Scientists now believe that the dinosaurs died when the Earth was hit with a large comet or meteor, about 65 millions years ago. The following comes from Compton's Interactive Encyclopedia:
The Extinction of the Dinosaurs
All of the remaining families of dinosaurs, along with a variety of other animals, died out some 65 million years ago. Traditionally, paleontologists believed that the number of dinosaurs slowly declined for millions of years before they eventually disappeared. Many studies of fossilized plants point to a slow change in environmental conditions, leading to a generally cooler climate, and thus the gradual extinction of dinosaurs by starvation.
Today, however, many scientists think that the extinction was sudden and catastrophic. This line of thought originated with the American scientist Luis Walter Alvarez, who in the late 1970s discovered evidence for an abrupt end to the Age of Dinosaurs. Right at the end of the Cretaceous period, traces of the chemical element iridium appear to have been deposited in soil around the world, along with what is known as shocked quartz a form of quartz produced only in colossal explosions. This deposit shows up today in excavations as a thin layer of boundary clay, so called because it separates sediments from the Cretaceous period from those of the Tertiary. Iridium is rare on the Earth's surface today, but it occurs in higher concentrations deep in the Earth and in meteorites that fall to Earth from space. It is therefore possible that an asteroid or comet, similar to meteorites in its composition, smashed into the Earth at the end of the Age of Dinosaurs. The dust thrown into the atmosphere would have caused a long period of darkness and cold during which a great many of the world's life forms would have died.
Here is a chart showing the history of the Dinosaurs.
Excerpted from Compton's Interactive Encyclopedia. Copyright (C) 1994, 1995 Compton's NewMedia, Inc.
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