What is a KT boundary?

The K-T boundary is the dividing line between the Cretaceous and Paleogene (formerly Tertiary) periods that occurred around 66 million years ago. It is designated by the K-T extinction event, which was a mass extinction that wiped out the non-avian dinosaur species previously dominant on the planet.

While the exact cause of the extinction event is unknown, the leading hypothesis points to at least one massive asteroid impact that caused global climate change. The Chicxulub Crater in Mexico shows evidence of an enormous impact at around the same time as the extinction, and the presence of iridium in contemporary geologic layers suggests that debris from the impact spread around the globe.