Dinosaurs might still roam Earth if it weren't for the asteroid, study suggests

The dinosaurs were not in decline before the asteroid hit, a new study finds. Instead, poor fossilization conditions and unexposed late Cretaceous rock layers mean they're either not preserved or hard to find.

By Richard Pallardy

published 1 May 2025

in News

an illustration of Tyrannosaurus rex, Edmontosaurus annectens and Triceratops prorsus in a floodplain — An illustration of dinosaurs — *Edmontosaurus annectens* (left), *Tyrannosaurus rex* and *Triceratops prorsus* — on a floodplain from the late Maastrichtian age about 66 million years ago. A recent study suggests that poor or unexposed fossil conditions explain the low number and diversity of dinosaur fossils from the ages before the extinction.

(Image credit: Davide Bonadonna)

Dinosaurs weren't in decline when an asteroid smashed into Earth and wiped them out, scientists say. Instead, the idea that dinosaur diversity was declining before the asteroid struck 66 million years ago is likely based on faulty fossil data, according to a study that looked at nearly 18 million years of fossil evidence.

Fossil discoveries have long indicated that dinosaurs were shrinking in numbers and diversity prior to the asteroid impact at the end of the Cretaceous period. Previously, some researchers believed this was a sign that dinosaurs were already on the road toward extinction even before the cataclysmic encounter with a space rock. However, this idea has long been controversial, with other researchers arguing that dinosaur diversity was doing just fine at the time of their demise.

Rather, the Maastrichtian may have had poorer geological conditions for fossilization, the researchers suggested. Events such as the retreat of the Western Interior Seaway, which once ran from the Gulf of Mexico up through the Arctic, and the rise of the Rocky Mountains starting around 75 million years ago, may have impeded or disrupted fossilization, making it appear as if there were fewer dinosaurs and less diversity during that time.

The team also found that geological outcrops from the Maastrichtian of North America were not exposed, or were covered by vegetation. In other words, rock from this time that might hold dinosaur fossils was not readily accessible to researchers who were searching for the remains. Because half of the known fossils from this period are from North America, the study's findings may have global implications as well.

Among the 8,000 fossil records examined, the team found that Ceratopsians — a group that includes horned dinosaurs like Triceratops and its relatives — were the most common, probably because they inhabited plain regions that were most conducive to preservation during the Maastrichtian. Hadrosaurians — duck-billed dinosaurs — were the least common, possibly due to their preference for rivers. Reductions in river flow may have led to fewer depositions of sediment that could have preserved these dinosaurs, the researchers wrote in the study.

Richard Pallardy is a freelance science writer based in Chicago. He has written for such publications as National Geographic, Science Magazine, New Scientist, and Discover Magazine.