Many hypotheses have been suggested as to what killed off the dinosaurs at the end of the Cretaceous Period (140 to 65 million years ago). The most famous hypothesis for the extinction of the dinosaurs is that they were killed off by a large asteroid impact. The Asteroid Impact Theory was first introduced by Luis Alvarez and his son Walter.
The Asteroid Extinction theory states that a large asteroid impact would have caused great changes to the earth. The impact would cause fires which would consume vegetation, while using up oxygen and creating carbon dioxide (greenhouse gasses) and other dangerous pollutants. These air-borne pollutants could lead to the production of acid rain, which would do further harm to the vegetation and animals. An asteroid impact could also trigger tsunamis (tidal waves), earthquakes, and volcanic eruptions.
The impact would also throw up large amounts of dust into the atmosphere, which would cause less sunlight to reach the earth, lowering the global temperature, and causing further decimation to the plantlife, and leave herbivorous (plant eating) animals with little to eat. As herbivorous animals died out, carnivorous (meat eating) animals would be left with less to eat as well. The dust in the air would also cause climatic changes, which could lead to storms, leading to further changes in the environment.
There is much evidence to corroborate the Asteroid extinction theory. Sizeable craters have been found which would be the result of an asteroid impact at the right time. A layer of Iridium (an element which is rare on Earth, but found more commonly in asteroids) has been found that also dates to the Cretaceous-Tertiary Boundary. Shocked quartz crystals (quartz crystals whose shape have been changed due to high heat and pressure, such as caused by an asteroid impact) have also been found which date to the appropriate time period.
Other factors may have also contributed to the Cretaceous-Tertiary extinction. Continental drift caused the breakup of the supercontinent Pangaea at this time; the movement of the resulting continents led to changes in regional climates, as the continents moved away from the Equator towards the Poles.
The mammals had appeared during the Triassic Period (230 to 195 million years ago), and were spreading and diversifying at the end of the Cretaceous Period. It is not clear whether the rise of the mammals led to the demise of the dinosaurs, or if the extinction of the dinosaurs allowed the mammals to take over. The angiosperms (flowering plants) appeared in the mid-Cretaceous, and their rise would have led to changes in the environment and the diets of the herbivorous dinosaurs.
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