Natural selection caused the extinction of the non-avian dinosaurs. Earth’s environment changed abruptly after the impact of the Chixulub bolide, and most of the dinosaurs could not survive under these changed conditions. Other species (in particular, mammals and crocodilians) were better adapted to the harsh post-impact environment, and so they survived to occupy the ecological niches formerly occupied by the dinosaurs while the latter starved and died.
One small group of winged, feathered dinosaurs did manage to survive, and we call them “birds” today.
