Which mass extinction event marked the end of the non-avian dinosaurs?

The Cretaceous–Paleogene extinction event marked the end of the non-avian dinosaurs.

Usually called the K–Pg extinction, this global catastrophe occurred about 66 million years ago at the boundary between the Cretaceous and Paleogene periods. It eliminated all non-avian dinosaur groups, while one dinosaur lineage—modern birds—survived. The event also wiped out many marine reptiles, ammonites, and other organisms, with roughly three-quarters of species disappearing according to widely used estimates.

The leading cause was the impact of a large asteroid near today’s Chicxulub crater on Mexico’s Yucatán Peninsula. The impact threw dust, sulfate aerosols, and debris into the atmosphere, reducing sunlight and disrupting photosynthesis. Fires, shock waves, tsunamis, cooling, and the collapse of food webs added to the devastation. Large animals with high food demands were particularly vulnerable.

The extinction is sometimes called the K–T event, but “K–Pg” is now preferred because the formal geologic time scale replaced the old “Tertiary” designation with Paleogene. Scientists still debate whether some dinosaur groups were already declining before the impact, but the boundary marks their definitive global disappearance.