WHAT IS EXTINCTION: Animal populations are all classified by biologists down to groups capable of reproducing fertile offspring (as well as bigger groups of classifications, such as a genus or family). When no more individuals of a species can be found anywhere on earth, the species is considered extinct. Many animals have been as added to the endangered species list because their populations are close to becoming extinct. If one animal relies on an endangered animal for its food or protection, it too 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 after a large asteroid's impact with the Earth. That was when all the non-bird-like dinosaurs went extinct.