It was at this time when something happened that caused dinosaurs to become extinct. While there are several ideas, one that many scientists believe is that a huge comet or asteroid 6 to 12 miles wide slammed into the region that is now part of the eastern coast of Mexico, but at that time was under water.
The impact of this object is believed to have caused darkness over the entire earth for many months, due to the huge amounts of dust that were thrown into the atmosphere. A global wildfire would have destroyed over half of all living things. Water would have been poisoned in most places, and the earth would have sunk into a deep freeze while the dust was in the air.
Even through all this, some plants and animals survived, including some insects, fishes, frogs, crocodiles, turtles, birds, and mammoths.
This may have just been part of a series of changes that caused the extinction of the dinosaurs. Before the asteroid/comet hit the earth, massive eruptions of volcanoes had caused earth's climate to be changed. At about the same time, sea levels dropped dramatically, opening new land bridges, changing ocean currents, and affecting the climate. These changes in climate likely reduced the ability of the dinosaurs to adapt, and the impact from the asteroid/comet was the last straw.
The creatures that were able to survive all these changes came to dominate the landscape. Mammals grew larger, and moved into new areas, taking over locations that had previously been the habitat of dinosaurs.
Changes in sea levels, ocean currents, and other events were also bringing in a new climatic cycle to the earth. Huge ice sheets would begin to cover large areas of the earth on a periodic basis. These swings in climate would have a major effect on animal habitats.