CretaceousPaleogene

Cretaceous-Paleogene, commonly abbreviated K-Pg, refers to the boundary between the Cretaceous and Paleogene periods about 66 million years ago, marking a major mass extinction event in Earth history. It is also known as the K-Pg boundary, replacing the older K-T terminology. The boundary is identified in the rock record by abrupt changes in fossil assemblages and a distinctive global stratigraphic layer.

The most widely supported cause is the impact of a large asteroid or comet, evidenced by the Chicxulub crater on the Yucatán Peninsula and a worldwide iridium-rich layer, along with other impact markers such as shocked quartz and tektites. Large-scale volcanism in the Deccan Traps in India may have contributed to climatic stress before, during, or after the impact, potentially amplifying environmental upheaval.

Extinction was profound among many life forms, including non-avian dinosaurs, pterosaurs, many marine reptiles, ammonites, and numerous plankton groups; about three-quarters of all species disappeared. Some lineages survived, such as crocodilians, turtles, birds, and most mammals. The event ended the Mesozoic era and paved the way for the Cenozoic, a period characterized by mammalian and avian diversification and the rise of modern ecosystems.

Stratigraphically, the boundary is a key marker layer worldwide, used to synchronize geological and paleontological records. Ongoing research continues to refine the timing, causes, and ecological dynamics of the K-Pg extinction and its aftermath.