K–T extinction

K–T extinction, a global mass 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 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 (251.9 million to 66 million years ago), including nearly all of the dinosaurs and many marine invertebrates. The event receives its name from the German word Kreide, meaning “chalk” (which references the chalky sediment of the Cretaceous Period), 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.

(Read E.O. Wilson’s Britannica essay on mass extinction.)

The only lines of archosaurs—the group of reptiles that contains the dinosaurs, birds, and crocodilians—that survived the extinction were the lineages that led to modern birds and crocodilians. Of the planktonic marine flora and fauna, only about 13 percent of the coccolithophore and planktonic foraminiferal genera remained alive. Among free-swimming mollusks, the ammonoids and belemnoids became extinct. Among other marine invertebrates, the larger 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 (or partially buried) life habit, such as Exogyra and Gryphaea. The stratigraphically important inoceramids also died out.

The mass extinction was quite different between, and even among, other marine and terrestrial organisms. Land plants appear to have fared better than land animals; however, there is evidence of widespread species extinctions of angiosperms and other dramatic shifts among North American plant communities. It is important to note that some groups of reptiles died out well before the K–T boundary, including flying reptiles (pterosaurs) and sea reptiles (plesiosaurs, mosasaurs, and ichthyosaurs). Among surviving reptile groups, turtles, crocodilians, lizards, and snakes were either not affected or affected only slightly. Effects on amphibians and mammals were also relatively mild. These patterns seem odd, considering how environmentally sensitive and habitat-restricted many of those groups are today.

Many hypotheses have been offered over the years to explain dinosaur extinction, but only a few have received serious consideration. The extermination of the dinosaurs has been a puzzle to paleontologists, geologists, and biologists for two centuries. Proposed causes include disease, heat waves and resulting sterility, freezing cold spells, the rise of egg-eating mammals, and X-rays from a nearby exploding supernova. Since the early 1980s, however, much attention has been focused on the so-called “asteroid theory” formulated by American scientists Walter Alvarez and Luis Alvarez. This theory states that the impact of a large asteroid on Earth may have triggered a mass extinction event some 66 million years ago by ejecting a huge quantity of rock debris into the atmosphere, enshrouding Earth in darkness for several months or longer. With no sunlight able to penetrate this global dust cloud, photosynthesis ceased, resulting in the death of green plants and the disruption of the food chain.

There is much evidence in the rock record that supports this hypothesis. A huge crater 180 km (112 miles) in diameter dating to the end of the Cretaceous was discovered buried beneath sediments of the Yucatán Peninsula near Chicxulub, Mexico. A second, smaller crater, which predates the one at Chicxulub by about 2,000 to 5,000 years, was discovered at Boltysh in Ukraine in 2002. Its existence raises the possibility that the K–T extinction was the result of multiple bolide impacts. In addition, tektites (fractured sand grains characteristic of meteorite impacts) and the rare-earth element iridium, which is common only deep within Earth’s mantle and in extraterrestrial rocks, have been found in deposits associated with the extinction. There is also evidence for some spectacular side effects of bolide impact, including an enormous tsunami that washed up on the shores of the Gulf of Mexico and widespread wildfires triggered by a fireball from the impact.

Despite this strong evidence, the asteroid theory has met with skepticism among some paleontologists, 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 Traps, occurred in India at the end of the 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.