Why Dinosaurs Became Extinct
Dinosaurs were extinct for millions of years before humans appeared. There is no complete explanation for why the dinosaurs became extinct. Among the many theories that have been proposed for the extinction are the following: (1) dinosaurs evolved such awkward bodies that they could not breed; (2) infectious diseases wiped out the dinosaurs; (3) small mammals fed on dinosaur eggs; and (4) sea levels dropped, creating land bridges that allowed dinosaurs previously separated to invade each other's territory, upsetting the balanced ecological relationships that existed between species. Most of these explanations fail to consider that the dinosaurs were well adapted to their environment, living successfully for millions of years, and that many other creatures became extinct at about the same time as the dinosaurs.
The most likely explanation for the extinction of the dinosaurs is that there was some type of drastic global change, to which the dinosaurs were extremely vulnerable. Such a change could have resulted from a catastrophe that affected the climate and vegetation, reducing the food supply, or decreased the oxygen concentration of the atmosphere. Some scientists believe that the dinosaurs had already been dying off gradually (possibly for some of the reasons listed in the preceding paragraph) and the catastrophe greatly accelerated their decline.
Scientists have proposed several theories to describe how such a global catastrophe might have occurred, including: (1) an extremely large object from space crashed into the earth, producing an explosion that would have created immense clouds of dust and ignited vast forest fires; (2) massive volcanic eruptions ignited fires and emitted volcanic gases and ash; and (3) a combination of the first two theories---that is, an object from space hit a weak spot in the earth's crust, causing violent volcanic eruptions. The clouds of dust or ash resulting from these catastrophes would have blocked out sunlight for months or years, chilling the land and killing plants. The dinosaurs and certain other species would have died from a lack of food and warmth.
In 1991, scientists discovered evidence in the Yucatn of a very large crater dating from the time the dinosaurs became extinct. The crater, which is now filled in and buried, was probably formed by the impact of a comet or asteroid. Many scientists believe this impact could have produced a global catastrophe leading to the dinosaurs' extinction.