Dinosaur Diversity Before Extinction: Fossil Record Gaps, Not Decline, May Explain Rarity - New Study

A study published in *Current Biology* on April 8, 2025, suggests that the perceived scarcity of dinosaur fossils before the asteroid impact 66 million years ago might be due to inadequate fossil preservation and limited exposure of late Cretaceous rock layers, rather than an actual decline in dinosaur populations. This challenges the idea that dinosaurs were declining in numbers and diversity prior to their extinction. The researchers analyzed approximately 8,000 fossils from North America, dating back to the Campanian (83.6 to 72.1 million years ago) and Maastrichtian (72.1 to 66 million years ago) ages. The study focused on four dinosaur families: Ankylosauridae, Ceratopsidae, Hadrosauridae, and Tyrannosauridae. Initially, the analysis indicated a peak in dinosaur diversity around 76 million years ago, followed by a decrease until the asteroid impact. However, the researchers found that this pattern likely reflects a decrease in fossil discovery due to fewer accessible rocks, rather than a true drop in dinosaur populations. They found no evidence of environmental factors or other conditions that would explain this decline, suggesting that the fossil record itself may be the primary cause of the apparent rarity. The lead author, Dr. Chris Dean from UCL Earth Sciences, noted that the probability of finding dinosaur fossils decreases in the final six million years before the asteroid impact, while the likelihood of dinosaurs having lived in these areas remained stable.