Extinction of the Dinosaurs
Perhaps the most notable event of the Cretaceous was its conclusion. About 65 million years ago the second greatest mass extinction in Earth history occurred, resulting in the loss of the dinosaurs as well as nearly 50% of all the world’s species. Though not nearly as severe as the end-Permian mass extinction, the end-Cretaceous extinction is the most famous mass extinction in Earth history. Other great animals also went extinct at that time, including flying reptiles (pterosaurs) and the last mosasaurs and plesiosaurs. Many mollusks, including rudistid and inoceramid clams, ammonites, and belemnites, also became extinct, as did many species of microscopic marine plankton. Terrestrial plants also suffered a major extinction at this time; in some regions up to 60% of latest Cretaceous plant species were absent in the subsequent Paleocene. Terrestrial insects also suffered a high level of extinction, especially those that were highly specialized to feed on one or a few types of plants. In fact, the level of insect herbivory—both generalized and specialized—did not recover to latest Cretaceous levels until the Paleocene-Eocene boundary, approximately 9 million years later. In spite of the severity of extinctions at the end of the Cretaceous, many types of animals and plants survived and gave rise to new groups of organisms in the Paleocene.

The causes of the end-Cretaceous extinction are still being debated by paleontologists. Researchers agree that a major factor was an asteroid about 10 kilometers in diameter that struck what is now the Yucatán peninsula in Mexico. The effects of the impact were catastrophic, probably including global forest fires, possibly a period of cold weather due to sunlight-blocking dust and smoke, and a subsequent period of hot climate caused by the high levels of CO₂ released into the atmosphere by the impact. Evidence for the devastation of terrestrial vegetation comes in the form of a thin rock layer deposited just after the impact that is dominated by fossil plants whose present-day relatives recover well after fires or other disturbances. Some paleontologists argue that dinosaurs were already in decline before the asteroid impact, so that its environmental effects merely hastened their extinction. Alternatively, others point to the high abundance and variety of dinosaur species recorded even in the sediments deposited just below the asteroid impact layer in the Hell Creek Formation of western North America.

Regardless of what caused the disappearance of the dinosaurs, the mass extinction at the end of the Cretaceous led the way for the rapid rise to dominance of new groups of organisms during the following time period, the Paleocene. In particular, Paleocene mammals would spread and evolve into the many ecological niches left open by the extinction of the dinosaurs.