K–T boundary

The K-T boundary marks the transition between the Cretaceous and Tertiary periods of geologic time, approximately 66 million years ago. This boundary is characterized by a mass extinction event that eliminated about 80 percent of all species, including nearly all dinosaurs and many marine invertebrates.12

The name "K-T" comes from the German word for chalk (Kreide) for the Cretaceous period and "Tertiary" for the subsequent period. While traditionally called the K-T extinction, it is now more commonly referred to as the Cretaceous-Paleogene (K-Pg) extinction event.23

The leading hypothesis for this extinction event is the impact of a large asteroid, which would have ejected debris into the atmosphere, blocking sunlight and disrupting the food chain. Evidence supporting this includes a large crater near Chicxulub, Mexico, and the presence of iridium, an element rare on Earth but common in extraterrestrial rocks, in deposits from this time. Other proposed causes include volcanic activity and climate change due to continental drift.2