In Earth's 4.5-billion-year history, more than 5 billion species have gone extinct, some of them at the same time. How can 75-95 percent of species on Earth disappear at one time? What event could cause such a massive die off? These are the questions that scientists have asked for decades as they explore the causes of extinction. In Extinction: What Happened to the Dinosaurs, Mastodons, and Dodo Birds? readers ages 9 to 12 learn about the scientific detective work scientists perform to answer these questions and find the culprit behind mass extinctions.