Non-avian dinosaurs (those dinosaur species that did not evolve into birds) vanished abruptly about 65 million years ago. Many other forms of animals and plants also disappeared in this mass mortality, known as the Cretaceous-Paleogene extinction event. (It was formerly known as the Cretaceous-Tertiary extinction event). It is not known for certain why they became extinct, but there is evidence of a global environmental stress at that time.
Scientists have learned this from the evidence of the geologic record. During the approximately 4 billion years of Earth's existence, layers of soil and rock have slowly accumulated. Geologists can estimate the ages of the various layers. Generally, the lower the layer, the older it is.
Sometimes the layers contain fossils of animals and plants. How long ago these organisms died can be estimated by dating the layer in which their traces are found.
The absence of traces of an organism in layers above those in which traces do appear indicates that the species became extinct.
Evidence in the geologic record shows many kinds of organisms existed until about 65 million years ago. Around that time, about half of the world's species (those present at the time) disappeared. More of those living in the sea died than did those living in freshwater lakes and streams. No land animal larger than 25 kg (55 lb.) is known to have survived, and most survivors were much smaller—about the size of rats.
The Cretaceous-Paleogene (K-Pg) boundary (formerly called the Cretaceous-Tertiary or K-T boundary) is drawn after the last, or stratigraphically highest, occurrence of dinosaur bones. The K-Pg boundary also can be recognized on land by dramatic changes in fossil plants; in some areas, 60% of Cretaceous Period terrestrial plants went extinct at the K-Pg boundary.
In some places, an unusually high concentration of the element iridium, a heavy metal that is also found in meteorites, marks the K-Pg boundary. Called the Iridium Anomaly, it may represent an extraterrestrial source of the cause of the mass extinction: were the organisms (dinosaurs included) exterminated through environmental disturbances caused by fallout from the impact of a meteorite, collision with a comet, a giant solar flare, an interstellar cloud, or an exploding star?
Although the K-Pg extinction event was catastrophic, there have been other mass extinction events throughout the Earth’s history, including one some 250 million years ago at the end of the Permian Period that is considered by scientists to have been more severe in its impact on global biodiversity.