65 million years ago a mass extinction event ended the age of dinosaurs. The end of the Cretaceous period is marked over much of the Earth by a discontinuity with high Iridium levels and an abrupt change in flora and fauna. Approximately 50% of all genera went extinct during this event. There are a number of theories as to what caused this event, detailed below.
LW Alverez first discovered the high Iridium levels at the Cretaceous-Tertiary (KT) boundry in 1980. He hypothesized that an asteriod hitting the earth was responsible. Iridium is very rare on the Earth's surface, but much more common in the Earth's interior as well as in extraterrestrial objects, such as asteriods and comets. The discovery of a crater burried under Chicxulub in the Yucatan as well as varios types of debris in North America have lended credibility to this theory. Most paleontologists agree that an asteroid did hit the Earth 65 million years ago, there is still disagreement over whether that impact caused the extinction.
A minority of scientists think the extensive volcanic activity in India known as the Deccan Traps may have been responsible for, or contributed to, the extinction. It has also been suggested that this is a secondary effect of the impact.
The impact and volcanic theories can be labled "fast extinction" theories. There are also a number of slow extinction theories. Studies of the diversity and population of species have shown that the dinosaurs were in decline for a period of about 10 million years before the asteriod hit. Slower mechanisms are needed to explain such extinctions. Climatic change, a change in the Earth's magnetic field, and desease have all been sugested as possible slow extinction theories.
It is worth noting that the Cretaceous extinction is not the only mass extinction in Earth's history. It is not even the worst. Other extinctions occurred in the Permian (the largest ever), the Triassic, and the Jurassic.