The question why non-avian dinosaurs went extinct 66 million years ago (Ma) remains unresolved because of the coarseness of the fossil record. A sudden extinction caused by an asteroid is the most accepted hypothesis but it is debated whether dinosaurs were in decline or not before the impact.
The evidence for a catastrophic asteroid impact at the end of the Cretaceous period - the Chicxulub (K/Pg or K-T) event - is now considered indisputable. However, the widely accepted theory that the impact put an almost immediate end to the (non-avian *) dinosaurs has recently been questioned.
An open-access 2021 report published in Nature Communications, which analysed fossil records of more than 200 dinosaur species, suggests that they had begun substantially and progressively declining in numbers about 10 million years before the Chicxulub impact. The authors speculate that the decline may have been driven by global climate cooling and herbivorous diversity drop.
See : Dinosaur biodiversity declined well before the asteroid impact, influenced by ecological and environmental pressures Nat Commun 12, 3833 (2021)
* Note : Although all the non-avian (i.e. not birdlike) dinosaurs were forced into extinction, the avian (birdlike) species did somehow survive. The reasons for their survival are unknown. See :
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