The fossil record suggest that sudden, catastrophic events that had abruptly decimated life on Earth, and that new life forms emerged following these ______ _______.
Following conventional wisdom of the time, they concluded that the extinctions of old life forms and emergence of new ones occurred _____, as a result of gradually changing conditions.
From studies of this type, scientists learned that at least __ times in earth history, catastrophic extinctions decimated life on earth.
The most dramatic extinction occurred during the ______ period. (90 percent of ocean species died out, 30 percent of insects vanished. A modern scientist said that this extinction event was "the closest life has come to complete extermination since its origin"
In 1977, the father-and-son team of _____ and _____ _____ were studying rocks that formed at the time the _____ and so many other life forms became extinct. They found high contents of the element ______, which comes from meteorites. This event, called the ______ _____ extinction, killed off the dinosaurs and many other life forms.
Volcanoes erupt materials capable of causing both atmospheric ____ and _____. This may be another reason why the dinosaurs became extinct. Another is because all of the continents were together so too much carbon dioxide came from the sea.