The Cretaceous–Tertiary extinction event was the large-scale mass extinction of animal and plant species in a geologically short period of time, approximately 65.5 million years ago (mya). It is associated with a geological signature, usually a thin band dated to that time and found in various parts of the world, known as the K–T boundary. The event marks the end of the Mesozoic Era, and the beginning of the Cenozoic Era. Non-avian dinosaur fossils are only found below the K–T boundary and became extinct immediately before or during the event. Mosasaurs, plesiosaurs, pterosaurs and many species of plants and invertebrates also became extinct. Mammalian and bird clades passed through the boundary with few extinctions, and radiation from those Maastrichtian clades occurred well past the boundary.
Many scientists theorize that the K-T extinctions were caused by one or more catastrophic events such as massive asteroid impacts or increased volcanic activity. Several impact craters and massive volcanic activity in the Deccan traps have been dated to the approximate time of the extinction event. These geological events may have reduced sunlight and hindered photosynthesis, leading to a massive disruption in Earth's ecology. Other researchers believe the extinction was more gradual, resulting from slower changes in sea level or climate.
During the Maastrichtian stage of the Cretaceous, there was already a progressive decline in biodiversity prior to the ecological crisis indicated by the K–T boundary. After the K–T event, biodiversity required substantial time to recover, despite the existence of abundant vacant ecological niches.
Even though the boundary event was severe, there was significant variability in the rate of extinction between and within different clades. Because atmospheric particles blocked sunlight, reducing the amount of solar energy reaching the earth's surface, species which depended on photosynthesis declined or became extinct. Photosynthesizing organisms, including phytoplankton and land plants, formed the foundation of the food chain in the late Cretaceous as they do today. Evidence suggests that herbivorous animals died out when the plants they depended on for food became scarce; consequently, top predators such as Tyrannosaurus rex also perished.
Coccolithophorids and molluscs, including ammonites, rudists, freshwater snails and mussels, and those organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, it is thought that ammonites were the principal food of mosasaurs, a group of giant marine reptiles that became extinct at the boundary.
Omnivores, insectivores and carrion-eaters survived the extinction event, perhaps because of the increased availability of their food sources. At the end of the Cretaceous there seem to have been no purely herbivorous or carnivorous mammals. Mammals and birds which survived the extinction fed on insects, worms, and snails, which fed on dead plant and animal matter. Scientists hypothesize that these organisms survived the collapse of plant-based food chains because they fed on detritus.
In stream communities, few groups of animals became extinct; because stream communities rely less directly on food from living plants and more on detritus that washes in from land, buffering them from extinction.
Similar, but more complex patterns have been found in the oceans. Extinction was more severe among animals living in the water column, than among animals living on or in the sea floor. Animals in the water column are almost entirely dependent on primary production from living phytoplankton, while animals living on or in the ocean floor feed on detritus or can switch to detritus feeding.
The largest air-breathing survivors of the event, crocodilians and champsosaurs, were semi-aquatic and had access to detritus. Modern crocodilians can live as scavengers and can survive for months without food, and their young are small, grow slowly, and feed largely on invertebrates and dead organisms or fragments of organisms for their first few years. These characteristics have been linked to crocodilian survival at the end of the Cretaceous.