End of the dinosaurs. Illustration of a Tyrannosaurus rex dinosaur in a depiction of the event that caused a mass extinction 65 million years ago. The Cretaceous-Paleogene (or K-T) extinction event marked the boundary between the Cretaceous (K) and Tertiary (T) periods. It is thought to have been caused by a catastrophic asteroid impact that took place at sea. Water vapour thrown into the atmosphere lowered global temperatures, and plant and then animal life began to die off. The dinosaurs never recovered, and mammals rose to become the dominant form of life.