The extinction event that killed the dinosaurs effected the entire earth! Plants and animals on both land and in water were affected. The disappearance of other living things was so great that scientists knew about the extinction 30 years before the first dinosaur was discovered. 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. There are two groups of extinction theories: catastrophic extinction and gradual extinction. Catastrophic extinction would have been caused by a sudden, external event, such as the collision of the earth with an asteroid, or the eruption of a series of gigantic volcanoes. Gradual extinction would have been the result of changes in the earth's land mass and climate shifts. It could also have been because new and better animals won in the struggle for existence.
The extinction event did not kill all animal and plant life. Many kinds of animals survived, including fishes, frogs, turtles, crocodilians, birds, and mammals. Scientists must take the fossil record and find reasons for all extinctions.