New Evidence Debunks the Theory That Dinosaurs Were Declining Before the Asteroid

Dinosaurs were living their best life, so legend goes, until the unwelcome arrival of a killer asteroid — Chicxulub — triggered an extinction-level event 66 million years ago. That was until more recent research suggested dinosaurs were already in a state of decline and the asteroid was just the final nail in the coffin. 

Now, new findings turn this theory on its head once again, arguing that the dinosaurs’ supposed decline may, in fact, be the result of a poor fossil record.

“It’s been a subject of debate for more than 30 years. Were dinosaurs doomed and already on their way out before the asteroid hit?” lead author Chris Dean, Research Fellow in Paleontology at University College London (UCL), in the U.K., said in a press release.

According to research published in Current Biology, probably not.

If you were to look at the fossil records without greater context, it may appear as though the number of dinosaur species peaked approximately 75 million years ago and underwent a period of decline in the nine million years before the arrival of the killer asteroid. 

However, Dean and his fellow researchers argue that this has less to do with what was really happening in North America at the time and more to do with the poor quality of fossils in the lead up to Chixculub. 

“Our findings hint that, in this region at least, dinosaurs may have been doing better than previously suggested in the lead-up to the asteroid impact, potentially with a higher diversity of species than we see in the raw rock record,” Dean said in a press release.

For this study, the team analyzed North America’s fossil record (an impressive collection of more than 8,000 specimens) in the 18 years leading up to the asteroid collision (84 million to 66 million years ago). The researchers also used a technique called occupancy modelling that enabled them to determine the likelihood of a species' presence in a given area.

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The research team split the dinosaurs into four groups (or clades): Ankylosauridae (armoured herbivores), Ceratopsidae (three-horned herbivores, including Triceratops), Hadrosauridae (duck-billed herbivores), and Tyrannosauridae (carnivorous dinosaurs like the infamous Tyrannosaurus Rex). 

Meanwhile, the team divided North America into a grid that took into consideration the geology, geography, and climate as it was at the time. The researchers then estimated the likelihood of each of the dinosaur clades occupying the different grid cells at four time points within the final 18 million years of the Cretaceous period.

Their results suggest that, contrary to the fossil record, the proportion of land occupied by each of the four groups remained stable  — and, significantly, their risk of extinction was low. In fact, Ceratopsidaes may have been even more prolific towards the tail end of the Cretaceous period, thanks to their predilection for green plains away from rivers, a habitat in increasing supply at this time.

In other words, their modelling suggests the dinosaurs were not in decline. However, according to the researchers, the likelihood of finding fossils towards the end of the Cretaceous period is lower, primarily as a result of a shortage of accessible rock.

“The probability of finding dinosaur fossils decreases, while the likelihood of dinosaurs having lived in these areas at the time is stable. This shows we can’t take the fossil record at face value,” Dean said in the press release.

While it might not be enough to settle the debate once and for all, it does suggest that what might look like a decline could be the result of a poor (and misleading) fossil record.

“Dinosaurs were probably not inevitably doomed to extinction at the end of the Mesozoic,” said Alfio Alessandro Chiarenza, a paleontologist at UCL who co-authored the paper, in a press release. 

“If it weren’t for that asteroid, they might still share this planet with mammals, lizards, and their surviving descendants: birds.”

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Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• Current Biology. The structure of the end-Cretaceous dinosaur fossil record in North America

Rosie is a freelance writer living in London. She has covered science and health topics for publications, including IFLScience, Newsweek, and Health.