K–Pg boundary

The geological boundary marking the transition from the Cretaceous to the Paleogene about 66 million years ago. It records a mass extinction that ended non-avian dinosaurs and reshaped life on Earth.

Chicxulub impactor: The asteroid or comet whose impact produced the Chicxulub crater and contributed to the end-Cretaceous mass extinction. Its impact is the leading explanation for the abrupt environmental disruption recorded at the boundary.
Non-avian dinosaurs: All dinosaurs except the lineage of birds, which survived the end-Cretaceous extinction. Their disappearance is the boundary’s most familiar biological consequence.
Walter Alvarez: An American geologist who helped develop the impact hypothesis for the end-Cretaceous extinction. His work linked the boundary’s iridium anomaly to a catastrophic extraterrestrial event.
Cenozoic: The geological era beginning after the Cretaceous and continuing to the present. The boundary marks the start of the era in which modern ecosystems developed.
Deccan volcanism and the end-Cretaceous extinction: The study of how Deccan Traps eruptions affected climate and ecosystems near the end of the Cretaceous. Its role relative to the impact remains debated, especially for pre-extinction environmental change.
Chicxulub crater: A large impact structure buried beneath Mexico’s Yucatán Peninsula, formed about 66 million years ago. Its age, size, and location connect a physical impact site to boundary deposits worldwide.
Ammonites: Extinct marine cephalopods with coiled shells that were abundant in Cretaceous seas. Their fossil record ends at the boundary, documenting the loss of a successful marine group.
Luis Alvarez: An American physicist who coauthored the 1980 impact hypothesis with Walter Alvarez and colleagues. His expertise helped interpret the boundary’s iridium as evidence of an impactor.
Mammal evolution: The evolutionary history of mammals, including their diversification after the end-Cretaceous extinction. Mammals expanded into ecological roles left vacant by extinct groups.
Paleoclimate proxy: A measurable feature of natural archives used to infer past climate conditions. Proxy records help distinguish volcanic climate effects from abrupt impact-related changes.

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