Death of the dinosaurs. Artwork of two Tyrannosaurus rex dinosaurs attacking a Triceratops (lower right), oblivious to the asteroids about to strike the Earth behind them. The impact of a huge asteroid or comet may have caused the extinction of the dinosaurs and 70% of all species on Earth. The impact occurred about 65 million years ago at the end of the Cretaceous geological period. The object, thought to be 10-20 kilometres across, struck the Earth at Chicxulub on the Yucatan peninsula, Mexico. It threw huge amounts of debris into the upper atmosphere, blocking out sunlight and causing global climate changes.