The Cretaceous–Tertiary extinction event was a mass extinction which occurred an estimated 65.5 million years ago, wiping out a substantial number of the species on Earth. Estimates of the severity of the event vary, with some biologists suggesting that as many as 85% of plant and animal species might have been lost during this period in Earth's history. Other estimates are more conservative, but still quite remarkable.
Among the lay public, the Cretaceous–Tertiary extinction event is probably most notable because it marked the end of the dinosaurs. Many other organisms were lost, however, including a wide range of marine animals. In a sense, it cleared the decks for the rise of birds and mammals, who emerged from the event largely unscathed, as did fungi and ferns, which experienced a notable resurgence after the Cretaceous–Tertiary extinction event.
For geologists, the Cretaceous–Tertiary extinction event is very easy to identify, because it left behind a distinctive band known as the K-T boundary. The K-T boundary occurs all over the Earth and in a variety of rock deposits, suggesting that whatever caused the Cretaceous–Tertiary extinction event, it was widespread, leaving no corner of the Earth untouched. Study of the K-T boundary also reveals a characteristic mineral signature, and often the rock above and below the band is markedly different.
The cause of the Cretaceous–Tertiary extinction event is a subject of some debate amongst scientists, and it may never be fully resolved. Some suggest that it was caused by a series of catastrophes, such as volcanic eruptions, meteor collisions, and supernovae which dramatically changed the Earth's climate, leading to widespread death among organisms which could not survive in Earth's radically altered climate. Some geological evidence does support this theory, with several large meteor craters around the world dating to the right time period.
Others believe that the event was more gradual, although they also link it to climate change. It may have been caused, for example, by radical changes in sea level, which would have changed the Earth's environment. This theory is also supported, as fossil evidence suggests that biodiversity was already on the decline before the Cretaceous–Tertiary extinction event, and the limited array of fossils available for study makes it difficult to precisely determine the amount of time involved in this historic mass extinction.