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Recent research led by University College London (UCL) challenges the long-held belief that dinosaurs were on a downward trajectory prior to the catastrophic asteroid event 66 million years ago. This study suggests that the perceived decline may be attributed to gaps and biases in the fossil record rather than an actual reduction in dinosaur diversity.
Published in Current Biology, the study scrutinized the fossil records from North America over an 18-million-year period leading up to the asteroid impact at the close of the Cretaceous period, covering time frames between 66 and 84 million years ago.
The team’s findings indicate that, based on the analysis of over 8,000 fossils, the number of dinosaur species appears to have peaked roughly 75 million years ago, followed by a decline in diversity in the nine million years preceding the asteroid strike. However, these conclusions may be misleading.
Dr. Chris Dean, the lead author, explained, “For over three decades, the question has lingered — were dinosaurs already facing extinction before the asteroid’s arrival?” He pointed out that the research revealed a significant decline in the quality of the fossil record for four major dinosaur groups during the last six million years before the impact. This deterioration means that while the likelihood of finding dinosaur fossils decreased, it does not reflect a true decline in their existence or diversity in those areas.
“In North America, which provides half of the fossil samples from this period, our results suggest that dinosaur diversity may have been underestimated,” Dr. Dean noted. “It indicates that they potentially thrived more than what the fossil evidence suggests leading up to the asteroid impact.”
The study focused on four specific dinosaur groups: the Ankylosauridae, armored herbivores like the Ankylosaurus; the Ceratopsidae, which includes the well-known three-horned Triceratops; the Hadrosauridae, featuring duck-billed herbivores such as Edmontosaurus; and the carnivorous Tyrannosauridae, represented by Tyrannosaurus Rex.
To conduct their analysis, the research team employed a technique known as occupancy modeling, typically used in ecological studies, to estimate the probability of dinosaur habitation across North America during the final 18 million years of the Cretaceous. The continent was segmented into a grid, and the authors assessed how environmental changes in geology, geography, and climate might have influenced the presence of these dinosaur clades over different time intervals.
The results indicated that, despite the drop in fossil detectability, the extent of land likely inhabited by these four groups remained stable. This suggests that their available habitats did not notably shrink and that their chances of extinction were low.
The researchers examined factors affecting fossil discovery, including accessibility of land covered by vegetation and the search efforts conducted in various regions. They found a marked decline in the detectability of fossils over the study period, largely influenced by how accessible, exposed rock formations were.
Interestingly, the study also revealed that, unlike the other groups, Ceratopsian dinosaurs demonstrated an increased likelihood of detection in later years of this timeframe, potentially inhabiting a broader range of areas. This was attributed to their affinity for green plains, which became more dominant as environmental conditions changed due to the retreat of an inland sea that once divided the continent.
Co-author Dr. Alessandro Chiarenza also addressed the implications of their findings, stating, “If we interpret the fossil record without careful scrutiny, it might support the idea of a pre-extinction decline in dinosaur populations.” He emphasized that the apparent decline is more likely attributed to geological shifts affecting the accessibility of fossil-bearing layers rather than reflecting true biodiversity changes.
“Dinosaurs weren’t necessarily destined for extinction at the end of the Mesozoic era. Absent that asteroid impact, they might still coexist with mammals, lizards, and their avian descendants, the birds, in today’s world,” Dr. Chiarenza concluded.
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