Cretaceous–Paleogene boundary

The geological boundary about 66 million years ago marking the end of the Cretaceous Period and the start of the Paleogene Period. It coincides with a mass extinction that eliminated non-avian dinosaurs and many other groups.

Chicxulub impactor: The asteroid whose impact at Chicxulub, Mexico, formed a large crater about 66 million years ago. Its impact is the leading cause of the abrupt extinction associated with the boundary.
K–Pg boundary clay: A thin clay-rich layer found at many sites that records the transition between Cretaceous and Paleogene rocks. Its unusual chemistry and position mark the boundary in widely separated sedimentary sequences.
Non-avian dinosaurs: Dinosaurs outside the bird lineage, all of which became extinct at the end of the Cretaceous. Their disappearance distinguishes the boundary from earlier geological transitions.
Luis Alvarez: An American physicist who proposed that an asteroid impact caused the end-Cretaceous mass extinction. His impact hypothesis grew from the iridium measurements at boundary sites.
Deccan Traps volcanism hypothesis: The hypothesis that extensive Deccan Traps eruptions drove or contributed substantially to the end-Cretaceous extinction. It offers a competing or complementary cause to the Chicxulub impact explanation.
Chicxulub crater: A buried impact structure beneath the Yucatán Peninsula, formed about 66 million years ago. Its age and location connect the impact directly to the boundary event.
K–Pg boundary section at Gubbio: A limestone succession near Gubbio, Italy, containing a thin iridium-rich layer at the extinction boundary. This site supplied one of the key early measurements of the boundary iridium anomaly.
Ammonites: Marine cephalopods with coiled shells that flourished for hundreds of millions of years before going extinct at the boundary. Their extinction shows how strongly the crisis affected marine ecosystems.
Walter Alvarez: An American geologist who helped discover the boundary iridium anomaly and develop the impact hypothesis. His fieldwork connected a thin rock layer to a global extinction problem.
Gradualism: The view that geological and biological change generally occurs through slow, continuous processes. The abrupt boundary evidence challenged gradual explanations for this extinction.

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