The extinction event that killed the dinosaurs was worldwide. It affected many plant and animal groups, both on land and in water. Dinosaurs were only a small part-the disappearance of other living things was so great that scientists knew about the extinction 30 years before the first dinosaur was described.
The victims of the Cretaceous extinction included dinosaurs, ammonites (mollusks related to the octopus and the chambered nautilus), pterosaurs, and certain plant groups. But many other animal groups, even some large-bodied reptile groups like champsosaurs, were not affected.
The image of the last majestic dinosaurs passing away and leaving a world of shrew-like mammals and cold-blooded reptiles is false. Instead, many of the major modern land animals were already living in the Cretaceous. Dinosaurs shared their last million years with modern creatures.
Since more than just dinosaurs became extinct, reasons that only explain why dinosaurs died can be ruled out. For instance, there is one theory that disease caused the extinction of dinosaurs. But a disease could not have caused the extinction of plants and animals over the whole world.