What is extinction: Animals are all classified by biologists into separate species (as well as bigger groups of classifications, such as a genius or family.) When no more individuals of a species can be found anywhere on earth, the species is considered extinct. Many animals have been classified as on the endangered species list because their populations are close to becoming extinct. If one animal relies on another for its food or protection, it can become part of the extinction chain. Possibly the most famous extinction happened at the end of the Cretaceous period, about 65 million years ago, when most of the species on Earth were wiped out by a large asteroid's impact with the Earth. That was when all the non-bird-like dinosaurs went extinct.