cretaceous
At the end of the Cretaceous Period, or 65 million years ago, all dinosaurs except those which led to birds went extinct. Some scientists believe a possible cause is a collision between the Earth and a comet or asteroid. The collision could have caused tidal waves, changed the climate, increased atmospheric dust and clouds, and reduced sunlight by 10-20%. A decrease in photosynthesis would have resulted in less plant food, leading to the extinction of the dinosaurs.