A new study led by researchers from University College London, published in Current Biology, analyzed the fossil record of North America covering the 18 million years leading up to the asteroid impact at the end of the Cretaceous period (between 66 and 84 million years ago).
The fossils, comprising more than 8,000 specimens, suggest that the number of dinosaur species peaked around 75 million years ago and then declined during the nine million years prior to the asteroid impact.
However, the research team found that this trend was due to the lower likelihood of fossil discoveries from that period, largely because there were fewer locations with exposed and accessible rocks from the late Cretaceous.
The lead author of the study, Chris Dean from UCL Earth Sciences, explained that his team examined the fossil record and found that the quality of the record for four dinosaur groups (clades) worsened during the last six million years before the asteroid hit.
“The likelihood of finding dinosaur fossils decreases, while the likelihood of dinosaurs having lived in those areas at that time remains stable. This shows that we cannot take the fossil record at face value,” he emphasized.
According to Dean, half of the fossils from this period were found in North America.
“Our findings suggest that, at least in this region, dinosaurs may have been living better lives than previously suggested before the asteroid impact, possibly with greater species diversity than what we see in the raw rock record,” he stated.
For the study, the research team analyzed the dinosaur clades Ankylosauridae (armored herbivores like the club-tailed Ankylosaurus), Ceratopsidae (large herbivores with three horns, including the Triceratops), Hadrosauridae (duck-billed herbivores such as the Edmontosaurus), and Tyrannosauridae (carnivores like the Tyrannosaurus rex).
They employed a technique known as occupancy modeling, previously used in ecology and biodiversity studies to estimate the probability of a species inhabiting a specific area.
The team divided North America into a grid and, based on the period’s geology, geography, and climate, estimated how many of these grid cells the four types of dinosaurs likely occupied at four different points during the last 18 million years of the Cretaceous.
During this time frame, they noted that the proportion of land likely occupied by the four dinosaur clades remained generally constant, suggesting their potential habitat area was stable and their risk of extinction remained low.
Simultaneously, they estimated the probability of detecting the four types of dinosaurs in each area, based on factors such as the amount of land accessible to researchers (i.e., whether it is covered by vegetation), the amount of relevant exposed rock, and how frequently researchers attempted to find fossils in that area.
The team discovered that detection probability declined over the four-time periods, with the most influential factor being the amount of relevant rock that was exposed and accessible.
The study concludes that dinosaurs were not inevitably doomed to extinction by the end of the Mesozoic era.
Had it not been for this asteroid, they might still share the planet with mammals, lizards, and their surviving descendants: birds.
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