Dinosaurs’ apparent decline before asteroid due to bad fossil records

Dinosaurs likely weren’t declining before an asteroid wiped them all out; instead, there may just be limited fossils from that time period, according to a new study.

It has been hotly debated whether dinosaur populations were thriving or dwindling when a huge asteroid slammed into the planet about 66 million years ago. Specifically, a drop in the availability of dinosaur fossils from the years leading up to the asteroid has led some scientists to believe the giants were doomed regardless of the impact.

Christopher Dean at University College London and his team analysed a dataset of more than 8000 fossils from four types of dinosaurs that lived between 84 million and 66 million years ago in North America, including the famed Tyrannosaurus rex and Triceratops. They found many fossils of dinosaurs from 84 million to 75 million years ago – and then that number drops in the following 9 million years leading up to the Chicxulub impact. But there was more.

When calculating how much land is currently accessible to palaeontologists from the years leading up to the asteroid’s impact and how many excavation expeditions have been undertaken in those areas, Dean’s team found there simply aren’t many of the right rocks available for today’s scientists to study.

Because palaeontologists look for fossils in ancient layers of Earth’s crust that have since been exposed to the surface, it is like working on “a puzzle where half the pieces are missing,” says Dean.

When the team used ecological models to estimate the plausible number of dinosaurs in those areas — including information about the geology and geography at the time — their calculations suggested that overall dinosaur numbers stayed stable before the asteroid impact. There weren’t fewer dinosaurs at the time; we are just less likely to find them, says Dean: “It looks like our ability to detect dinosaurs is influencing the patterns that we see in the fossil records more than anything else.”

This adds to the growing body of research suggesting there is a bias in how many fossils palaeontologists can access from North America in the 9 million years leading up to the asteroid hit, according to Manabu Sakamoto from Reading University in the UK, who was not involved in the study. Yet, he says, this doesn’t change the bigger picture of dinosaurs being in decline before the asteroid hit.

Even if dinosaurs were still populous and dominant towards the end of the Cretaceous period, there doesn’t seem to be a lot of variation in their species. Sakamoto’s research suggests that, during the 175 million years dinosaurs roamed Earth, the rate at which new species of dinosaurs appeared was slowing down overall, leading to more dinosaur species going extinct than new ones evolving.

This long-term decline in dinosaur diversity still holds true, says Sakamoto, despite the new research suggesting a bias in the available fossils: “Those two things are not mutually exclusive of each other.”