Diversity is central to one of the biggest questions paleontologists have about dinosaurs. Namely, were these ancient creatures already on the decline when that fateful asteroid came crashing out of the sky? A group of Earth scientists believes they probably were not.
While it’s true that fossils from the final dinosaur era are comparatively rare compared to those that came before, it seems the fossil record may not be telling the entire story. The point may seem moot: one way or the other, dead is dead, and this mass die-off occurred more than 66 million years ago. But among paleontologists, it’s a bit of a hot-button issue.
For decades, scientists have argued both for and against the idea that extinction rates among dinosaurs were accelerating towards the end of their reign. Those who believe dinosaurs were dwindling claim that new species were appearing with less frequency compared to previous eras. Some reachers have presented evidence suggesting a shifting climate was partially responsible, as was the emergence of hadrosaurs who proved better at competing for food than other herbivores. But those on the other side believe their colleagues simply aren’t looking hard enough for signs that dinosaurs remained just as diverse at the end as they were in the millions of years that preceded it.
The latest shot in the ongoing battle comes from a collaboration of scientists from the U.S., U.K., and China. Their argument, which they made in the journal Current Biology, is that while there is a drop in the number of fossils hailing from the final 9 million years of the dinosaur age, this is more a matter of geology and a shifting planet than it is a sign that the dinosaurs were actually on the decline.
To make their point, the Earth scientists looked at dinosaur fossils from North America that date from the last 18 million years of dinosaur existence, a span that took place between 84 and 66 million years ago. They focused in particular on dinosaurs belonging to the Ankylosauridae, Ceratopsidae, Hadrosauridae, and Tyrannosauridae families. They chose North American fossils in particular because nearly 50% of all fossils discovered from that era have been found in North America, particularly in the Western Interior Basin (WIB), where an ancient body of water once split the continent in half. The WIB “represents an unprecedented dataset for discerning patterns” in dinosaur evolution, the team wrote.
After creating a map of North America as it would have appeared at the time, including geology, geography, and climate, they divided it into grids. They then used mathematical formulas that are common in studies on modern biodiversity to calculate the likelihood of each of these families inhabiting the different grids over time. The results showed that population levels would have remained fairly stable over time.
To find an explanation for the discrepancy between this finding and the fossil record, they ran another test, this time to determine if the areas where the dinosaurs would have thrived would also be conducive to fossils being discovered, as the terrain has dramatically changed in the time since the dinos roamed. They looked at factors such as how much exposed rock exists in these areas today, how much of the area is inaccessible to humans, and how often paleontologists have bothered to look in a given area.
According to their calculations, the dinosaurs didn’t decline, but the odds of finding evidence of them did.
“The probability of finding dinosaur fossils decreases, while the likelihood of dinosaurs having lived in these areas at the time is stable,” said Chris Dean, a paleontology research fellow at University College London, who led the research, in a statement. “This shows we can’t take the fossil record at face value. Half the fossils we have from this time were found in North America. 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.”
While the research is compelling, it likely isn’t a smoking gun. If nothing else, it could point to new areas to explore in the quest to better understand the full scale of dinosaur history. It should be fun to see what the next punch thrown in this paleontologic fistfight brings.
