Cretaceous species decline. Last year, a paper noted that most evidence comes from western North America. Changing conditions west of the sea lane that divided the continent at the time meant fewer fossil-bearing deposits between 75 and 66 million years ago, creating an illusion of species loss. Moreover, European sites have never been so indicative of a downward trend.

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