Why did the dinosaurs become extinct? What happened over 65 million years ago to kill a species that had survived for 150 million years? The answer is that no one knows for sure, but everyone seems to have their own favorite theory. Whatever caused the death of the dinosaurs also caused the death of about 70% of all of the species on the Earth. Now we'll introduce just a few of the popular theories dealing with the dinosaur extinction at the the end of the Cretaceous Period.

Asteroid Impact
Asteroids very seldom pass through the earth's atmosphere, but when they do they cause mass destruction. One of the most common extinction theories is that an asteroid 4-9 miles (10 km) wide struck the earth on the tip of the Yucatan Peninsula, in the Gulf of Mexico dealing the dinosaurs a fatal blow. Upon striking, the comet would have sent dust and steam into the atmosphere and caused huge forest fires, tidal waves, severe storms, and eliminated all sunlight. It would be six months before sunlight would reach the ground after such a blow. The dinosaurs, even in their prime, stood no chance without sunlight and plant life.

Climate Change
The climate of the Cretaceous period was tropical. Scientists know this because nearly one half of the plants in this time were tropical plants. Many scientists believe that at the end of the Cretaceous period the temperature dropped. If this drop in temperature is correct, many of the plants that the plant eating dinosaurs ate would die and thus many of the plant eaters would die as well. If the plant eaters died there would be nothing for the meat eaters to eat and they would eventually die out also. Plankton also seemed to decrease in number during the Tertiary period which could explain the death of the marine dinosaurs that fed on them. This popular theory just possibly could have caused the dinosaurs demise.

Volcanic Eruptions
Could numerous volcanic eruptions have caused such mass destruction? Possibly. When volcanos erupt, they send huge clouds of dust, sulfuric acid, and poisonous gases into the air making the air almost impossible to breath. The dust and ash in the atmosphere would also change the climate and sunlight levels. On top of that, the ground would be covered with layers and layers of molten rock. The dinosaurs would never have survived such a hostile, poisoned environment!

Disease
Disease has also been proposed as the killer of the dinosaurs. As the Cretaceous period went on, more and more land bridges started to appear on the earth. The oceans started to dry up and the dinosaurs were able to walk across the land that was there. As the dinosaurs searched for food they would walk across these new land bridges and find relatives of theirs from when the lands of the world were one. But land bridges brought more than dinosaurs looking for food into the new land. They brought all of the diseases that those dinosaurs had inside of them. The diseases did not hurt those dinosaurs because they were immune to them, but to the other dinosaurs they could be fatal and the other way around. In most cases disease in a species is not fatal and is a positive thing because it will wipe out the weak so only the strong survive to pass their genes on to the next generation, but in the dinosaurs case it might not have been that way.