Death of the dinosaurs. Illustration of an asteroid above a T. rex dinosaur. The Cretaceous-Paleogene (K-Pg), or K-T, extinction event 65 million years ago marked the boundary between the Cretaceous (K) and Tertiary (T) periods. It is thought to have been caused by a catastrophic asteroid, or comet, impact that took place at sea, just off the Yucatan Peninsula. 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.