Paleontology is the study of life-forms from the distant past, as revealed primarily through the record of fossils left on and in the earth. It is a subdiscipline of both the biological and the earth sciences, one that brings to bear the techniques of geologic study and several areas of biology, including botany and zoology. But the term paleontology, as used in the present context, also provides a convenient means of encompassing, in a single word, the study of the distant biological past. Such study, of course, is related intimately to the investigation of evolutionary processes and phenomena. In the present context, however, our concern is not so much with the theory and principles of evolution, discussed elsewhere in this book, but rather with a relatively short overview of biological history. This area of biological investigation calls upon such concepts as mass extinction and fossilization as well as the study of plant and animal forms that scientists know only from scientific reconstruction of the past rather than from direct experience. Chief among these life-forms are the dinosaurs, which dominated Earth for a period of more than 100 million years, ending about 65 million years ago. This span of time, impressive as it seems on the human scale, is minuscule compared with the entire history of life on Earth.