qnli question: What is the largest extinction of species in Earth's history known as? sentence: The upper (Cretaceous) boundary is set at the Cretaceous–Tertiary (KT) extinction event (now more accurately called the Cretaceous–Paleogene (or K–Pg) extinction event), which may have been caused by the impactor that created Chicxulub Crater on the Yucatán Peninsula.