Cretaceous Third of the three periods included in the Mesozoic Era. It began approximately 145.6 Ma ago and ended about 65 Ma ago. It is noted for the deposition of the chalk of the White Cliffs of Dover, England, and for the mass extinction of many invertebrate and vertebrate stocks. Among these were the dinosaurs, mosasaurs, ichthyosaurs, and plesiosaurs. See K/T BOUNDARY EVENT.

K/T boundary event The impact of an asteroid, about 10–11 km in diameter, that struck the Earth about 65 Ma ago near Chicxulub, on the coast of northern Yucatan, Mexico. Initial evidence for the impact was an iridium-enriched layer of clays, found in many parts of the world. The Chicxulub crater, now filled with sediments, is about 180km in diameter. Many scientists have concluded that the impact caused the mass extinction at the end of the Cretaceous. This is not accepted by all, and how the impact caused the extinction is not known.