A new study suggests that dinosaurs likely weren't in decline before an asteroid wiped them out 66 million years ago; instead, there may just be limited fossils from that time period, challenging the theory of pre-impact decline, according to Live Science.
Dr. Christopher Dean from UCL Earth Sciences and his team found that fewer rock formations from the final part of the Cretaceous are exposed and accessible today, making fossils from that time much harder to find.
"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?" said Dean, according to SciTechDaily.
The study, published in the journal Current Biology, examined the North American fossil record from the last 18 million years of the Cretaceous period, between 84 and 66 million years ago. Dean and his team analyzed a dataset of more than 8,000 fossils from four types of dinosaurs that lived during this time, focusing on families like Ankylosauridae and Ceratopsidae.
Among the 8,000 fossil records examined, Ceratopsians, including horned dinosaurs like Triceratops, were the most common, which may indicate a thriving population prior to the asteroid impact. However, the number of dinosaur fossils found drops in the following nine million years leading up to the Chicxulub impact.
The researchers found that this pattern likely reflects a decrease in fossil discovery, rather than a true drop in dinosaur populations. "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 added.
They adopted a technique called occupancy modeling, previously used in ecology and biodiversity studies. The researchers divided North America into a grid and estimated the likelihood of the four dinosaur types being detected in each area, based on the geology, geography, and climate of the time.
They found that during this period, the proportion of land the four dinosaur clades likely occupied remained constant overall. This suggests their potential habitat area remained stable and the risk of extinction stayed low. The team discovered that the likelihood of detection declined over the four time periods, mainly due to how much relevant rock was exposed and accessible.
"If we take the fossil record at face value, we might conclude dinosaurs were already experiencing a decline before their final extinction. In this study, we show that this apparent decline is more likely a result of a reduced sampling window, caused by geological changes in these terminal Mesozoic fossil-bearing layers—driven by processes such as tectonics, mountain uplift, and sea-level retreat—rather than genuine fluctuations in biodiversity," said co-author Dr. Alessandro Chiarenza from UCL Earth Sciences.
Events such as the retreat of a large inland sea that split the continent in two and river systems feeding the sea drying up contributed to these changes. These geological shifts may have impeded or disrupted fossilization, leading to fewer fossils being available from the period before the asteroid impact.
This research opens the possibility that if not for the asteroid, dinosaurs might still roam the Earth. "Dinosaurs were probably not inevitably doomed to extinction at the end of the Mesozoic [252 million to 66 million years ago]. If it weren't for that asteroid, they might still share this planet with mammals, lizards, and their surviving descendants: birds," said Chiarenza, a paleontologist at University College London, according to Live Science.
The study suggests that the apparent drop in dinosaur diversity may be due to gaps in the fossil record, not an actual decline in species. By accounting for the incomplete nature of the fossil record, the researchers argue that dinosaurs were likely flourishing until the catastrophic asteroid impact ended their reign.
The article was written with the assistance of a news analysis system.