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Dinosaurs inhabited the earth for over 165 million years， during the Mesozoic Era.
Most dinosaur species lived and died out during this period， but at the end of the Cretaceous period （65 million years ago）， all remaining types of dinosaurs （with the exception of birds which had already evolved， and are widely believed to be descended from dinosaurs） died out. At the same time many other creatures also became extinct. In fact， all animals weighing more than 55 pounds （25 kilograms） seem to have died out！
Scientists have proposed many different possible explanations for what may have happened， and why this mass extinction might have happened：
Asteroid Impact Hypothesis
An asteroid hit the earth. Apart from the immediate impact of fires and flooding （if it hit the sea）， this would have caused catastrophic （災难性的） changes in the environment because of the dust thrown up into the air blocking out sunlight.
Volcanism Hypothesis
Volcanoes （there is known to have been very active volcanic around this time， especially in India in a region that is today known as the Deccan Traps） could have caused catastrophic changes in the environment.
The dust from volcanoes would have affected the climate， sulphur （硫磺） from volcanoes could have caused immensely strong acid rain， and poisonous substances such as selenium （硒） could have been released into the atmosphere.
Supernova Hypothesis
A nearby star exploded as a supernova. A nearby supernova would have bathed the earth in deadly radiation （辐射）.
Climatic Change Hypothesis
Changes in the Earth s climate caused the mass extinction. This could have been caused by gradual changes in the positions of continents effecting ocean currents and winds， or by changes in the Earth s orbit around the Sun， or even by a relatively sudden greenhouse effects.
Flowers Drugged the Dinosaurs Hypothesis
The first flowering plants appeared during the Cretaceous period， and if dinosaurs ate them they could have been drugged by them.
Mammals Ate the Dinosaurs Eggs Hypothesis
Mammals ate the dinosaurs eggs. This used to be a popular hypothesis—but does not explain the extinction of many other species， especially marine species. It also does not explain why dinosaurs were very successful for millions and millions of years， during most of which time， mammals were also around.
Caterpillars （毛虫） Ate All the Plants Hypothesis
Caterpillars ate all the plants and there was not enough food for herbivores （食草动物）， and eventually not enough meat for carnivores.
Disease Epidemic Hypothesis
The dinosaurs could have been wiped out by a disease epidemic （流行病）.
Although many scientists now favor the asteroid impact hypothesis， it is not yet actually certain that this is the reason why the dinosaurs died out. It is possible that one of the other theories might be the correct one. It is also possible， that there could have been several events which happened together， and between them， killed all the dinosaurs.
Most dinosaur species lived and died out during this period， but at the end of the Cretaceous period （65 million years ago）， all remaining types of dinosaurs （with the exception of birds which had already evolved， and are widely believed to be descended from dinosaurs） died out. At the same time many other creatures also became extinct. In fact， all animals weighing more than 55 pounds （25 kilograms） seem to have died out！
Scientists have proposed many different possible explanations for what may have happened， and why this mass extinction might have happened：
Asteroid Impact Hypothesis
An asteroid hit the earth. Apart from the immediate impact of fires and flooding （if it hit the sea）， this would have caused catastrophic （災难性的） changes in the environment because of the dust thrown up into the air blocking out sunlight.
Volcanism Hypothesis
Volcanoes （there is known to have been very active volcanic around this time， especially in India in a region that is today known as the Deccan Traps） could have caused catastrophic changes in the environment.
The dust from volcanoes would have affected the climate， sulphur （硫磺） from volcanoes could have caused immensely strong acid rain， and poisonous substances such as selenium （硒） could have been released into the atmosphere.
Supernova Hypothesis
A nearby star exploded as a supernova. A nearby supernova would have bathed the earth in deadly radiation （辐射）.
Climatic Change Hypothesis
Changes in the Earth s climate caused the mass extinction. This could have been caused by gradual changes in the positions of continents effecting ocean currents and winds， or by changes in the Earth s orbit around the Sun， or even by a relatively sudden greenhouse effects.
Flowers Drugged the Dinosaurs Hypothesis
The first flowering plants appeared during the Cretaceous period， and if dinosaurs ate them they could have been drugged by them.
Mammals Ate the Dinosaurs Eggs Hypothesis
Mammals ate the dinosaurs eggs. This used to be a popular hypothesis—but does not explain the extinction of many other species， especially marine species. It also does not explain why dinosaurs were very successful for millions and millions of years， during most of which time， mammals were also around.
Caterpillars （毛虫） Ate All the Plants Hypothesis
Caterpillars ate all the plants and there was not enough food for herbivores （食草动物）， and eventually not enough meat for carnivores.
Disease Epidemic Hypothesis
The dinosaurs could have been wiped out by a disease epidemic （流行病）.
Although many scientists now favor the asteroid impact hypothesis， it is not yet actually certain that this is the reason why the dinosaurs died out. It is possible that one of the other theories might be the correct one. It is also possible， that there could have been several events which happened together， and between them， killed all the dinosaurs.