K–T extinctionabbreviation of Cretaceous–Tertiary extinction, also called K–Pg extinction or Cretaceous–Paleogene extinctiona global extinction event responsible for eliminating approximately 80 percent of all species of animals at or very close to the boundary between the Cretaceous and Paleogene periods, about 65.5 66 million years ago. The K–T extinction was characterized by the elimination of many lines of animals that were important elements of the Mesozoic Era (251 252.2 million to 65.5 66 million years ago), including nearly all of the dinosaurs and many marine invertebrates. The event receives its name from the German word Kriede Kreide, meaning “chalk,” and the word Tertiary, which was traditionally used to describe the period of time spanning the Paleogene and Neogene periods. The K–T extinction ranks third in severity of the five major extinction episodes that punctuate the span of geologic time.

Of the dinosaurs, only the archosaurs, a lineage that gave rise to modern birds and crocodilians, survived the extinction. Of the planktonic marine flora and fauna, only about 13 percent of the coccolithophore and planktonic foraminiferan foraminiferal genera remained alive. Among free-swimming mollusks, the ammonoids and belemnoids became extinct. Among other marine invertebrates, the larger foraminiferans foraminifers (orbitoids) died out, and the hermatypic corals were reduced to about one-fifth of their genera. Rudist bivalves also disappeared, as did bivalves with a reclining life habit, such as Exogyra and Gryphaea. The stratigraphically important inoceramids also died out.




The asteroid theory has met with skepticism among paleontologists who prefer to look to , with some agitating for terrestrial factors as the cause of the extinction and others claiming that the amount of iridium dispersed by an impact was caused by a smaller object, such as a comet. A huge outpouring of lava, known as the Deccan trapsTraps, occurred in India during the latest Cretaceous. Some paleontologists believe that the carbon dioxide that accompanied these flows created a global greenhouse effect that greatly warmed the planet. Others note that tectonic plate movements caused a major rearrangement of the world’s landmasses, particularly during the latter part of the Cretaceous. The climatic changes resulting from such continental drift could have caused a gradual deterioration of habitats favourable to the dinosaurs and other animal groups that suffered extinction. It is, of course, possible that sudden catastrophic phenomena such as an asteroid or comet impact contributed to an environmental deterioration already brought about by terrestrial causes.