This all happened 65m years ago at the end of the Cretaceous period, when dinosaurs still roamed the earth. Suddenly, between 60 and 80% of all living species became extinct. Until the 1980s, this catastrophic loss of life was a mystery, but then scientists found a clue ?" traces of the element iridium in rocks of this age. Iridium generally falls to Earth with extraterrestrial objects. This suggested a massive asteroid collided with the planet and that this could be responsible for the mass extinction.