Flexi Says: Extinction is the complete dying out of a species. Once a species goes extinct, it can never return. More than 99 percent of all the species that ever lived on Earth have gone extinct. Five mass extinctions have occurred in Earth’s history. They were caused by major geologic and climatic events. The fifth mass extinction wiped out the dinosaurs 66 million years ago. Woolly Mammoth, Sabre-toothed Cats, Steller's Sea Cow and Sumatran Rhino are few examples of extinct animal species.