Biostratigraphy is a method of relative dating that utilizes the presence of fossil organisms. It is based upon the idea that a particular time interval can be characterized by a distinctive assemblage, or group, of organisms. For example, if one knows that dinosaurs lived from 228 to 65 Ma, then any rock containing a dinosaur fragment must fall within that age range. Although biostratigraphy cannot provide ages in years before present, the fact that many species of organisms have existed on Earth for 1-2 million-year intervals enables them to be

Figure 2.3. Bone between two dated horizons. As we know the ages of the two horizons, the age of the bone can be interpolated between them (see the text).

used as powerful dating tools. For example, Tyrannosaurus rex lived for only about 2 million years, from 67 to 65 Ma. Therefore if we found a T. rex fossil (a good find, indeed), we would know that, no matter where that dinosaur was found, it could be correlated with T. rex-bearing sediments in North America that have been well dated at 67-65 Ma.

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