Cretaceous

noun

(L. creta: chalk) Third of the three periods included in the Mesozoic Era. It began approximately 145.6 million years ago and ended about 65 million years 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.